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Scab15280APSnet Features - October 2000: Scab<br> <table cellspacing="0" cellpadding="0" width="340" align="right" border="0"> <tbody> <tr> <td width="230" height="137"> <p><a title="" href="/edcenter/apsnetfeatures/Article%20Images/PumpkinFig4_Lead.jpg"><span class="ms-rteCustom-PhotoCaption"><img style="border-right&#58;0px solid;border-top&#58;0px solid;border-left&#58;0px solid;width&#58;200px;border-bottom&#58;0px solid;height&#58;137px;" src="/edcenter/apsnetfeatures/PublishingImages/2000%20Features/Pumpkin4_Lead.jpg" border="0" /></span></a><br><span class="ms-rteCustom-PhotoCaption">Scab lesions on pumpkin rind with active sporulation in the sunken lesions. Courtesy <br>T. A. Zitter. (Click image for </span><a href="/online/feature/pumpkin/images/scabintro.htm"><span class="ms-rteCustom-PhotoCaption">larger view</span></a><span class="ms-rteCustom-PhotoCaption">)</span></p> <td width="100"></td></td></tr></tbody></table> <p style="margin-top&#58;0px;margin-bottom&#58;0px;"><br><br>Contributed by<br><b>T. A. Zitter</b></p> <p>Dept. of Plant Pathology<br>Cornell University<br>Ithaca, NY 14853-4203<br><br><br><br><br></p> <p>&#160;</p> <p>Although scab, or gummosis, is rarely a problem now for slicing and pickling cucumbers, because of the availability of many resistant cultivars, it remains a significant problem for <br>summer and winter squash, pumpkin, melon, and watermelon. The disease was first described <br>in the United States in New York in 1887. It has been reported in many cool, temperate parts <br>of North America, Europe, and Asia. </p> <h2>Symptoms</h2> <table cellspacing="0" cellpadding="0" width="210" align="left" border="0"> <tbody> <tr> <td width="200"><a title="" href="/edcenter/apsnetfeatures/Article%20Images/PumpkinFig4_1.jpg"><img style="border-right&#58;0px solid;border-top&#58;0px solid;border-left&#58;0px solid;width&#58;200px;border-bottom&#58;0px solid;height&#58;135px;" src="/edcenter/apsnetfeatures/PublishingImages/2000%20Features/PumpkinFig4_1.jpg" border="0" /></a><br><span class="ms-rteCustom-PhotoCaption">Fig. 1.Scab caused by <i>Cladosporium cucumerinum</i>. Lesions on zucchini squash leaf and fruit. Courtesy T. A. Zitter. (Click image for </span><a href="/online/feature/pumpkin/images/scab1.htm"><span class="ms-rteCustom-PhotoCaption">larger view</span></a><span class="ms-rteCustom-PhotoCaption">.)</span></td> <td width="10"></td></tr></tbody></table> <p>The fungus can attack any aboveground portion of the plant, including leaves, petioles, stems, and fruits. On leaves and runners, pale green, water-soaked areas are the initial symptoms. These spots gradually turn gray to white and may become &quot;shot-holed&quot; in appearance. A chlorotic halo appears around the lesion (Fig. 1). If weather conditions are favorable for disease development, scab can deform young leaves, especially of summer squash. The apical runners of young plants such as melons can be killed.<br><br><br><br></p> <table cellspacing="0" cellpadding="0" width="220" align="right" border="0"> <tbody> <tr> <td width="10"></td> <td width="200"><br><a title="" href="/edcenter/apsnetfeatures/Article%20Images/PumpkinFig4_2.jpg"><img style="border-right&#58;0px solid;border-top&#58;0px solid;border-left&#58;0px solid;width&#58;200px;border-bottom&#58;0px solid;height&#58;157px;" src="/edcenter/apsnetfeatures/PublishingImages/2000%20Features/PumpkinFig4_2.jpg" vspace="10" border="0" /></a><span class="ms-rteCustom-PhotoCaption"><br>Fig. 2. Extensive early infection of pumpkin by scab caused malformation of the rind surface and rather deep lesions. Courtesy T. A. Zitter.&#160;(Click image for </span><a href="/online/feature/pumpkin/images/scab2.htm"><span class="ms-rteCustom-PhotoCaption">larger view</span></a><span class="ms-rteCustom-PhotoCaption">).</span></td> <td width="10"></td></tr></tbody></table> <p><br>Scab can produce the greatest damage on fruit. The appearance of fruit lesions varies in different crops, depending on the susceptibility of the crop. Spots first appear as small, sunken areas similar to insect stings, about 3–4 mm in diameter (Fig. 1). A sticky substance may ooze from the infected area, especially on fleshy fruit. Secondary soft-rotting bacteria may also invade the cavities and lead to foul-smelling decay. On pumpkin fruit, lesions may appear as sunken craters and can lead to misshapen areas on the fruit (Fig. 2); these lesions can be confused with those caused by anthracnose. On more resistant watermelon fruits, lesions appear as raised blisters or superficial pimples.</p> <h2>Causal Organism</h2> <table cellspacing="0" cellpadding="0" width="220" align="right" border="0"> <tbody> <tr> <td width="20"></td> <td width="200"> <p><br><a title="" href="/edcenter/apsnetfeatures/Article%20Images/PumpkinFig4_3.jpg"><img style="border-right&#58;0px solid;border-top&#58;0px solid;border-left&#58;0px solid;width&#58;200px;border-bottom&#58;0px solid;height&#58;135px;" hspace="5" src="/edcenter/apsnetfeatures/PublishingImages/2000%20Features/PumpkinFig4_3.jpg" border="0" /></a><br><span class="ms-rteCustom-PhotoCaption">Fig. 3. Profuse sporulation on squash fruit. Courtesy T. A. Zitter. (Click image for </span><a href="/online/feature/pumpkin/images/scab3.htm"><span class="ms-rteCustom-PhotoCaption">larger view</span></a><span class="ms-rteCustom-PhotoCaption">.)<br></p></span></td></tr></tbody></table> <p>Scab is caused by <i>Cladosporium cucumerinum</i> Ellis &amp; Arth. Extensive presence of the fungus is not evident on infected leaves and stems, but sporulation can be profuse on fleshy fruit (Fig. 3). The fungus consists of septate and branching mycelium which appears hyaline when young and turns greenish to black with age. Conidia are oblong, colored, mostly continuous (or, in some cases, one-septate), and borne terminally on short, branched, dark conidiophores (Fig. 4). The one-septate conidia measure 4.6–5.7 × 16.4-22.5 µm. Structures intermediate between conidiophores and conidia also become detached and germinate. These are larger than conidia, have thicker walls, and have one to many cells.<br><br></p> <table align="right"> <tbody> <tr> <td width="20"></td> <td><br><br><br><a title="" href="/edcenter/apsnetfeatures/Article%20Images/PumpkinFig4_4.jpg"><img style="border-right&#58;0px solid;border-top&#58;0px solid;border-left&#58;0px solid;width&#58;200px;border-bottom&#58;0px solid;height&#58;135px;" src="/edcenter/apsnetfeatures/PublishingImages/2000%20Features/PumpkinFig4_4.jpg" border="0" /></a><br><span class="ms-rteCustom-PhotoCaption">Fig. 4. Conidia of <i>C. cucumerinum</i>. <br>Courtesy L. W. Hsu. (Click image for <br></span><a href="/online/feature/pumpkin/images/scab4.htm"><span class="ms-rteCustom-PhotoCaption">larger view</span></a><span class="ms-rteCustom-PhotoCaption">).<br><br><br></span></td></tr></tbody></table> <p> <h2>Disease Cycle</h2> <p></p> <p>The scab organism survives in soil on squash, melon, and pumpkin vines and reportedly may grow extensively as a saprophyte. The fungus may also be seedborne. It is disseminated on clothing and equipment and by insects. The conidia can survive long-distance spread in moist air. The most favorable weather conditions for disease development are wet weather (valley fogs, heavy dews, and light rains) and temperatures near or below 21°C, which usually occur after mid-season in the northern United States. At 17°C the growing tips of young plants are killed. Conidia germinate and enter susceptible tissue within 9 hr. A spot may appear on leaves within 3 days, and a new crop of spores is produced by the fourth day.</p> <h2>Control</h2> <p>The use of scab-resistant cultivars is an effective means of control in cucumber. Although scab resistance or tolerance has been reported in <i>Cucurbita pepo, C. maxima,</i> and <i>C. moschata,</i> no resistant cultivars are commercially available.</p> <p>Because the scab fungus appears to overseason well, rotation of cucurbit crops with nonhost crops is necessary, with two or more years between cucurbit crops. The use of disease-free seed is important. Select sites that have well-drained soils and are conducive to good air drainage to allow for rapid drying of the foliage.</p> <p>Protectant fungicides are recommended for control. However, fungicide sprays are less effective during extended periods of cool, wet weather, because of the short disease cycle.</p> <h2>Selected References</h2> <p><span class="ms-rteCustom-Reference">Crossan, D. F., and Sasser, J. M. 1969. Effect of rotation with corn on cucurbit scab. Plant Dis. Rep. 53&#58;452-453.</span></p> <p><span class="ms-rteCustom-Reference">Morton, D. J., Crossan, D. F., and Manning, W. J. 1967. Reduction of cucurbit scab by a one-year rotation with corn. Plant Dis. Rep. 51&#58;495-497.</span></p> <p><span class="ms-rteCustom-Reference">Strider, D. L., and Konsler, T. R. 1965. An evaluation of the <i>Cucurbita</i> for scab resistance. Plant Dis. Rep. 49&#58;388-391.</span></p> <p><span class="ms-rteCustom-Reference">Walker, J. C. 1950. Environment and host resistance in relation to cucumber scab. Phytopathology 40&#58;1094-1102.</span></p><a title="" href="/edcenter/apsnetfeatures/Pages/PumpkinsandCucurbits.aspx">RETURN TO APSnet FEATURE STORY</a><br>2/13/2019 7:39:30 AMScab lesions on pumpkin rind with active sporulation in the sunken lesions Extensive early infection of pumpkin by scab caused malformation of the rind surface and
Apple scab96264Education Center - Introductory Plant Disease Lessons<p><span class="ms-rteCustom-EdCtrDOI">Gauthier, Nicole. 2018. Apple scab. <em>The Plant Health Instructor</em>. DOI&#58; 10.1094/PHI-I-2000-1005-01<br><em>Updated 2018. Previous version by <span class="ms-rteCustom-EdCtrDOI">Vaillancourt, L.J. and J.R. Hartman</span>.</em></span></p> <p><strong><span class="ms-rteCustom-EdCtrLessonHeader">DISEASE&#58;</span>&#160;Apple scab </strong></p> <p><strong><span class="ms-rteCustom-EdCtrLessonHeader">PATHOGEN&#58;</span>&#160;<em>Venturia inaequalis</em></strong></p> <p><strong><span class="ms-rteCustom-EdCtrLessonHeader">HOSTS&#58;</span>&#160;</strong>&#160;<strong><span>Apples and flowering crabapples (<i>Malus </i>spp.), hawthorn (<i>Crataegus </i>spp.), mountain ash (<i>Sorbus </i>spp.), firethorn (<i>Pyracantha </i>spp.), and loquat (<i>Eriobotrya japonica</i>). Pear (<i>Pyrus </i>spp.) is infected by a related fungus, <i>Venturia pirina</i>, which causes similar symptoms.<span>&#160; </span>However, the apple scab pathogen will not infect pear, and the pear scab pathogen will not infect apple.</span><span></span></strong></p> <h3 align="center">Author</h3> <p align="center">Nichole Gauthier<br>University of Kentucky </p> <p align="center"><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab01.jpg"><img border="0" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab01sm.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;" /></a><br>&#160; <span><span class="ms-rteCustom-PhotoCaption"><strong>Figure 1</strong>.<span>&#160; </span>Typical “scabby” lesions are diagnostic symptoms <br>of advanced apple scab on fruit.</span></span></p> <p dir="ltr"><span>Apple scab occurs everywhere in the world where apples are grown and results in more losses than any other apple disease. It is most serious in areas that have cool, wet spring weather and may not be economically important in warm and/or dry climates.</span></p> <h2><font size="4">Symptoms and Signs</font></h2> <p><span>Apple scab results in symptoms on most upper plant parts, most notably leaves and fruit.<span>&#160; </span>Petioles, flowers, sepals, pedicels, young shoots, and bud scales can also become infected.</span></p> <h3>Apple leaf and fruit symptoms</h3> <p><span>Apple scab infections are initiated in early spring on emerging and young leaves.<span>&#160; </span>Early lesions appear 10 days later as lighter green areas compared to the surrounding leaf tissue.<span>&#160; </span>Lesions increase in size and become olive-colored and velvety as a result of asexual spore production (conidia) (<a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab02.jpg">Figures 2</a> and <a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab03.jpg">3</a>). Scab lesions that form on young leaves may expand to more than 1 cm in diameter. Ontogenetic resistance of older leaves, however, usually results in smaller lesions or no visible symptoms. Affected tissues eventually may become distorted and puckered, and leaf lesions become cracked and torn. Severely infected leaves drop from trees.<span>&#160; </span>Two to three consecutive defoliation events can weaken trees, resulting in a greater susceptibility to other stresses such as freeze damage, insect injury, and other diseases.</span></p> <p><span>Fruit lesions are generally blistered and &quot;scabby&quot; in appearance, with a distinct margin (<a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab04.jpg">Figures 4</a> and <a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab05.jpg">5</a>). The earliest noticeable symptom on fruit is water-soaked areas that rapidly develops into velvety, green to olive-brown lesions. Infections of young fruit cause fruit distortion as healthy tissue continues to grow (<a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab05.jpg">Figure 5</a>). Severely infected fruit often drop prematurely from trees. </span></p> <table cellpadding="4" align="center"> <tbody> <tr align="center" valign="bottom"> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab02.jpg"><img border="0" alt="Figure 2. Apple scab lesions on young apple leaves. (Courtesy W. E. MacHardy)" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab02sm.jpg" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;100px;" /></a><br><span><span class="ms-rteCustom-PhotoCaption"><strong>Figure 2.