Posted online March 27, 2006
Heat stable antibacterial agents from shiitake mushroom. A. K. BRAR and R. P. Pacumbaba. Department of Plant and Soil Science, Alabama A & M University, Normal, AL 35762.
Mycelial leachate of shiitake contain antibiotic and antibacterial agents. Mycelial leachate was obtained by soaking the spawn in water for one week. The antibiotic was named Pentathiodecane (heat labile). Antibacterial agents from the mycelial leachate were lyophilized, rehydrated, and autoclaved (heat stable). The heat stable antibacterial agents were each treated with chloroform and the ethyl acetate, lyophilized, and resuspended with 0.01 M TRIZMA and passed through a DEAE Sephadex™ A-25 column. The chloroform and ethyl acetate fractions were tested for growth inhibition of Ralstonia solanacearum, Curtobacterium flaccumfaciens pv. flaccumfaciens, Pseudomonas syringae pv. glycinea, P. syringae pv. tabaci, Xanthomonas campestris pv. vesicatoria, Erwinia amylovora, Bacillus cereus, Escherichia coli, Listeria monocytogenes, Salmonella typhimurium, and Staphylococcus aureus. Part of chloroform and ethyl acetate fractions inhibited the above bacteria in the laboratory. Both fractions are to be tested for inhibition of bacterial wilts of tomato and beans in the greenhouse and the chemical identity of the antibacterial agents.
Use of chlorpyrifos to supplement full- and reduced-input fungicide programs for control of peanut stem rot. T. B. BRENNEMAN (1) and S. L. Brown (2). (1) Dept. Plant Pathology, and (2) Dept. Entomology, University of Georgia, Tifton, GA 31794.
The insecticide chlorpyrifos (2.18 kg/ha) has fungitoxic traits and was evaluated for control of peanut stem rot (Sclerotium rolfsii), either alone or with tebuconazole (two or four applications at 0.23 kg/ha) or azoxystrobin (two applications at 0.22 or 0.33 kg/ha) in four field tests from 2003–2005. Nearly all treatments reduced stem rot incidence, but the fungicides alone increased yield in only 11 of 16 comparisons, versus 15 of 16 when chlorpyrifos was also used. Yield in control plots was between 2822 and 3123 kg/ha, and chlorpyrifos added 642 kg/ha. The mean yield increase for the fungicides alone was 802 kg/ha, and 1167 for chlorpyrifos plus the fungicides. In two of eight comparisons, yield was lower in the two versus four sprays of tebuconazole, but yields were always similar for the two rates of azoxystrobin. Insect damage was minimal and did not affect yield, confirming the potential to obtain additional control of stem rot with chlorpyrifos.
Identification of Armillaria species on daylily based on ribosomal ITS and IGS-1 sequences. K. E. BUSSEY and G. Schnabel. Dept. of Entomology, Soils and Plant Sciences, Clemson University, Clemson, SC 29634.
Daylily was not reported to be a host of Armillaria species, until the recent discovery of infected daylily plants in Walhalla, SC. The aim of this study was to characterize the Armillaria species infecting daylily based on comparisons of ribosomal ITS and IGS-1 nucleotide sequences with those available on GenBank. A single isolate (SC.FR.04) was found responsible for the daylily infection and analysis of the ITS1-5.8s-ITS2 region revealed that this isolate is most similar to A. gallica and A. calvescens sequences from GenBank. Sequence electropherograms also indicated heterogeneity within the IGS-1 region of the rDNA. Twelve clones from the IGS-1 amplicon were sequenced and five divergent copies were discovered. The copies were most similar to A. gallica and A. sinapina. Our results indicate that the disease is most likely caused by A. gallica based on ribosomal DNA analysis. Sexual compatibility tests using single-spore tester strains from North American Armillaria species are currently under way in order to confirm the molecular identification.
Distribution of methyl isothiocyanate (MITC) in Vapam-treated plastic-mulched sandy soil beds. B. L. CANDOLE (1), A. S. Csinos (1), and D. Wang (2). (1) University of Georgia-Tifton; (2) University of Minnesota, St. Paul.
The distribution and efficacy of MITC against soilborne plant pathogens in drip-irrigated sandy soil beds were studied. MITC levels were monitored from four sites (10 and 20 cm below the emitter, 20 cm away [and 10 cm deep] from the emitter, and the airspace between the soil surface and plastic film) in beds treated with Vapam (42% metam sodium EC) at 285 l/ha. MITC was monitored at 3, 12, 24, 48, 72, 120, and 240 h after chemigation. Highest MITC levels were observed at 20 cm below the emitter and lowest at the airspace. MITC peaked at 2200 and 1312 µg/l soil atmosphere 12 h after chemigation at 20 and 10 cm below the emitter, respectively. Survival of P. capsici, R. solani from artificially infested beet seeds and yellow nutsedge nutlets embedded for 10 d at 10 cm below and at 20 cm away from the emitter were assessed. Lower MITC concentrations, higher pathogen and yellow nutsedge survival at 20 cm away from the emitter indicated poor lateral diffusion of MITC in sandy soil bed and consequently reduced efficacy against soilborne pathogens and weeds. Improved application techniques are needed to improve the efficacy of MITC.