</strong> <span>Early lesions are olive-colored (A) and velvety <br>with no distinct margins (B).</span></span></span></td> <td valign="bottom"><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab03.jpg"><img border="0" alt="Figure 3. Blistered scab lesions on apple leaves. (Courtesy W. E. MacHardy)" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab03sm.jpg" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;100px;" /></a><br><span><span class="ms-rteCustom-PhotoCaption"><strong>Figure 3.</strong> <span>Affected leaves <br>become distorted and puckered.</span></span></span></td></tr> <tr align="center" valign="bottom"> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab04.jpg"><img border="0" alt="Figure 4. Blistered scab lesions on apple leaves. (Courtesy W. E. MacHardy)" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab04sm.jpg" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;125px;" /></a><br><span><span class="ms-rteCustom-PhotoCaption"><strong>Figure 4.</strong>&#160;<span>Fruit lesions appear blistered and scabby <br>(A) with distinct margins (B).</span></span></span></td> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab05.jpg"><img border="0" alt="Figure 5. Apple fruits which are distorted from apple scab infection. (Courtesy W. E. MacHardy)" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab05sm.jpg" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;125px;" /></a><br><span><span class="ms-rteCustom-PhotoCaption"><strong>Figure 5. </strong><span>Infections of young <br>fruit cause distortion as fruit mature.</span></span></span></td></tr></tbody></table><br> <h3>Apple blossoms</h3> <p><span>Blossom infections usually develop into small, dark green lesions at the base of flowers, on sepals, and on stem pedicels before and during bloom. Developing fruit may drop from infected pedicels, resulting in lower fruit yield.<span>&#160; </span>Reduced and late-return bloom can result from infections that had occurred in preceding years.<span>&#160; </span></span></p> <h2><font size="4">Pathogen Biology</font></h2> <p><i><span>Venturia inaequalis</span></i><span> is an ascomycete fungus; it produces sexual spores (ascospores) in a sac-like structure called an ascus (plural asci). The mycelium of <i>V. inaequalis</i> is septate, and the nuclei are haploid.</span></p> <h3>Sexual Reproduction</h3> <p><i><span>Venturia inaequalis</span></i><span> overwinters in fallen leaf and fruit debris as pseudothecial initials.<span>&#160; </span>These structures emerge in early spring at approximately the same time as host plants break dormancy.<span>&#160; </span></span></p> <p><span>Mating between the two different mating types takes place in debris, and both mating types must be present in order for sexual reproduction to be initiated. Mating consists of the fusion of a male organ (antheridium) formed from a hyphal tip of one mate to a female receptive hypha (trichogyne) from the opposite mate. The trichogyne is attached to a coil of hyphae called the pseudothecial initial. During fertilization, nuclei pass from the antheridium through the trichogyne into a cell at the base of the pseudothecial initial. </span></p> <p><span>After fertilization, the pseudothecial initial develops into a pseudothecium (<a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab06.jpg">Figure 6</a>), a cavity located within a dense mat of fungal mycelia called a stroma. Inside this cavity, asci and ascospores are formed (<a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab07.jpg">Figure 7</a>). The very brief diploid stage in the life cycle of <i>V. inaequalis</i> occurs within the pseudothecium in single hyphal cells (croziers), which give rise to the haploid ascospores following meiosis. Asci are elongated, sac-like structures, each of which contains eight ascospores in a linear arrangement (<a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab08.jpg">Figure 8</a>). The ascospores are brown, two-celled spores, and have a characteristic &quot;footprint&quot; shape (<a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab09.jpg">Figure 9</a>). The shape of ascospores inspired the Latin name for apple scab, &quot;inaequalis&quot;, which refers to the unequal size of the cells. Ascospores measure between 5 and 7 µm wide and between 11 and 15 µm long. </span></p> <p><span>During wet conditions, mature pseudothecia swell and protrude from the surface of fallen leaves (<a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab10.jpg">Figure 10</a>). Ascospores are released and carried to blossoms and leaves by rain and wind. There is only one cycle of ascospore production and infection per season.</span></p> <table cellpadding="4" align="center"> <tbody> <tr align="center" valign="bottom"> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab06.jpg"><img border="0" alt="Figure 7. Pseudothecium of Venturia inaequalis embedded in leaf tissue. The arrows point to hyphae which may represent the two m" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab06sm.jpg" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;150px;" /></a><br><span class="ms-rteCustom-PhotoCaption"><span class="ms-rteCustom-PhotoCaption"><span class="ms-rteCustom-PhotoCaption"><strong>Figure 6</strong>. </span><span><span>Pseudothecial initial develops into <br>a pseudothecium.<span>&#160; </span>Arrows show hyphae, <br>which may represent the two mating types.&#160;<span>&#160;</span></span><span>&#160;</span></span></span></span></td> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab07.jpg"><img border="0" alt="Figure 8. Cross section of a pseudothecium of Venturia inaequalis. The section has been stained so that the asci and ascospores " src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab07sm.jpg" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;150px;" /></a><br><span><span class="ms-rteCustom-PhotoCaption"><strong>Figure 7. </strong><span>Asci and ascospores form inside a <br>pseudothecium.<span>&#160; </span>Asci and ascospores may <br>be stained (not shown) for enhanced viewing.&#160;<span>&#160;</span></span></span></span></td></tr> <tr align="center" valign="bottom"> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab08.jpg"><img border="0" alt="Figure 9. Asci of Venturia inaequalis containing ascospores (arrows). (Courtesy W. E. MacHardy)" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab08sm.jpg" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;150px;" /></a><br><span><span class="ms-rteCustom-PhotoCaption"><strong>Figure 8. </strong>Asci of <i>Venturia inaequalis</i> containing eight ascospores. Arrows highlight individual ascospores. </span></span></td> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab09.jpg"><img border="0" alt="Figure 10. Two-celled ascospores of Venturia inaequalis. (Courtesy W. E. MacHardy)" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab09sm.jpg" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;150px;" /></a><br><span><span class="ms-rteCustom-PhotoCaption"><span><strong>Figure 9.</strong> Two-celled ascospores of <i>Venturia inaequalis</i> are footprint-shaped.&#160;<span>&#160;</span></span></span></span></td></tr> <tr align="center" valign="bottom"> <td colspan="2"><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab10.jpg"><img border="0" alt="Figure 11. View of a fallen apple leaf, showing the mature pseudothecia of Venturia inaequalis protruding above the surface read" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab10sm.jpg" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;150px;" /></a><br><span><span class="ms-rteCustom-PhotoCaption"><span><strong>Figure 10. </strong>View of a fallen apple leaf, showing the mature <br>pseudothecia of <i>Venturia inaequalis</i> protruding above the <br>surface ready to release ascospores.&#160;</span><span>&#160;</span></span></span></td></tr></tbody></table><br> <h3>Asexual Reproduction</h3> <p><i><span>V. inaequalis</span></i><span> reproduces asexually by spores called conidia. Conidia are single-celled, uninucleate, and narrower at one end than the other (<a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab11.jpg">Figure 11</a>). In mass, conidia appear brown or olive, but they are lighter when viewed individually under a microscope.<span>&#160; </span>Conidia measure between 6 and 12 µm wide and 12 and 22 µm long and are produced by specialized short hyphae called conidiophores. Conidiophores are formed on a dense mat of mycelia that pushes up through and ruptures the leaf cuticle (<a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab12.jpg">Figure 12</a>). It is this mass of conidia and conidiophores that causes the velvety appearance of young scab lesions. </span></p> <p><span>Conidia are produced nine to thirty days after initial leaf infection, depending upon temperature. They are disseminated by wind and by wind-driven rain. Both ascospores and conidia require a period of wetness in order to germinate. The germination hypha penetrates the cuticle and establishes a new infection. There can be many cycles of conidial production and infection within a single growing season.</span></p> <table cellpadding="4" align="center"> <tbody> <tr align="center" valign="bottom"> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab11.jpg"><img border="0" alt="Figure 12. Conidia of Spilocaea pomi (Venturia inaequalis). (Courtesy J. Hartman)" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab11sm.jpg" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;150px;" /></a><br><span><span class="ms-rteCustom-PhotoCaption"><strong>Figure 11</strong>.<span>&#160;</span>Conidia are asexual, <br>single-celled spores<br><br></span></span></td> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab12.jpg"><img border="0" alt="Figure 13. Cross section through a leaf infected with apple scab. Arrow indicates the leaf cuticle which has been ruptured and p" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab12sm.jpg" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;150px;" /></a><br><span><span class="ms-rteCustom-PhotoCaption"><strong>Figure 12. </strong><span>Cross section through a leaf infected with apple scab. Arrow indicates the leaf cuticle, which has been ruptured and pushed back by the mass of erupting conidia and conidiophores.&#160;<span>&#160;</span></span></span></span></td></tr></tbody></table><br> <h2><font size="4">Disease Cycle and Epidemiology</font></h2> <p align="center"><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScabdiscycle.jpg"><img border="0" alt="Disease Cycle" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScabdiscycle.jpg" style="width&#58;450px;" /></a><br><span class="ms-rteCustom-PhotoCaption"><strong>Figure 13.</strong> Disease cycle of apple scab.</span></p> <h3>Disease Cycle</h3> <p><span>During the growing season, the apple scab fungus is found only in the area between the host cuticle and the epidermis. Once infected leaves fall from trees and die, the fungal mycelia colonize them completely. <i>Venturia inaequalis</i> usually overwinters in fallen leaves as mycelia and pseudothecial initials. In milder climates, the fungus may overwinter in infected buds and produce conidia that serve as primary inoculum. In leaf debris, most pseudothecial initials form within a month after leaves fall, and then enter a period of dormancy.</span></p> <p><span>In early spring, mating (sexual reproduction) takes place in leaf debris between two mating types (as described above). In response to rain events, asci expand through the ostiole, a hole at the top of the pseudothecia. Ascospores are forcibly discharged and spread by wind and by wind-driven rain. Sufficient moisture must be present for ascospores to infect blossoms and young leaves. In most years, ascospore dissemination coincides with the several-week period between budbreak and the end of bloom.</span></p> <p><span>Lesions caused by ascospores (primary infections) produce asexual conidia within 9 days and up to 30 days later.<span>&#160; </span>Conidia are dispersed to healthy leaves and developing fruit, where they establish secondary infections. Up to 100,000 conidia can be produced by a single lesion. Lesions from primary or secondary infections expand at a rate that is determined partly by temperature and partly by characteristics of the host tissue, including genotype and age. Lesion expansion, in turn, affects the rate at which new spores are produced. During cooler conditions, or on more resistant cultivars, lesions expand more slowly and may be smaller in size. As a result, the number of secondary cycles will be fewer.</span></p> <p><span>Infection and spore production are dependent upon available moisture.<span>&#160; </span><span>&#160;</span>Infection by ascospores and conidia is highly dependent upon how long the leaves or fruit stay wet, as well as on the average temperature. The Mills table relates leaf wetness duration and temperature to determine the likelihood that conidial infection will occur (Figure 13). For example, at an average temperature of 18°C (65°F), light infection will result if leaves remain wet for 9 h. Lesions can produce conidia after 9 days if the temperature averages 18°C (65°F), but not until 17 days if the temperatures are lower, averaging only 8°C (49°F). The Mills table continues to be revised as more data are gathered from different regions, and modifications by A.L. Jones comprise the most commonly accepted version.</span></p><span><span class="ms-rteCustom-PhotoCaption"><b>Table 1.</b>&#160;</span></span><span style="font-size&#58;12pt;font-family&#58;&quot;times new roman&quot;, serif;line-height&#58;107%;"><span class="ms-rteCustom-PhotoCaption">Wetting period (in hours) required for apple scab infection at different air temperatures, and time required for development of conidia by lesions at different air temperatures. </span></span> <table bordercolor="#808080" cellspacing="2" cellpadding="3" bgcolor="#feecd3" border="1" style="border-width&#58;1px;border-style&#58;initial;border-color&#58;initial;"> <tbody> <tr> <td colspan="6" align="center"><b>Wetting period (HOURS)</b></td></tr> <tr align="center"> <td align="center"><b>Average Temperature (F)</b></td> <td align="center"><b>Average Temperature (C)</b></td> <td align="center"><b>Light Infection</b></td> <td align="center"><b>Moderate Infection</b></td> <td align="center"><b>Heavy Infection</b></td> <td align="center"><b>Incubation Period (days)</b></td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">78</td> <td align="center">25.6</td> <td align="center">13</td> <td align="center">17</td> <td align="center">26</td> <td align="center">...</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">77</td> <td align="center">25.0</td> <td align="center">11</td> <td align="center">14</td> <td align="center">21</td> <td align="center">...</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">76</td> <td align="center">24.4</td> <td align="center">9.5</td> <td align="center">12</td> <td align="center">19</td> <td align="center">...</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">63-75</td> <td align="center">17.2-23.9</td> <td align="center">9</td> <td align="center">12</td> <td align="center">18</td> <td align="center">9</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">62</td> <td align="center">16.7</td> <td align="center">9</td> <td align="center">12</td> <td align="center">19</td> <td align="center">10</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">61</td> <td align="center">16.1</td> <td align="center">9</td> <td align="center">13</td> <td align="center">20</td> <td align="center">10</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">60</td> <td align="center">15.6</td> <td align="center">9.5</td> <td align="center">13</td> <td align="center">20</td> <td align="center">11</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">59</td> <td align="center">15.0</td> <td align="center">10</td> <td align="center">13</td> <td align="center">21</td> <td align="center">12</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">58</td> <td align="center">14.4</td> <td align="center">10</td> <td align="center">14</td> <td align="center">21</td> <td align="center">12</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">57</td> <td align="center">13.9</td> <td align="center">10</td> <td align="center">14</td> <td align="center">22</td> <td align="center">13</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">56</td> <td align="center">13.3</td> <td align="center">11</td> <td align="center">15</td> <td align="center">22</td> <td align="center">13</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">55</td> <td align="center">12.8</td> <td align="center">11</td> <td align="center">16</td> <td align="center">24</td> <td align="center">14</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">54</td> <td align="center">12.2</td> <td align="center">11.5</td> <td align="center">16</td> <td align="center">24</td> <td align="center">14</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">53</td> <td align="center">11.7</td> <td align="center">12</td> <td align="center">17</td> <td align="center">25</td> <td align="center">15</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">52</td> <td align="center">11.1</td> <td align="center">12</td> <td align="center">18</td> <td align="center">26</td> <td align="center">15</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">51</td> <td align="center">10.6</td> <td align="center">13</td> <td align="center">18</td> <td align="center">27</td> <td align="center">16</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">50</td> <td align="center">10.0</td> <td align="center">14</td> <td align="center">19</td> <td align="center">29</td> <td align="center">16</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">49</td> <td align="center">9.4</td> <td align="center">14.5</td> <td align="center">20</td> <td align="center">30</td> <td align="center">17</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">48</td> <td align="center">8.9</td> <td align="center">15</td> <td align="center">20</td> <td align="center">30</td> <td align="center">17</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">47</td> <td align="center">8.3</td> <td align="center">17</td> <td align="center">23</td> <td align="center">35</td> <td align="center">17</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">46</td> <td align="center">7.8</td> <td align="center">19</td> <td align="center">25</td> <td align="center">38</td> <td align="center">17</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">45</td> <td align="center">7.2</td> <td align="center">20</td> <td align="center">27</td> <td align="center">41</td> <td align="center">17</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">44</td> <td align="center">6.6</td> <td align="center">22</td> <td align="center">30</td> <td align="center">45</td> <td align="center">17</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">43</td> <td align="center">6.1</td> <td align="center">25</td> <td align="center">34</td> <td align="center">51</td> <td align="center">17</td></tr> <tr align="center" bgcolor="#fdfae3"> <td align="center">42</td> <td align="center">5.5</td> <td align="center">30</td> <td align="center">40</td> <td align="center">60</td> <td align="center">17</td></tr></tbody></table> <p>&#160;</p> <h2><font size="4">Disease Management</font></h2> <h3>Chemical Management</h3> <p><span>Management of apple scab is focused on the prevention of primary infection by ascospores. This is because early infection by ascospores may result in poor fruit set and will result in more secondary inoculum throughout the season. Fungicide applications are therefore timed to coincide with the spring release of ascospores (between bud break and petal fall). Maturing plant tissue is somewhat resistant to infections, and scab-targeted fungicides are reduced or sometimes eliminated from spray schedules unless conditions are conducive for disease (wet weather, susceptible cultivars, and/or high disease pressure).<span>&#160; </span>Later-season fungicide sprays are often targeted toward other fungal diseases, but also may be effective against secondary inoculum. Disease severity and distribution varies, so growers should utilize local university sources when developing spray schedules.<span>&#160; </span></span></p><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab16.jpg"></a> <table align="center" border="0"> <tbody> <tr align="center" valign="bottom"> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab14.jpg"><img border="0" alt="Figure 15. An airblast sprayer being used to apply fungicides in a commercial orchard. (Courtesy J. Hartman)" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab14sm.jpg" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;150px;" /></a><br><span><span class="ms-rteCustom-PhotoCaption"><strong>Figure 14. </strong><span>Contact fungicides require complete, uniform coverage.<span>&#160; </span>Leaf appears to <br>be completely covered with residue.</span></span></span></td> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab15.jpg"><img border="0" alt="Figure 16. Apple leaf protected from scab infections by a residue of fungicide. (Courtesy J. Hartman)" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab15sm.jpg" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;150px;" /></a><br><span><span class="ms-rteCustom-PhotoCaption"><strong>Figure 15.</strong> <span>An airblast sprayer being used to apply fungicides in a commercial orchard.<br></span></span></span></td></tr></tbody></table> <p>&#160; <br><span>Various fungicide chemistries are available for management of apple scab. Most products can be classified as contact or systemic.<span>&#160; </span>Contact fungicides form an external barrier on plant surfaces.<span>&#160; </span>They are typically not rainfast (they wash off readily with rain) and do not translocate through plant tissue.<span>&#160; </span>Systemic fungicides, in contrast, penetrate epidermal tissue and can translocate within plant tissue.<span>&#160; </span>They are more rainfast than contact fungicides, and they usually have longer reapplication intervals.</span></p> <p><span>Both contact and systemic fungicides are applied before infection to prevent fungal spores from germinating or penetrating host tissue. Fungicides do not cure disease, even though some older recommendations may refer to systemic fungicides as curative products.<span>&#160; </span>Thorough spray coverage and uniform deposition are essential, especially for contact fungicides (<a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab14.jpg">Figure 14</a>). To ensure coverage of newly emerging tissues and due to breakdown of active ingredients, fungicides must be re-applied on a regular schedule. This application schedule is dependent upon product formulation and systemic properties; consult product label for individual&#160;&#160; m<span>recommendations. In commercial orchards, airblast sprayers (<a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab15.jpg">Figure 15</a>) are typically used to apply fungicides to ensure sufficient coverage.<span>&#160; </span></span></span></p> <p><span>Fungicides are classified according to their mode of action.<span>&#160; </span>The Fungicide Resistance Action Committee (FRAC) has categorized fungicides according to their mode of action and, accordingly, their risk for resistance development.<span>&#160; </span>Many newer fungicides have a high risk for resistance, so it is important to rotate between FRAC groups to prevent development of resistant populations. </span></p> <p><span>Timing of fungicide applications is critical for effective scab management. When fungicides are combined with cultural practices and are based on environmental conditions (e.g. prediction models such as a Mills table-based weather monitoring system, <a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab16.jpg">Figures 16</a>, <a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab17.jpg">17</a>) disease is more effectively managed. Fungicide applications based on weather monitoring systems can reduce numbers of sprays and/or more accurately time applications, thereby reducing inputs and increasing grower profits. <span>&#160;</span>Several university programs and private industries have developed regionally-specific prediction models that utilize weather forecasts to estimate leaf wetness and temperature conditions to evaluate risk for spore release and infection.<span>&#160;&#160;</span></span>&#160;</p> <table align="center" border="0"> <tbody> <tr align="center" valign="bottom"> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab16.jpg"><img border="0" alt="Figure 17. A field-based microcomputer and weather station used to monitor apple scab infection. (Courtesy J. Hartman)" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab16sm.jpg" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;150px;" /></a><br><span class="ms-rteCustom-PhotoCaption"><strong>Figure 16</strong></span></td> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab17.jpg"><img border="0" alt="Figure 18. Using weather monitoring instruments to record apple scab infection events. (Courtesy J. Hartman)" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab17sm.jpg" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;150px;" /></a><br><span class="ms-rteCustom-PhotoCaption"><strong>Figure 17</strong></span></td></tr></tbody></table> <h2><span><font size="4"><br>Biological Control</font></span></h2> <p><span>There are a limited number of biological control products available for management of apple scab.<span>&#160; </span>To date, many of these products have below-average efficacy ratings when compared to traditional fungicides; thus, use of cultural controls and resistant cultivars are vital components of a biological control disease management program.</span></p> <h3><span></span>Genetic Resistance</h3> <p><span>Scab-resistant (or more appropriately scab-tolerant) apples and crabapples are widely available. Levels of resistance/tolerance vary with cultivar, although most scab-resistant apple cultivars carry the <i>Vf </i>resistance gene from <i>M. floribunda </i>821.<span>&#160; </span>For many years, resistance resulted in reduction and/or elimination of fungicide use for scab management.<span>&#160; </span>However, reports of loss of host resistance were reported in Europe in 1993 and in North America in 2007.<span>&#160; </span></span></p> <p><span>In many areas, resistance remains intact.<span>&#160; </span>Risk for loss of host resistance (due to adaptability of the pathogen) is high when growers rely on resistance alone.<span>&#160; </span></span></p> <p><span>In all cases, an integrated management (IPM) program helps prevent resistance breakdown, as well as manage disease in orchards where resistance has been compromised.<span>&#160; </span>Cultural practices (improved air circulation and sanitation) are critical for reduction of inoculum.<span>&#160; </span>Additionally, use of fungicides (synthetic, organic, and/or biological) during primary infection periods also reduce inoculum and help break the disease cycle.<span>&#160; </span>Resistant and susceptible cultivars should not be planted in close proximity.<span>&#160; </span>Integrated use of these and other management practices help protect resistance and reduce disease pressure. Resistant or tolerant apple cultivars are gaining acceptance by both growers and consumers. Resistant cultivars are listed in <a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScabtable02.jpg">Table 2</a>.