Timing applications of ASM in tobacco for TSWV management. A. S. CSINOS (1), M. G. Stephenson (2), L. L. Hickman (1), and S. Mullis (1). (1) Dept. of Plant Pathology, (2) Crop and Soil Sciences Dept., University of Georgia, CPES, Tifton, GA 31793.
Tomato spotted wilt virus (TSWV) continues to be one of the most serious diseases of solanaceous crops in the Southeast. In particular, tobacco has no resistant commercial cultivars, and thus only agronomic and chemical means are available for management. Post plant applications of acibenzolar-S-methyl (ASM) were made 0, 14, and 28 days pp in 2003, 0, 14, 28, and 42 days pp in 2004 and 0, 7, 14, 21, 28, 35, and 42 days pp in 2005, following float house treatments of imidacloprid plus ASM. The greatest disease suppression and highest yield were for plots treated 28 days (24 April 2003), 28 days (27 April 2004) and 42 days (11 May 2005) pp. Yields were increased 40, 76, and 82% respectively for 2003, 2004, and 2005 over the non treated controls. These dates closely correspond to the beginning of the epidemics for each of the years. ELISA data mirrored symptomatic plant data, and thrips populations began to peak at that point in the season. These data may provide a key trigger that growers may use to best optimize applications of ASM for TSWV management in flue cured tobacco.
Genetic diversity of Tennessee Sclerotinia homoeocarpa isolates delineated by vegetative compatibility and AFLP analysis. R. E. DEVRIES (1), R. N. Trigiano (1), M. T. Windham (1), A. Windham (1), and T. A. Rinehart (2). (1) University of Tennessee, Dept. of Entomology and Plant Pathology, Knoxville, TN 37996; (2) USDA/ARS, Southern Horticultural Laboratory, Poplarville, MS 39470.
Sclerotinia homoeocarpa lesions cause serious problems by tainting the uniformity and aesthetic value of putting greens. Genetic variation between fungicide resistant dollar spot isolates collected from golf courses across Tennessee was examined by vegetative compatibility pairings, sequencing of conserved genes, and AFLP analysis. Isolates were paired against six tester strains and each other on potato dextrose agar. Most isolates were incompatible with each other and only a few were placed into vegetative compatibility groups. Sequencing of conserved genes showed 100% homology between isolates. AFLPs were detected by capillary gel electrophoresis and isolates were determined to be 80–90% similar. Although incompatibility may indicate high genetic diversity, little variability was demonstrated using molecular methods. Increased sampling of the genome will be performed to uncover sources of variation.
New disease of Syagrus romanzoffiana (queen palm) in Florida. M. L. ELLIOTT. Fort Lauderdale Research and Education Center, University of Florida, Fort Lauderdale, FL 33314.
Syagrus romanzoffiana is a popular landscape ornamental palm grown throughout subtropical southern Florida. Mature queen palms, with symptoms not observed previously, have been dying prematurely in Broward, Collier, Orange, and Palm Beach counties. Neither insect damage nor nutritional deficiencies account for the symptoms, which begin with premature browning of the oldest leaves. Affected leaves do not break or hang down. The next younger 1 to 2 leaves turn varying shades of yellow. Within 2 months after onset of initial symptoms, the entire canopy desiccates and turns brown as if freeze-dried in situ. Patches of reddish-brown discoloration are consistently observed on petioles where affected leaves bend outwards from the trunk. While petiole cross-sections revealed internal discoloration, no rotted tissues were evident. The apical meristem is not affected, even after the entire canopy has turned brown. Several candidate pathogens have been isolated from symptomatic petiole tissue. Of these, Fusarium spp. have been consistently isolated. However, inoculations of juvenile queen palms have not, as yet, confirmed pathogenicity.
Pathogenicity of a diverse group of isolates of Phytophthora capsici from Florida on four main vegetable crops. R. D. FRENCH-MONAR, D. C. Schultz, and P. D. Roberts. Plant Pathology Department, IFAS-SWFREC, University of Florida, Immokalee, FL 34142.
Thirty-two isolates of Phytophthora capsici were collected from bell pepper (10), summer squash (10), tomato (5), and watermelon (7) from several counties in Florida. Fifty-three percent of the isolates were of the A2 mating type. Pathogenicity tests were carried out on seedlings of four plant hosts under greenhouse conditions. Isolates from pepper and tomato were highly pathogenic on pepper seedlings. None of the isolates tested were highly pathogenic on tomato seedlings. No disease symptoms were observed on tomato for 21% of the isolates tested. Although most isolates were moderately pathogenic on watermelon seedlings, 14% of isolates were weakly pathogenic. All isolates tested on summer squash were highly pathogenic. In at least one repetition for all isolates tested, 100% mortality was observed on summer squash. Based on these findings, the P. capsici isolates tested were highly pathogenic on squash and except for two isolates recovered from watermelon and one from squash, were equally pathogenic on bell pepper and less pathogenic on watermelon and tomato.