</span></p> <p align="center"><span><span class="ms-rteCustom-PhotoCaption"><strong>Table 2. </strong>List of popular scab-resistant apple cultivars.</span></span><br><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScabtable02.jpg"><img border="0" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScabtable02.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;width&#58;250px;" /></a></p> <p><span>Susceptible cultivars are still widely grown due to popularity and consumer demand.<span>&#160; </span>Additionally, these cultivars often can be grown easily in drier regions where apple scab is not a problem. Cultivars that are highly susceptible are listed in <a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScabtable03.jpg">Table 3</a>. </span></p> <p align="center"><span><span class="ms-rteCustom-PhotoCaption"><strong>Table 3</strong>. List of popular apple cultivars that are susceptible to scab.</span><br><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScabtable03.jpg"><img border="0" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScabtable03.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;width&#58;250px;" /></a></span></p> <p><span><span>Scab-resistant flowering crabapples provide greater bloom potential because there is no risk for defoliation (<a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab18.jpg">Figures 18</a> and <a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab19.jpg">19</a>). Many scab-resistant cultivars are available, and many new ones are being introduced.<span>&#160; </span>Table 4 includes examples of crabapples from the Morton Arboretum (http&#58;// that exhibit excellent resistance.</span></span></p> <table align="center" border="0"> <tbody> <tr align="center" valign="bottom"> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab18.jpg"><img border="0" alt="Figure 19. Some scab resistant apple cultivars. (Courtesy Purdue University)" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab18sm.jpg" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;150px;" /></a><br><span class="ms-rteCustom-PhotoCaption"><strong>Figure 18</strong></span></td> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab19.jpg"><img border="0" alt="Figure 20. Defoliation of a crabapple tree caused by early apple scab infection. (Courtesy J. Hartman)" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab19sm.jpg" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;150px;" /></a><br><span class="ms-rteCustom-PhotoCaption"><strong>Figure 19</strong></span></td></tr></tbody></table> <p align="center"><br><span><span class="ms-rteCustom-PhotoCaption"><strong>Table 4.</strong> <span>List of popular scab-resistant crabapple on <br>display at the Morton Arboretum near Chicago, IL.</span>.</span><br></span><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScabtable04.jpg"><img border="0" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScabtable04.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;width&#58;250px;" /></a></p> <h3>Cultural Practices</h3> <p><span>Cultural practices should be combined with fungicides (chemical and/or biological) for an integrated pest management system.<span>&#160; </span>Two common cultural practices used to reduce apple scab are sanitation and increased air circulation.<span>&#160; </span>Sanitation eliminates inoculum during the growing season and during the overwintering season.<span>&#160; </span>Commercial growers use various methods to speed up leaf decomposition, thereby decreasing the amount of primary inoculum the following season.<span>&#160; </span>These include&#58; applying urea to trees, just before leaf drop, applying urea to fallen leaves, and tilling fallen leaves into the soil or chopping them into small pieces. <span>&#160;</span>Another effective cultural practice is to lower wetness and humidity in tree canopies so that the environment becomes unfavorable for disease.<span>&#160; </span>Pruning and wide tree spacing can enhance air movement and allow sunlight to penetrate, which speeds up drying of leaves and fruit.</span></p> <h2><font size="4">Significance</font></h2> <p><span>Apple scab has long been a problem on apples; symptoms of the disease can be recognized on fruit in paintings from the fifteenth and sixteenth centuries, as well as the 1824 painting by American James Peale, shown in <a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab20.jpg">Fig. 20</a>. </span></p> <table cellpadding="4" align="center"> <tbody> <tr align="center" valign="bottom"> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/AppleScab20.jpg" target="_blank"><img border="0" alt="Fruit Still Life with Chinese Export Basket, 1824. National Gallery of Art." src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/AppleScab20sm.jpg" align="bottom" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;height&#58;200px;" /></a></td></tr> <tr align="center" valign="bottom"> <td><span><span class="ms-rteCustom-PhotoCaption"><span>&#160; <span><strong>Figure 20. </strong><span>&#160;</span>Fruit Still Life with Chinese Export Basket, <br>painted in 1824 by American James Peale (1749–1831). Note the <br>symptoms of scab on the apple on the table at the left side of the painting.</span><span></span></span></span></span></td></tr></tbody></table> <p><br>&#160; <span>The frequent historical depictions of scab-infected apples suggest that the disease was common and that the affected fruit was acceptable in earlier times. All the commonly grown apple cultivars were susceptible to the disease, and there were no chemical treatments until the late 1800s. At that time, copper- and sulfur-based fungicides served as protectants, but the treatments often caused substantial damage to the apple foliage. Even today, in spite of the highly effective chemicals and the resistant apple cultivars that are available, apple scab causes greater economic losses of apples in North and South America, Europe, and Asia than any other apple disease.</span></p> <p><span></span><i><span>Venturia inaequalis</span></i><span> was one of the first ascomycetes to be subjected to genetic analysis; the heritability of pathogenicity and of sexual compatibility was investigated as early as the 1930s. It has served as and continues to be a valuable tool for basic genetic research and for studies of the inheritance of pathogenicity. Among the features that make <i>Venturia inaequalis</i> so amenable for genetic research is the fact that it is similar to many obligate parasites that infect young tissues and live in a close association with them, without visibly harming them, for an extended period of time. Yet unlike obligate parasites, it can be cultured on artificial media, and matings can be made <i>in vitro</i>. Other advantages of <i>V. inaequalis</i> for genetic studies include its considerable diversity, with many pathotypes or races occurring in the natural population; the stability of phenotypes and genotypes in culture over many years; the fact that it is haploid, allowing the effects of alleles to be studied directly; the uninucleate conidia which give rise to colonies in which all the nuclei are genetically uniform; and the ability to isolate all eight of the ascospores from a single ascus (Figure 9). The eight spores are the result of a single meiotic event, and so segregation of traits can be studied directly rather than statistically. Furthermore, the eight ascospores are arranged in order, allowing reconstruction of the meiotic divisions and mapping of centromeres as genetic loci. Many of these advantages are shared by other Ascomycetes, but they are uniquely brought together in <i>V. inaequalis</i>.</span></p> <p><span>Many years of study have demonstrated that the genetics of pathogenicity in <i>V. inaequalis</i> are not simple. There are multiple genes involved in pathogenicity to various apple species and cultivars. In several of these interactions, pathogenicity is conditioned by single genes. In some cases, there appears to be a gene-for-gene relationship between the host and the pathogen. In a gene-for-gene interaction, each gene for resistance in the host is matched by a gene for pathogenicity (or virulence) in the pathogen. Resistance will only occur if the host has the dominant allele for the resistance gene and the pathogen has the corresponding dominant allele conditioning non-pathogenicity (or avirulence); all other combinations will result in disease. Gene-for-gene interactions are found in numerous host-pathogen interactions, and are thought to be the consequence of co-evolution of the host and the pathogen.</span></p> <h2><font size="4">Selected References</font></h2> <p><span class="ms-rteCustom-Reference">Beckerman, J. 2009.<span>&#160; </span>Managing scab-resistant apples (BP-76-W).<span>&#160; </span>Purdue University.<span>&#160; </span>Purdue Extension Education Store. Available for download from&#58; <span><a href="https&#58;//"><span>https&#58;//</span></a></span></span></p> <p><span class="ms-rteCustom-Reference">Fungicide Resistance Action Committee.<span>&#160; </span><span><a href="http&#58;//"><span></span></a></span>.<span>&#160; </span></span></p> <p><span class="ms-rteCustom-Reference">Gessler, C., and Pertot, I.<span>&#160; </span>2012.<span>&#160; </span><i>Vf</i> scab resistance of <i>Malus </i>in&#58; Trees (2012) 26&#58;95.<span>&#160; </span>Doi 10.1007/s00468-011-0618-y.</span></p> <p><span><span><span class="ms-rteCustom-Reference">Jones, A. L., Lillevik, S. L., Fisher, P. D. and Stebbins, T. C. 1980. A microcomputer-based instrument to predict primary apple scab infection periods. Plant Dis. 64&#58;69-72.</span></span></span></p> <p><span class="ms-rteCustom-Reference">Merwin, I. A., Brown, S. K., Rosenberger, D. A., Cooley, D. R., and Berkett, L.P. 1994. Scab-resistant apples for the Northeastern United States&#58; New prospects and old problems. Plant Disease 78&#58;4-10.</span></p> <p><span class="ms-rteCustom-Reference">MacHardy, W. E. 1996. <i>Apple Scab Biology, Epidemiology, and Management</i>. APS Press St. Paul 545 pp.</span></p> <p><span class="ms-rteCustom-Reference">MacHardy, W. E. and Gadoury, D. M.<span>&#160; </span>1989.<span>&#160; </span>A revision of Mills’ criteria for predicting apple scab infection periods.<span>&#160; </span>Phytopathology 79&#58;304-301.</span></p> <p><span class="ms-rteCustom-Reference">Morton, V., and Staub, T.<span>&#160; </span>2008.<span>&#160; </span>A short history of fungicides.<span>&#160; </span>Online.<span>&#160; </span>APSnet Features.<span>&#160; </span>Doi 10.1094/APSnetFeature-2008-0308.</span></p> <p><span><span class="ms-rteCustom-Reference">Parisi, L., Lespinasse, Y., Guillaumes, J., and Kruger, J. 1993.<span>&#160; </span>A new race of <i>Venturia inaequalis </i>virulent to apples with resistance due to the <i>Vf </i>gene. Phytopathology 83&#58; 533–537.</span><span></span></span></p> <p><span><span class="ms-rteCustom-Reference">Rosenberger, D. A. 2003. Susceptibility of new apple cultivars to common apple diseases. <i>New York Fruit Quarterly</i>, 11(2)&#58; 17-22.</span></span></p> <p><span><span class="ms-rteCustom-Reference">Stensvand, A., Gadoury, D. M., Amundsen, T., Semb, L., and Seem, R. C. 1997. Ascospore release and infection of apple leaves by conidia and ascospores of <i>Venturia inaequalis</i> at low temperatures. Phytopathology 87&#58;1046-1053.</span></span></p> <p><span><span class="ms-rteCustom-Reference">Sutton, T. B., Aldwinckle, H. S., Agnello, A. M., and Walgenbach, J. F., eds. 2014. Compendium of Apple and Pear Diseases, 2<sup>nd</sup> Edition.<span>&#160; </span>APS Press, St. Paul 224 pp.</span></span></p> <p class="MsoNormal" style="margin-bottom&#58;0pt;line-height&#58;normal;"><span><span class="ms-rteCustom-Reference">Travis, J.W., and J. Ritter. 2006. Scab Resistant Cultivars, Table 1-6. <i>Penn State Tree Fruit Production Guide</i>.</span></span></p>3/6/2019 7:46:50 PMDISEASE: Apple scab However, the apple scab pathogen will not infect pear, and the pear scab pathogen will not infect apple
Pecan Scab96286APS Education Center - Plant Disease Lesson - Pecan Scab<h6>Hoefnagels, M.H. and B.D. Mason 2016. Pecan Scab. <em>The Plant Health Instructor.</em> DOI&#58; 10.1094/PHI-I-2016-0620-01<br><br></h6> <p><strong><span class="ms-rteCustom-EdCtrLessonHeader">DISEASE&#58;</span>&#160;Pecan scab</strong></p> <p><strong><span class="ms-rteCustom-EdCtrLessonHeader">PATHOGEN&#58;</span>&#160;<em>Fusicladium effusum </em>(former names include <em>Fusicladosporium effusum, Fusicladium caryigenum, Cladosporium caryigenum, </em>and <em>Cladosporium effusum</em>)</strong></p> <p><strong><span class="ms-rteCustom-EdCtrLessonHeader">HOST AND RANGE&#58;</span> Host species include pecan (<em>Carya illinoinensis</em>) and other <em>Carya </em>species, including bitter pecan (<em>C. aquatica</em>), bitternut hickory (<em>C. cordiformis</em>), pignut hickory (<em>C. glabra</em>), shagbark hickory (<em>C. ovata</em>), and mockernut hickory (<em>C. tomentosa</em>). Pecan scab is widespread in the southeastern United States but has also been reported in other parts of the United States and in Canada, Mexico, Central America, South America, and New Zealand.</strong></p> <h3 align="center">Authors</h3> <p align="center"><strong>Mariëlle H. Hoefnagels</strong><sup>1</sup> and <strong>Bonnie D. Mason</strong><sup>2</sup> <br><sup>1</sup>Associate Professor, Department of Microbiology and Plant Biology, University of Oklahoma<br><sup>2</sup>Former student, Department of Microbiology and Plant Biology, University of Oklahoma</p> <h2 align="center"><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/PecanScabFig3.jpg"><img border="0" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/PecanScabFig3.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;width&#58;300px;" /></a></h2> <p align="center"><span class="ms-rteCustom-PhotoCaption"><span>Nuts infected with pecan scab.</span></span></p> <h2 align="left">Signs and Symptoms</h2> <p align="left">Symptoms of infection are similar on all parts of an infected plant. Pecan scab affects the leaves, shucks, and twigs of the infected plant and manifests as small (1 to 5 mm), circular, black or olive green lesions (<span class="ms-rteCustom-PhotoCaption">Figure 1</span>).&#160; Less commonly, catkins and dormant buds may be affected.&#160; Lesions may coalesce and form larger blackened areas. In the early stages of infection, the lesions appear velvety, due to production of conidia on their surface.&#160; As the infection progresses the lesions harden and turn a dark grey to silvery-brown color, and can become dry, crack, and drop out of the leaf.&#160;&#160; Lesions on young shoots appear sunken due to the swelling of the tissue at the margins of the lesions (<span class="ms-rteCustom-PhotoCaption">Figure 2</span>). These lesions may persist for several seasons after the initial infection. Similar lesions occur on the nut shucks (<span class="ms-rteCustom-PhotoCaption">Figure 3</span>). These lesions may be slightly raised. A black, velvety, cushion-like mass called a stroma (plural&#58; <em>stromata</em>) that forms in these lesions provides the basis for overwintering, and gives rise to reproductive structures called conidiophores in the following spring.&#160;&#160;&#160;&#160;&#160; </p> <p align="center"> </p><table style="height&#58;181px;width&#58;68.63%;"> <tbody> <tr> <td>&#160;<a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/PecanScabFig1.jpg"><img border="0" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/PecanScabFig1sm.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;" /></a><br><span class="ms-rteCustom-PhotoCaption">Figure 1</span></td> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/PecanScabFig2.jpg"><img border="0" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/PecanScabFig2s.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;width&#58;87px;" /></a><br><span class="ms-rteCustom-PhotoCaption">Figure 2</span>&#160;</td> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/PecanScabFig3.jpg"><img border="0" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/PecanScabFig3sm.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;" /></a><br><span class="ms-rteCustom-PhotoCaption">Figure 3</span>&#160;</td></tr></tbody></table><p></p> <p align="left">Pecan scab affects nuts in several ways. If severe, infection can result in defoliation and a reduction of the size and quality of the nut; if the infection occurs early in nut development, the nuts will abort. In addition, if <em>F. effusum </em>reaches deep tissues in the shuck, it can cause the shuck to cling to the shell of the nut (a condition called “stick-tight”). Separating the nut from the shell is impossible, which is problematic for shelling. The surface of severely infected nuts may also crack, giving a point of entry for secondary infections. In particular, the pink mold fungus, <em>Cephalothecium roseum</em>, can invade old lesions on the shucks. </p> <h2 align="left">Pathogen Biology</h2> <p align="left"><em>Fusicladium effusum </em>is an ascomycete fungus in the class Dothideomycetes.&#160; Dothideomycetes reproduce both sexually and asexually, but only the asexual phase has been observed in <em>F. effusum</em>.&#160; Asexual spores (conidia) initiate infection on susceptible host tissue. Conidia may be present in simple or branched chains two to nine cells long, with individual cells being 10–24 μm long × 5–10 μm wide (<span class="ms-rteCustom-PhotoCaption">Figure 4</span>). Conidia are light brown, clavate, fusiform, ovate or almost cylindrical in shape. Under favorable conditions, these spores germinate and form germ tubes that penetrate the host tissue. Germ tubes may enlarge to form an appressorium immediately adjacent to the conidium, or elongate and form an appressorium distally.&#160; The appressoria penetrate the cuticle of the host tissue and initiate subcuticular hyphal growth.&#160; Conidiophores arise from the hyphae and rupture the cuticle, forming the visible part of the lesion on the tissue surface. Conidiophores may be solitary structures or form loose fascicles. Conidiophores are long, unbranched or apically branched cells, septate and pale to dark brown, and 22-130 × 4-6 µm in size. Conidia are discharged from the tips of the conidiophores and are dispersed by wind and rain.</p> <p align="center">&#160; </p><table style="height&#58;140px;width&#58;96.39%;"> <tbody> <tr> <td><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/PecanScabFig4.jpg"><img border="0" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/PecanScabFig4.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;width&#58;250px;" /></a>&#160;<br><span class="ms-rteCustom-PhotoCaption">Figure 4</span></td></tr></tbody></table><p></p> <h2 align="left">Disease Cycle and Epidemiology</h2> <p align="left"><em>Fusicladium effusum </em>overwinters in lesions on the leaves, shucks, and twigs from the previous year’s infection (<span class="ms-rteCustom-PhotoCaption"> Figure 5</span>).&#160; These overwintering sites serve as the primary source of inoculum, while the current year’s infections produce lesions that act as a secondary source of inoculum and provide the polycyclic dimension to the disease. </p> <p align="left"> </p><table align="center" style="height&#58;157px;width&#58;92.42%;"> <tbody> <tr> <td>&#160; <p align="center"><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/PecanScabFig5.jpg"><img border="0" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/PecanScabFig5.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;width&#58;600px;" /></a><br><span class="ms-rteCustom-PhotoCaption">Figure 5</span></p></td></tr></tbody></table>In spring, stromata in overwintering lesions begin producing conidiophores. Spore discharge is stimulated by sunlight and a rapid decrease in relative humidity and vegetative wetness. Spores are dispersed by wind as early as March and as late as December, with aerial spore concentrations being the highest in May and June. The conidia may land on developing nuts, newly emerging leaves, and twigs. <p></p> <p align="left">Surface moisture is critical for infection, and the optimum temperature range for infection is 20-30° C. Under favorable conditions, germination of conidia occurs within 3-24 h of inoculation. By 36 h post inoculation, the germ tube has typically penetrated the epidermis.&#160; Once the cuticle of the tissue is penetrated, hyphal growth occurs laterally within the tissue (<span class="ms-rteCustom-PhotoCaption">Figure 5</span>). Lesions appear 7-9 days after inoculation and the number of lesions increases with more prolonged leaf surface wetness.&#160; Conidiophores grow upward from the mycelial mass below the tissue surface and penetrate the epidermis (<span class="ms-rteCustom-PhotoCaption">Figure 5</span>). These conidiophores produce the conidia that act as a secondary inoculum throughout the growing season.</p> <p align="left">The pathogen primarily infects young, expanding leaves and shoots. The resulting lesions remain visible on older leaves; once leaves are fully expanded, however, they are effectively resistant to new infection. Fruit remain susceptible throughout their development and maturation. Therefore, infections established early in the season tend to be more severe and result in greatest crop loss. </p> <h2 align="left">Significance</h2> <p align="left">Pecan scab can have a significant economic impact due to crop yield reduction and loss in quality.&#160; The disease causes nut drop, with total crop loss possible in severe cases.&#160; Reduction in the size and quality of the nuts occurs (<span class="ms-rteCustom-PhotoCaption">Figure 6</span>). In addition, pecan scab lesions on foliage reduce the photosynthetic area of the tree, causing a reduction in the photosynthetic rate of the plant.&#160; This reduction, in turn, may enhance alternate bearing, which is the tendency to produce a heavy crop one season, followed by one or more years of little or no production. </p> <p align="left"> </p><table align="center" style="height&#58;134px;width&#58;89.9%;"> <tbody> <tr> <td>&#160; <p align="center"><a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/PecanScabFig6.jpg"><img border="0" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/PecanScabFig6.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;width&#58;275px;" /></a><br><span class="ms-rteCustom-PhotoCaption">Figure 6</span></p></td></tr></tbody></table>In addition to crop losses, pecan growers incur considerable expenses for costly preventative fungicides. As described in the next section, fungicides must be applied repeatedly throughout the growing season.<p></p> <h2 align="left">Management</h2> <p align="left"><strong><em>Host resistance</em></strong></p> <p align="left">The first line of defense against pecan scab is the selection of resistant cultivars.&#160; Cultivars may be rated on a scale of 1 to 5, with 1 meaning “no incidence of scab” and 5 meaning “very severe incidence.” Recommendations vary by region. For example, in old orchards in the southeastern United States, the Alabama Cooperative Extension lists Elliott (scab susceptibility rating = 1.4) and Davis (1.7) as the most scab-resistant cultivars; for new plantings, Jubilee (1.1) and Melrose (1.1) are even more resistant than Elliott, and Gloria Grande also shows good resistance (1.4). For commercial orchards in Georgia, the University of Georgia recommends Elliott and Kanza for excellent resistance and Sumner for good resistance; Gloria Grande is “not recommended for most situations.” This variation in cultivar resistance may reflect the existence of regional strains and high genetic variability in <em>F. effusum</em>.</p> <p align="left"><strong><em>Chemical control</em></strong></p> <p align="left">Thanks to the historic loss of host resistance to <em>F. effusum</em>, commercial growers rely on fungicides to manage pecan scab. Organic growers may spray trees with the Bordeaux mixture (copper sulfate and hydrated lime). This fungicide was commonly used on pecans in the 1920s but was eventually supplanted in commercial pecan orchards by modern fungicides. </p> <p align="left">Today, multiple fungicides are used to control pecan scab. Overall, these chemicals inhibit spore germination or hyphal growth, kill germinating spores, or prevent sporulation. The Fungicide Resistance Action Committee organizes fungicides by mode of action at the cellular level. Chemicals that are typically used in commercial pecan production include benzimidazoles (mitosis/cell division inhibitors), strobilurins and organometals such as tri-phenyl tin compounds (respiration inhibitors), and triazoles (sterol biosynthesis inhibitors). Other fungicides used on pecans include dithiocarbamates (“multi-site contact activity”) and guanidines and phosphorous acid (phosphites), both of which have unknown modes of action. Optimal timing varies among these fungicides. For example, in one study, phosphites protected foliage early in the growing season but did not deliver late-season protection.&#160; </p> <p align="left">The Fungicide Resistance Action Committee also classifies fungicides according to their risk of selecting for resistant fungi. Dithiocarbamates and phosphorous acid (phosphites) are assigned to the low-risk category; guanidines and organometals are assigned a low to medium risk; triazoles have a medium risk for resistance; and benzimidazoles and strobilurins have a high risk. Consecutive application or too many applications of the same fungicide in a growing season may exacerbate pathogen resistance. For example, the Alabama Cooperative Extension recommends mixing triazole fungicides with organometals or alternating triazoles with organometals to reduce the selection pressure for resistant fungi. </p> <p align="left">Large, commercial air-blast sprayers are needed to ensure adequate coverage of fungicides, but even these sprayers may not be sufficient to reach the foliage and fruits in very tall trees (<span class="ms-rteCustom-PhotoCaption">Figure 7</span>). Inadequate coverage is a concern because low doses of fungicides may accelerate the selection for resistant varieties of <em>F. effusum</em>.&#160; Growers should begin spraying susceptible cultivars in early spring, when developing leaves are first exposed to inoculum from the overwintered lesions. Between bud break and nut set, fungicides should be applied every 10 to 14 days; from nut set to shell hardening, fungicides should be applied every 10 to 21 days. More frequent spraying may be required when conditions favor disease development. Multiple online tools analyze weather patterns and help growers determine if and when they should spray. For example, the Oklahoma Mesonet has an online <a href="http&#58;//">Pecan Scab Advisor</a>, and the <a href="http&#58;//">AU-Pecan spray advisory </a>is available to growers in Alabama, Georgia and Louisiana.</p> <p align="left"> </p><table align="center" style="width&#58;100%;"> <tbody> <tr> <td> <p align="center"><strong><em>&#160;<a href="/edcenter/disandpath/fungalasco/pdlessons/Article%20Images/PecanScabFig7.jpg"><img border="0" src="/edcenter/disandpath/fungalasco/pdlessons/PublishingImages/PecanScabFig7.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;width&#58;200px;" /></a><br></em></strong><span class="ms-rteCustom-PhotoCaption">Figure 7</span></p></td></tr></tbody></table><strong><em>Cultural control</em></strong><p></p> <p align="left">When establishing new orchards, tree spacing and orientation are important considerations, because adequate exposure to sunlight and good airflow are two keys to keeping foliage dry. Selective pruning of damaged branches during the dormant season is also recommended to promote sun exposure and air circulation. Finally, good sanitation practices are recommended to limit the amount of primary inoculum that may cause infection. </p> <h2 align="left">Acknowledgments</h2> <p align="left">We thank Dr. William Reid (Kansas State University), Dr. Clive Bock (USDA-ARS), and Dr. Katherine Stevenson (University of Georgia) for their generosity in granting permission to use their images in this article. We also thank two anonymous reviewers for their helpful suggestions.</p> <h2 align="left">Selected References</h2> <p align="left">Alabama Cooperative Extension. 2005. <a href="http&#58;//">Commercial Pecan&#58; Insect, disease, and weed control recommendations for 2005.</a> Publication 2005IPM-27.&#160; </p> <p>Bock, C. H. 2013. <em><a href="http&#58;//"><em>Fusicladium effusum </em>(pecan scab).</a></em> CABI Invasive Species Compendium.</p> <p>Bock, C.H., Brenneman, T. B., Hotchkiss, M. W., and Wood, B. W. 2012. Evaluation of a phosphite fungicide to control pecan scab in the southeastern USA. Crop Protection 36&#58;58-64.</p> <p align="left">Bock, C.H., Cottrell, T. E., Hotchkiss, M. W., and Wood, B. W. 2013. Vertical distribution of scab in large pecan trees. Plant Disease 97&#58; 626-634.</p> <p align="left">Bock, C.H., Wood, B. W., Stevenson, K. L., and Arias, R. S. 2014. Genetic diversity and population structure of <em>Fusicladium effusum </em>on pecan in the United States. Plant Disease 98&#58; 916-923.</p> <p align="left">Brenneman, T., Brock, J., Culpepper, A. S., Hudson, W., Mitchem, W., and Wells, L. 2015. <a href="http&#58;//">Commercial Pecan Spray Guide. University of Georgia Cooperative Extension Service Bulletin No. 841. </a>Demaree, J. B. 1924. Pecan scab with special reference to sources of the early spring infections. Journal of Agricultural Research 28&#58; 321-330.</p> <p align="left">Demaree, J. B. 1928. Morphology and taxonomy of the Pecan-scab fungus, <em>Cladosporium effusum </em>(Wint.) comb. nov. Journal of Agricultural Research 37&#58;181-187.</p> <p align="left">Demaree, J. B., and Cole, J. R. 1926. <a href="https&#58;//">Commercial control of pecan scab.</a> United States Department of Agriculture Department Circular 386. Farr, D.F., and Rossman, A.Y. Undated. </p> <p align="left"><a href="http&#58;//">Fungal Databases – Nomenclature. Systematic Mycology and Microbiology Laboratory, ARS, USDA.</a> Retrieved December 21, 2015, from </p> <p align="left">Fungicide Resistance Action Committee (FRAC). 2016. <a href="http&#58;//">FRAC Code List 2016&#58; Fungicides sorted by mode of action (including FRAC Code numbering)</a>. </p> <p align="left">Gottwald, T. R. 1982. Spore discharge by the pecan scab pathogen, <em>Cladosporium caryigenum</em>. Phytopathology 72&#58; 1193-1197.</p> <p align="left">Gottwald, T. R. 1982. Taxonomy of the pecan scab fungus <em>Cladosporium caryigenum</em>. Mycologia 74&#58;382-390.</p> <p align="left">Gottwald, T. R. 1985. Influence of temperature, leaf wetness period, leaf age, and spore concentration on infection of pecan leaves by conidia of <em>Cladosporium caryigenum</em>. Phytopathology 75&#58;190-194.</p> <p align="left">Gottwald, T. R., and Bertrand, P. F. 1982. Patterns of diurnal and seasonal airborne spore concentrations of <em>Fusicladium effusum </em>and its impact on a pecan scab epidemic. Phytopathology 72&#58;330-335.</p> <p align="left">Gottwald, T. R., and Bertrand, P. F. 1983. Effect of time of inoculation with <em>Cladosporium caryigenum</em> on pecan scab development and nut quality. Phytopathology 73&#58; 714-718.</p> <p align="left">Gottwald, T. R., and Wood, B. W. 1985. Decreased net photosynthetic and dark respiration rates of pecan fruit and foliage in response to infection by <em>Cladosporium caryigenum</em>. Plant Disease 69&#58;800-803.</p> <p align="left">Hunter, R. E. 1983. Influence of scab on late season nut drop of pecans. Plant Disease 67&#58;806-807.</p> <p align="left">Isakeit, T. 2010. <a href="http&#58;//">Pecan scab&#58; understanding fungicide activity to prevent fungicide resistance. </a></p> <p align="left">Latham, A. J., and Goff, W. D. 1991. Pecan scab&#58; A review and control strategies. Pages 89-93 in Pecan Husbandry&#58; Challenges and Opportunities. First National Pecan Workshop Proceedings. Unicoi State Park, Georgia, July 23-24, 1990. </p> <p align="left">Latham, A. J., and Rushing, A. E. 1988. Development of <em>Cladosporium caryigenum </em>in pecan leaves. Phytopathology 78&#58;1104-1108.</p> <p align="left">Lee, J., Mulder, P., and Driever, G. 2013. <a href="http&#58;//">Commercial Pecan Insect and Disease Control. </a>Oklahoma Cooperative Extension Service. Bulletin CR-6209. </p> <p align="left">Littrell, R. H., and Bertrand, P. F. 1981. Management of pecan fruit and foliar diseases with fungicides. Plant Disease 65&#58; 769-774.</p> <p align="left">National List of Allowed and Prohibited Substances. 2000. Code of Federal Regulations, title 7, §205.601. Synthetic substances allowed for use in organic crop production.</p> <p align="left">Nolen, R. E. 1926. Pecan scab. Bulletin 181 of the University of Florida Agricultural Experiment Station. 26 pp.</p> <p align="left">Schubert, K., Ritschel, A., and Braun, U. 2003. A monograph of <em>Fusicladium </em>s. lat. (Hyphomycetes). Schlechtendalia 9&#58;1-132.</p> <p align="left">Smith, G.S., M.H. O'Day, and W. Reid. 1995. <a href="http&#58;//">Pecan Pest Management&#58; Insects and Diseases. </a>Bulletin MP711 of the University of Missouri Extension. </p> <p align="left">Stevenson, K. 1999. Fungicide resistance management in pecans. In&#58; McCraw, B.E., E. H. Dean, and B. W. Woods, eds. The Pecan Industry&#58; Current Situation and Future Challenges, Third National Pecan Workshop Proceedings, USDA Agricultural Research Service, 1998-04. USA&#58; USDA, pages 1-6.</p> <p align="left">Turechek, W.W. and K. L. Stevenson. 1998. Effects of host resistance, temperature, leaf wetness, and leaf age on infection and lesion development of pecan scab. Phytopathology 88&#58;1294-1301.</p> <p align="left">University of Georgia College of Agricultural and Environmental Sciences. Undated website. <a href="http&#58;//">Pecan Breeding&#58; Cultivar Information. Review of Scab Resistant Cultivars. </a></p> <p align="left">Vann, S. Undated. Home Pecan Diseases and Control. University of Arkansas Division of Agriculture Research and Extension. Technical bulletin, FSA7540. </p>3/7/2019 4:52:24 PMPecan scab is widespread in the southeastern United States but has also been reported in Pecan scab affects the leaves, shucks, and twigs of the infected plant and manifests as
​​When Weather Goes Wrong: Managing Apple Scab in a Changing Climate 37595APS Education Center: This case study introduces undergraduate students to the management of apple scab <p>​<a href="mailto&#58;">Elizabeth Garofalo</a> and <a href="mailto&#58;">Daniel Cooley</a><br>Stockbridge School of Agriculture<br>University of Massachusetts Amherst<br></p><h3> <span>INTRODUCTION</span></h3><p>This case study introduces undergraduate students to the management of apple scab (caused by the fungus <i>Venturia inaequalis</i>), a classic disease that drives most of the fungicide use in apples in northeastern North America. It teaches the relevant biology of the pathosystem, and exposes students to disease forecast models and using them in a decision support system.</p><p>A young apple grower, Laura Sagar, has adopted new cultural control (these include non-chemical means of reducing pest pressure, such as removing infected leaves or using disease resistant varieties) strategies to reduce the risk of scab, and weather-based disease models to forecast the risk of scab infections. In the past, Laura’s father had sprayed according to how long it had been since his last fungicide application, usually every five to seven days from early spring into early summer. Now, however, Laura, as well as her customers, want to keep fungicide use at a minimum. Cultural controls and disease forecasts offer ways to reduce the threat of scab outbreaks and the need for fungicide sprays while maintaining crop quality. The combination worked well in Laura’s first years managing the orchard, but then a devastating scab epidemic nearly destroyed her crop, causing her to ask whether she should return to her father’s calendar-based approach to scab management. </p><p>The apple scab management failure case asks students to learn the biology of apple scab, and to understand how knowledge of the epidemiology of scab has been used to design integrated pest management (IPM) approaches for scab. By determining how the scab epidemic in Laura Sagar’s orchard happened, students will learn about effective ecologically based tactics for managing plant diseases, such as physically decreasing pest populations, monitoring weather, and using disease risk forecasts.</p><h3> <span>OBJECTIVES</span></h3><p>The goal of this case study is to teach students modern methods for plant disease management by asking them to determine the reasons behind the failure of a management program. Students will learn the basics of two empirical disease forecast models, the importance of weather in such forecasts, and how they are used in an IPM program. The case study will also show that real IPM involves more than fungicide management.</p><p>After completing this case study, the student will&#58;</p><ul><li>Recognize apple scab symptoms.</li><li>Understand the apple scab disease cycle and the role played by weather in scab infections.</li><li>Understand the importance of primary inoculum and managing primary infections in a polycyclic disease.</li><li>Be able to weigh the advantages and limitations of timing fungicide applications using weather forecasts and related models.</li></ul><h3> <span>CAST OF CHARACTERS</span></h3><p> <b>Laura Sagar</b> – Apple orchard owner/manager. She inherited the orchard from her family.</p><p> <b>Noah Elma</b> - Extension fruit specialist in western Massachusetts, working to get fruit growers to use IPM, particularly cultural controls and disease forecasting models, in order to reduce pesticide use while maintaining crop quality. </p><p> <b>Jennifer Shea</b> – Plant pathologist.</p><h3> <span>THE CASE</span></h3><p>Laura Sagar had been learning the ropes of managing her family’s apple orchard from her father from the time she was a small child through high school and college, and since graduating she had been anxiously waiting to run the orchard herself. In 2009, when her father Jerry reached his 65<sup>th</sup> birthday, he sold the Sagar Family Orchard to her, and left the Massachusetts farm for Aruba. Laura had the independence she’d long wanted, but for the first time felt the full worry and stress of making her living from the farm.</p><table align="center"><tbody><tr><td> <a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure1.jpg" class="hidePseudo"> <img border="0" src="/edcenter/foreducators/TeachingNotes/PublishingImages/WeatherFigure1.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;" /></a></td></tr><tr><td> <span class="ms-rteCustom-PhotoCaption"> <strong>Figure 1. </strong>New apple scab infections on an apple <br>leaf early in the season (top left); older leaf infections <br>that have darkened (top right); lesions on young fruit <br>(bottom right) and scabbed fruit at harvest (bottom left).</span></td></tr></tbody></table><p align="left"> <br>High among those worries was managing apple diseases. Apples in Massachusetts get many diseases, most caused by fungi; the most important of these is apple scab (<a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure1.jpg">Figure 1</a>). The apple varieties McIntosh and Cortland, preferred by Laura’s customers, are especially susceptible to scab. In Massachusetts, a normal spring is rainy and cool, providing the perfect conditions for apple scab infections. If apple growers fail to manage the disease, scab can be devastating, destroying most or all of a crop. Laura’s father, like many conservative, older growers, had managed scab using frequent applications of fungicides, which are chemicals that can kill the pathogen that causes scab, the fungus <i>Venturia inaequalis. </i>To do this, he used a huge airblast sprayer (<a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure2.jpg">Figure 2</a>), a machine that sprayed a mist of pesticides into a fan the size of a small plane’s propeller, driving clouds of fungicide solution onto the trees. His strategy was simple&#58; keep the trees covered with fungicide from the time the first green leaflets emerged in spring until after trees had bloomed and fruit had set. Usually, he had to apply fungicide sprays targeting scab about ten times each year, sometimes more. Even then, occasionally there would be apples with scab at harvest, and every scabby apple was a worthless apple.</p><p align="left"></p><table align="center"><tbody><tr><td> <a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure2.jpg" class="hidePseudo"> <img border="0" src="/edcenter/foreducators/TeachingNotes/PublishingImages/WeatherFigure2.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;" /></a></td></tr><tr><td> <span> <strong> <span class="ms-rteCustom-PhotoCaption">Figure 2. </span></strong></span> <span> <span class="ms-rteCustom-PhotoCaption">Airblast sprayer applying pesticides <br>in an applied research apple orchard. </span></span></td></tr></tbody></table><p></p><p>Laura didn’t really enjoy driving the tractor up and down the rows, often in the night, with the roaring sprayer at her back, but there was really no alternative if she wanted to stop scab and other disease and insect problems. She knew her customers, and consumers in general, worried about pesticides on fruit. So when Laura heard about a way to cut sprays without increasing the risk of pest damage, it caught her interest. The approach, Integrated Pest Management or IPM, decreased reliance on pesticides, and incorporated other tactics to manage diseases and insects effectively. It meant spending more time every day gathering information about the insects and diseases, but she hoped it would reduce the time and money she spent spraying. Over the three years she had been using IPM, it had lived up to its promise.</p><p> <font color="#545454" face="Arial"> </font></p><table align="center"><tbody><tr><td> <a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure3.jpg" class="hidePseudo"> <img border="0" src="/edcenter/foreducators/TeachingNotes/PublishingImages/WeatherFigure3.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;" /></a></td></tr><tr><td> <span class="ms-rteCustom-PhotoCaption"> <span> <strong> <span class="ms-rteCustom-PhotoCaption">Figure 3. </span></strong></span> <span> <span class="ms-rteCustom-PhotoCaption">Apple scab disease cycle.</span></span></span></td></tr></tbody></table> <font color="#545454" face="Arial">&#160;<br></font>IPM programs require that a grower understand the biology of the pest to be managed - in this case, the fungus <i>Venturia inaequalis </i>and its disease cycle (<a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure3.jpg">Figure 3</a>). Laura had learned that scab epidemics happen in two phases, primary infections and secondary infections. Primary infections start an epidemic, and are caused by the apple scab fungus, which survives winter in old leaves on the orchard floor (<a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure4.jpg">Figure 4</a>). Usually, the first spores are ready to be released just as apple trees emerge from dormancy in the spring, pushing out new green leaves (<a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure5.jpg">Figure 5</a>). Wet weather is critical to the development and release of these fungal spores, called ascospores (<a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure6.jpg">Figure 6</a>), which float into the air. Spores that land on the new apple leaves germinate, producing a small tube called a hypha (<a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure7.jpg">Figure 7</a>). If the leaf stays wet for long enough, the tube penetrates the leaf’s cuticle. Once inside, the fungus continues to grow along the leaf surface, between the cuticle and epidermis. Eventually the fungus shoves its way back out through the cuticle, forming a fuzzy mat containing tens of thousands of new spores (<a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure8.jpg">Figure 8</a>). These spores, called conidia, can each start new, infections, termed secondary infections, which in turn can produce another generation of conidia, and in a few weeks, a scab epidemic can explode. <p></p><p></p><table align="center"><tbody><tr><td> <a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure4.jpg" class="hidePseudo"> <img border="0" src="/edcenter/foreducators/TeachingNotes/PublishingImages/WeatherFigure4.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;width&#58;300px;" /></a></td><td> <a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure5.jpg" class="hidePseudo"> <img border="0" src="/edcenter/foreducators/TeachingNotes/PublishingImages/WeatherFigure5.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;width&#58;140px;" /></a></td></tr><tr><td> <span class="ms-rteCustom-PhotoCaption"> <span> <strong>Figure 4.</strong> </span> <span>Right&#58; apple leaf from an orchard floor showing <br>scab infections from the previous growing season (dark areas). <br>Left&#58; magnified scab lesion in a leaf showing several fruiting <br>bodies (round, dark objects), called pseudothecia, with <br>one pseudothecium circled. </span></span></td><td> <span> <span> <span class="ms-rteCustom-PhotoCaption"> <span> <strong>Figure 5. </strong></span> <span>The green tip <br>growth stage on apple, <br>when buds break and begin <br>to form the first leaves. <br></span></span></span></span></td></tr></tbody></table><p></p><p>The amount of time it takes a scab infection to go from penetrating a leaf to producing conidia depends on temperature. At relatively warm temperatures, for example 65 to 75º F, it takes nine days; when it’s cold, say 45 to 55ºF, it takes 17 days. Before the new scab spots become visible, growers can’t see the fungus growing. It’s invisible. They have to depend on understanding the conditions that lead to infection in order to make good management decisions.</p><p>Laura knew the key to apple scab IPM is preventing primary infections. “If primary scab infections are prevented, or reduced,” the IPM specialist had explained, “there is no need to spray for secondary scab infections. There’s no inoculum.” </p><p></p><table align="center"><tbody><tr><td> <a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure6.jpg" class="hidePseudo"> <img border="0" src="/edcenter/foreducators/TeachingNotes/PublishingImages/WeatherFigure6.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;width&#58;210px;" /></a></td><td> <a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure7.jpg" class="hidePseudo"> <img border="0" src="/edcenter/foreducators/TeachingNotes/PublishingImages/WeatherFigure7.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;width&#58;140px;" /></a></td></tr><tr><td> <span> <span> <span class="ms-rteCustom-PhotoCaption"> <span> <strong>Figure 6.</strong> </span> <i> <span>Venturia inaequalis</span></i><span> pseudothecium <br>magnified under a microscope (400X) in a <br>prepared “squash mount” showing the three <br>important stages of ascospore maturation&#58; <br>immature asci with no spores or immature <br>spores; a mature ascus with mature <br>ascospores; and an empty ascus, which has <br>discharged spores. </span></span></span></span></td><td> <span> <span> <span> <span> <span class="ms-rteCustom-PhotoCaption"> <span> <strong>Figure 7.</strong> </span> <span>Venturia inaequalis <br>ascospore germinating, <br>producing hyphae which <br>can penetrate apple tissue.<br><br><br></span><br></span></span></span></span></span></td></tr></tbody></table><p></p><p></p><table align="center"><tbody><tr><td> <a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure8.jpg" class="hidePseudo"> <img border="0" src="/edcenter/foreducators/TeachingNotes/PublishingImages/WeatherFigure8.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;width&#58;370px;" /></a></td></tr><tr><td> <span> <span> <span> <span class="ms-rteCustom-PhotoCaption"> <span> <strong>Figure 8. </strong></span> <span>New scab lesions on a leaf (left) conidia that have developed <br>as a result of infection (right). </span></span></span></span></span></td></tr></tbody></table><p></p><p>The apple IPM program involved collecting a lot of information. For apple scab, Laura started by estimating how much inoculum would be in the orchard at the beginning of the season. This meant going through the orchard after harvest in the fall and systematically counting the number of infected leaves on a sample of trees after harvest. It took time when she was already quite busy selling apples, but it gave her a clear indication of the relative risk of scab for the following season. Ascospores from <i>V. inaequalis</i> usually don’t travel very far, about 100 feet or so. That means most, if not all, of the inoculum comes from leaves that fall within an orchard, rather than from orchards down the road or elsewhere. If there are very few scab-infected leaves at harvest, the risk of scab next year is low. Better yet, the risk of scab infections when leaves first emerged in spring can be so low that there’s no need to spray as soon as the first green apple tissue emerges, so the first fungicide application, or even two, that her father had always made could be skipped. On the other hand, if Laura found enough scab-infected leaves, she knew she should start scab management as soon as the first leaves emerged. </p><p>The next part of her IPM program involved <span> <a href="https&#58;//">sanitation</a></span>, which meant destroying as much potential primary inoculum (in the infected leaves) in the orchard as she could. To do this, Laura would spray the trees with a common nitrogen compound, urea, just before the leaves fell. If she was too busy, she could spray urea on the fallen leaves, either in the fall or the spring. After that, she would use a kind of mowing machine called a flail chopper to grind the leaves to bits. The nitrogen in the urea fed bacteria and other microbes that would quickly decay the leaves, while chopping further promoted leaf decay and disrupted fungal growth. It was a kind of insurance against inoculum that might be in the old leaves. </p><p>In spring, when the apples began to break buds, Laura kept careful track of the weather. Weather, especially temperature and moisture, is an important part of any pest’s development. She had purchased an electronic weather station that fed data into her office computer, then over the Internet to a computer at a university, where it was run through different <span> <a href="http&#58;//">pest forecasting models</a></span>. Laura could use a web app to look at the output. This app told her whether an infection had occurred, and using data from weather forecasts, whether an infection might be coming. All of the calculations happened virtually instantaneously. Using the information, Laura could decide whether she needed to spray. Basically, the app told her when the scab fungus was producing ascospores and when it stopped making them (<a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure9.jpg">Figure 9</a>), and if rainy weather might lead to a scab infection (<a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure10.jpg">Figure 10</a>). Laura didn’t understand the details of the models in the app, but they had worked well for her so far.</p><p></p><table align="center"><tbody><tr><td> <a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure9.jpg" class="hidePseudo"> <img border="0" src="/edcenter/foreducators/TeachingNotes/PublishingImages/WeatherFigure9.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;width&#58;370px;" /></a></td></tr><tr><td> <span> <span> <span> <span class="ms-rteCustom-PhotoCaption"> <span> <span class="ms-rteCustom-PhotoCaption"> <strong>Figure 9. </strong>Output from a decision support system (Network for the <br>Environment and Weather Applications, NEWA, an online tool that <br>includes pest forecasting models) for apple scab indicating estimated <br>ascospore maturation. At this site during this year, accumulated <br>ascospore maturity reached an estimated 95% on May 2 and an <br>estimated 100% on May 15.</span></span></span></span></span></span></td></tr></tbody></table><p></p><p></p><table align="center"><tbody><tr><td> <a href="/edcenter/foreducators/TeachingNotes/Article%20Images/WeatherFigure10.jpg" class="hidePseudo"> <img border="0" src="/edcenter/foreducators/TeachingNotes/PublishingImages/WeatherFigure10.jpg" alt="" style="border-width&#58;0px;border-style&#58;solid;border-color&#58;initial;width&#58;370px;" /></a></td></tr><tr><td> <span> <span> <span> <span class="ms-rteCustom-PhotoCaption"> <span> <span class="ms-rteCustom-PhotoCaption"> <strong>Figure 10. </strong>Output from a decision support system (NEWA) for <br>apple scab indicating infection events from Mar. 29 to 30, and <br>forecast to Apr. 5, indicating a high risk of infection on Apr. 1 to 3, <br>with relevant related data and forecasts.</span></span></span></span></span></span></td></tr></tbody></table><p></p><p>She did know that whether ascospores cause primary infections depends on wet leaves, so growers need to keep a close eye on “wetting periods” and their associated temperatures. Not all wetting periods cause infection. Wet periods that can cause infections are known as infection periods, or <span> <a href="http&#58;//">Mills Periods</a></span>, after a scientist who discovered that the time needed for infection varies with temperature. At cold temperatures, near freezing, it takes as long as two days for the scab spores to germinate and infect wet leaves. At warmer temperatures, around 60 to 75 ºF, it takes only nine hours of leaf wetting for the fungus to infect. If a weather forecast predicted infection conditions, it would show up on the app, and Laura would apply fungicide protection if she needed to. Alternatively, with some fungicides, it is possible to spray after an infection has started, and Laura would use these post-infection sprays if needed. In any case, she could use the app to overcome the invisibility problem with early scab infections, so she could tell here whether or not an infection had occurred. </p><p>In the fall of 2014, Laura found herself too busy with sales to spend time evaluating the amount of scab in her orchard. Recent springs had been very dry, and the orchard hadn’t had any significant scab for a couple of years. The search for one or two scab leaves in the orchard had gotten monotonous. She decided it would be okay to skip the inoculum monitoring that fall. And again, because she was busy, she decided to put off her sanitation treatment until spring.</p><p>The spring of 2015 began wet and muddy. Laura was a little nervous about not being able to get into the orchard to spray urea and chop leaves before the apple buds opened, but since she hadn’t had scab problems for several years, she didn’t think it would cause problems. </p><p>Buds swelled, burst open and started to produce flower buds; she watched the weather and the scab forecasts on her computer. As had become her practice in her low-inoculum orchard, she didn’t spray for the first infection period. </p><p>Then, abruptly, the rain disappeared, and a couple of days of hot dry weather dried the ground. A few days later, she put on a fungicide for scab when a cold front generated a couple of days of rain, enough for an infection according to the app forecast. After that, it stayed warm and dry for a couple of weeks.. When the petals began to fall from the apple flowers, petal fall, and the fruit begin to form, the rain returned for a week. Laura saw it coming using the app, and sprayed a fungicide to protect against infection. Normally, this would be the last primary scab spray Laura would need to apply that season. According to the app, ascospores had all matured. Primary scab was over. And this year looked more or less normal, if a little dry, the temperature-based model agreed with her father’s conventional wisdom, that inoculum had all been released a week after petal fall, and her fungicide protection should have dealt with the last infection. Laura heaved a mental sigh, deciding scab sprays were done. The bloom on her apples had been heavy, setting a good crop, and her harvest promised to be excellent.</p><p>Several weeks after the end of primary scab season, Laura was out checking for insect damage in the orchard, and was horrified to discover velvety, olive-colored spots on many of the leaves. This meant at least three, four or even more fungicide applications during late spring and summer would be needed to try to stop the epidemic from infecting fruit. Otherwise, come harvest, most of her apples would be unmarketable, and she would have a hard time making ends meet. Laura was angry and frustrated. She had followed the IPM strategy that had been working well for her for several years, yet the disease had hit her anyway. What had gone wrong???</p><h4> <span>Questions</span></h4><ol><li>What is the most critical period in the apple growing season for scab management?</li><li>What are two distinct stages in an apple scab epidemic, and what types of fungal spores are associated with each? </li><li>What two environmental factors are critical in apple scab infections? Which one drives the development of primary inoculum? Which is/are important in an individual infection of apple tissue?</li></ol><h3> <span>DISEASE MANAGEMENT</span></h3><p>In a bit of a panic, Laura called her local Extension tree fruit specialist, Noah Elma, with whom she had worked to develop the IPM plan for her orchard, installing the weather station and getting connected with the university. </p><p>First, Noah grilled Laura on how much fungicide she applied in her sprays, whether she had calibrated her sprayer to apply correct amounts, and whether wind might have been blowing hard enough to cause the fungicide to drift away from the apple trees where it was needed. Laura kept detailed records of all her pesticide applications. These records include information like date and time of application, what materials and rates she used and a brief description of what the weather looked like for any given day. Based on Laura’s records, it didn’t appear as though wind or the amount of fungicide used were at fault.</p><p>Then they pored over the records from Laura’s weather station. There had been four infection periods during the time that models predicted ascospores were available to cause infections, two early and two late in the primary scab season. The pair then checked Laura’s fungicide spray records from that spring to verify that the orchard had been treated at the right times for each of the infection events. </p><blockquote dir="ltr" style="margin-right&#58;0px;"><p>“You didn’t put anything on for the first infection?” asked Noah.</p><p>“No. It was muddy in the orchard. And for the last few years, I’ve skipped the first one. No problem.”</p><p>“Let’s go look,” said Noah.</p></blockquote><p>They went back to Laura’s orchard and pushed into the leaves on the trees to see exactly what the pattern of infection was. Right away, Noah saw that most infections were on leaves that had emerged relatively recently. In fact, some infections seemed to have happened after all inoculum should have been spent. It didn’t make sense, the model clearly showed primary scab inoculum depleted at least 10 days before some leaves had emerged and been hit with scab. Laura’s spray records indicated she had gotten a protective cover on in advance of each of the other primary infection events. </p><blockquote dir="ltr" style="margin-right&#58;0px;"><p>“Except for the first one”, observed Noah.</p><p>“I explained that. Shouldn’t have been a problem”, snapped Laura.</p><p>“What did your fall scab survey show?”</p><p>“Actually, I had to skip it last fall.”</p><p>“I’m thinking maybe that was a bad idea. You made a lot of assumptions.”</p></blockquote><p>Noah’s idea was that Laura had been hit by a combination of failing to monitor and bad luck. Laura’s orchard had likely suffered a little more scab than usual the previous year, and she hadn’t noticed because she hadn’t done her usual careful fall evaluation. Normally, her sanitation program would have greatly reduced or eliminated most inoculum. But it hadn’t happened. But, where had the late season infections come from? </p><blockquote dir="ltr" style="margin-right&#58;0px;"><p>“Two possibilities,” said Noah. “You got some early infections in that first infection period, and it didn’t explode to the point you saw it until now. The other is a little strange. I’ve heard Jenn Shea say that the model for predicting the end of primary season may not be that accurate in dry years.”</p></blockquote><p>Noah gave Jennifer Shea, a plant pathologist at the University of Massachusetts, a call. After hearing Laura’s tale of woe, Jennifer agreed. She had observed the actual growth and release of ascospores in order to check the model estimates. This year, the most commonly used model, the one Laura followed, had indicated primary season was done nearly two weeks before Jenn had stopped seeing mature spores. In that time, there had been a significant infection period. She had also fielded calls from other growers; Laura was not the only one with an apple scab outbreak, though hers was noticeably more severe. </p><p>The high number of infections in her orchard still surprised Laura. There shouldn’t have been much inoculum left at the end of ascospore development. Noah reminded her of the possibility of an infection very early in the season. </p><blockquote dir="ltr" style="margin-right&#58;0px;"><p>“The earlier scab starts, the worse the epidemic Laura. You know that.”</p></blockquote><p>With more inoculum than usual in her orchard, any mistake would lead to more scab than usual. Some of the mistakes had been Laura’s. The fall assessment would’ve warned her of a potential problem. <span> <a href="https&#58;//">Sanitation</a></span> would have helped reduce the impact, had she done it. If she had checked her “hot spots”, the places where scab was most likely to hit, during the spring, she might have seen symptoms earlier and been better able to stop them. </p><blockquote dir="ltr" style="margin-right&#58;0px;"><p>“Yeah, I got kind of complacent. But what about the end of scab season thing?”</p><p>“I don’t know. Hopefully Jenn will fix that soon. Meanwhile, with these climate change extremes, dry then wet, I’d play it conservative. The decision support recommendations are a useful guide, but they aren’t absolute reality.”</p></blockquote><p>It made sense, though it didn’t make Laura feel any better. It would be a rough year. She launched a fungicide program to kill off the scab infections and protect fruit from new, secondary infections. Ultimately, she sprayed twice as much as she normally had, and still suffered some loss from scab at harvest. It had cost her much more than usual. From then on, monitoring and the other IPM tactics she used for scab became a top priority&#58; evaluating inoculum in the fall, spraying and chopped leaves in the fall, and interpreting the app recommendations more conservatively. </p><h4> <span>Questions</span></h4><ol><li> <span>What are the three key factors that could have contributed to the scab epidemic in Laura’s orchard?</span></li><li> <span>How does evaluating inoculum in the fall contribute to an apple scab management program?</span></li><li> <span>Why is it important to know when all primary inoculum is spent?<span>&#160; </span>How might Laura use this information to inform scab management decisions?</span></li><li> <span>Why is it important for Laura to keep detailed records of her management decisions?</span></li></ol><h3> <span>References</span></h3><p> <span class="ms-rteCustom-Reference">Aylor, D.E. 1998. The aerobiology of apple scab. The American Phytopathological Society. Plant Disease 90&#58; 829-847.</span></p><p> <span> <span class="ms-rteCustom-Reference">Baskerville, G. L. and P. Emin. 1969. Rapid Estimation of Heat Accumulation from<span>&#160;&#160;&#160;&#160; </span>Maximum and Minimum Temperatures. Ecology 50(3)&#58;514-517</span></span></p><p> <span class="ms-rteCustom-Reference">Gadoury, D.M., R.C. Seem, W.E. MacHardy, W. F. Wilcox, D.A. Rosenberger, A. Stenzvend. 2004. Comparison of methods to estimate ascospore maturity and release of ascospores by <i>Venturia inaequalis<b>. </b></i>The American Pythopathological Society. 88&#58; 869-874.</span></p><p> <span class="ms-rteCustom-Reference">James, J.R., T.B. Sutton. 1982. Environmental factors influencing pseudothecial </span></p><p> <span class="ms-rteCustom-Reference">development and ascospore maturation of <i>Venturia inaequalis</i>. Phytopathology 72&#58; 1073-1080</span></p><p> <span class="ms-rteCustom-Reference">McHardy, W.E. and D.M., Gadoury. 1985. Forecasting seasonal maturation of ascospores of <i>Venturia inaequalis</i>. Phytopathology 75&#58; 381-385.</span></p><p> <span class="ms-rteCustom-Reference">Mills, W. D. 1944. Efficient use of sulfur dusts and sprays during rain to control apple scab.<i> </i> <span>Cornell Extension Bulletin, 630</span>(War Emergency Bulletin 114)</span></p><p> <span> <span class="ms-rteCustom-Reference">Stensvand, A., H. Eikemo, D.M. Gadoury, and Robert C. Seem. 2005. Use of a rainfall frequency threshold to adjust a degree-day model of ascospore maturity of&#160;<i>Venturia inaequalis</i>. Plant Disease 89&#58;198-202</span>.</span></p>​<br>6/21/2019 5:29:48 PMstrategies to reduce the risk of scab, and weather-based disease models to forecast the risk of scab infections
Plant Disease Lessons4181APS Education Center - Diseases and Pathogens - Fungal Diseases, Ascomycota - Plant Disease Lessons<p>​​</p><div class="ms-rtestate-read ms-rte-wpbox"><div class="ms-rtestate-notify ms-rtestate-read cff8f422-7da1-44be-a3e8-44944f56a7b8" id="div_cff8f422-7da1-44be-a3e8-44944f56a7b8" unselectable="on"></div><div id="vid_cff8f422-7da1-44be-a3e8-44944f56a7b8" unselectable="on" style="display&#58;none;"></div></div><p>​​<br></p>2/22/2019 7:10:03 AMEspañol) Apple powdery mildew Apple scab Black knot Black sigatoka Black sigatoka vine decline of melons Mummy Berry Pecan Scab Petal Blight of Camellia Phomopsis Stem
scab343613/3/2010 4:58:16 PM
Teaching Articles & Notes1887APS Education Center Teaching Articles & Notes<p></p><div class="ms-rtestate-read ms-rte-wpbox"><div class="ms-rtestate-notify ms-rtestate-read f37706a3-faee-4195-a477-88ca73a61a12" id="div_f37706a3-faee-4195-a477-88ca73a61a12" unselectable="on"></div><div id="vid_f37706a3-faee-4195-a477-88ca73a61a12" unselectable="on" style="display&#58;none;"></div></div><p>​​<br></p>6/21/2019 5:36:40 PMBecome a Member Divisions People & Directories News Capsule Phytopathology News When Weather Goes Wrong: Managing Apple Scab in a Changing Climate ​A Short Course in
U.S. Wheat & Barley Scab Initiative Request for Pre-Proposals Now Open for FY22 Funding Cycle114849The U.S. Wheat & Barley Scab Initiative (USWBSI) announces the opening of its Request for Pre-Proposal (RFP) process for the FY22 Funding Cycle.7/2/2021 4:51:58 PMBecome a Member Divisions People & Directories News Capsule Phytopathology News Diversify APS Online Community Vision & Overview Boards & Committees Governance History
2021113886<p>​​​​​​​<br></p>2/8/2021 11:47:21 PMBecome a Member Divisions People & Directories News Capsule Phytopathology News Diversify APS Online Community Vision & Overview Boards & Committees Governance History
2011 Meeting | Influence of nickel on severity of pecan scab157066/13/2011 6:59:32 PMof nickel (Ni) on the severity of pecan scab was examined in both field and lab studies in Ni can provide some protection against pecan scab when used alone at high concentration or
2016 Annual Meeting | Scab severity in relation to hedge pruning pecan trees in the Southeastern USA201949/27/2016 4:47:45 PMinvariably had a significant effect on scab severity, with up to 18-fold more severe scab on fruit at ≥12 m compared to 5 m
Recovery plan for Monilinia polystroma1012639/22/2016 5:09:04 PMSpecies native to North America > invasive to Europe and Asia Peach scab which appears as fuzzy slightly raised olive colored spots & Peach shot hole M. fructicola and M. Laxa: 5x1
2008 Annual Meeting Abstract | The status of powdery scab disease on potatoes in Egypt184377/19/2010 9:50:40 PMSymptoms of powdery scab on tubers were assessed visually by following the progress of the development of the powdery scab lesions on the infected tubers
2008 Annual Meeting Abstract | Combining sanitation practices with timing of scab sprays in organic apple production180617/19/2010 9:49:36 PMThe aim of this study was to evaluate scab control efficacy in integrated approaches of i Results on scab resistant cultivar Prima indicated that sprays against scab could
2011 Meeting | Evaluation of phosphite to control scab on pecan in the southeastern U.S.A.164506/13/2011 7:02:24 PM1 ha–1). Both phosphite and TPTH reduced scab provided equally good control of leaf scab, with the exception of one of the TPTH