Occurrence of race 4 of Phytophthora nicotianae in tobacco. C. A. GALLUP (1), M. J. Sullivan (1), W. C. Nesmith (2), and H. D. Shew (1). (1) Plant Pathology, NC State Univ., Raleigh, NC 27695; (2) Univ. of Kentucky, Lexington.
While conducting tests on a set of host differentials to identify races 0 and 1 of P. nicotianae, a new race, race 4, was identified based on the ability to overcome a resistance gene from Nicotiana longiflora but not the Ph gene from N. plumbaginifolia. Race 1 overcomes both genes and race 0 neither gene. Race 4 was recovered from field sites in NC and Kentucky where the N. longiflora gene was deployed. In addition, race 4 was recovered from multiple field sites in NC where varieties with only partial resistance from Fla. 301 were deployed. The development of race 4 on tobacco varieties with partial resistance was confirmed in greenhouse tests. Flats were planted with varieties with low, medium, or high levels of partial resistance and soil was infested with an isolate of race 0, 1, or 4. Isolates were recovered from soil after 140 days and inoculated onto host differentials. Races 0 and 4 were recovered from flats infested with race 4, and race 4 was recovered from all flats infested with races 0 and 1. Race 1 was not recovered from race 0 or race 4 flats. The mechanism for this previously unreported race shift is under investigation.
Effect of field resistance to the soybean cyst nematode on soybean sudden death syndrome development and yield components. S. L. GIAMMARIA and J. C. Rupe. Department of Plant Pathology, University of Arkansas, Fayetteville, AR 72701.
Soybean sudden death syndrome (SDS, Fusarium virguliforme) has been associated in the field with the soybean cyst nematode (SCN). Previous reports indicate that co-inoculations enhance the onset and severity of SDS. The role of cultivar resistance on SDS development and yield was studied in a 2-year microplot experiment. Four cultivars (Pioneer 9594 –SDS resistant, SCN susceptible; Asgrow 5603 –SCN resistant, SDS susceptible; Hartwig –SDS and SCN resistant; Essex –SDS and SCN susceptible) were inoculated with the fungus, SCN, both pathogens or not inoculated. SDS did not develop in SCN-only, uninoculated plots, or in any treatment in cv. Hartwig. AUDPC was higher for Essex, followed by A5603 and P9594 (P < 0.001). Co-inoculation increased SDS development and reduced pod and seed numbers, weight of total and 100 seeds in Essex and P9594 (P < 0.001) but not in A5603. Enhancement of SDS by SCN presence may require soybean cultivars that are susceptible to both pathogens. Varietal resistance to SCN may predict the performance of soybean cultivars in fields where both pathogens occur simultaneously.
Mandipropamid: A new fungicide for control of late blight and downy mildews. TYLER HARP (1), Gary Cloud (2), Brad Minton (3), and Alex Cochran (4). Syngenta Crop Protection, (1) Vero Beach, FL; (2) Tallahassee, FL; (3) Cypress, TX; and (4) Visalia, CA.
Mandipropamid is a new fungicide from Syngenta Crop Protection for control of late blight on tomato and potato, and downy mildews on cucurbits, leafy vegetables, brassica vegetables, bulb vegetables, hops, and tobacco. Mandipropamid is a representative of a novel class of chemistry, the mandelamides, and provides outstanding control of foliar diseases caused by oomycetes. The solo product is formulated as a 2.08 SC, and has a use rate of 5.5–8.0 oz per acre, with an application interval of 7–14 days. Several combination products are also under development. Mandipropamid belongs to Resistance Group 40, the carboxylic acid amides (CAA). Registration is anticipated late in 2007, with launch the following year.
Overview of the Asian soybean rust sentinel network for 2005. D. E. HERSHMAN (1) and L. J. Giesler (2). (1) Dept. of Plant Pathology, University of Kentucky, Princeton, KY 42445; (2) Dept. of Plant Pathology, University of Nebraska, Lincoln, NE 68583.
In response to the arrival of Asian soybean rust (ASR) (Phakopsora pachyrhizi) in the U.S. in the fall of 2004, an extensive ASR surveillance network was established in 31 states and Ontario, Canada during 2005. The network, which was coordinated by USDA and implemented by Land Grant Universities, was comprised of 824 soybean and 149 kudzu sentinel plots. Soybean plots were planted early relative to the main crop, and often involved early and late-maturing cultivars. Soybean plots were at least 15 m(^2). Plots were established on university research farms and in grower fields at strategic locations. Plots were scouted at least weekly. Soybean, kudzu, and other legumes were also observed for ASR in arbitrarily selected sites. ASR was detected in 136 counties in nine southern states between 23 Feb and 21 Nov. The disease always first observed in the lower canopy. All soybean detections were at the latter reproductive stages (R3 or later). ASR was also confirmed in Desmodium tortuosum in Georgia, but not in other legumes. The sentinel network was highly effective for monitoring ASR activity.
Phytophthora cambivora, a new dieback pathogen of rhododendron and Pieris in North Carolina. J. Hwang (1), C. Y. Warfield (2), and D. M. BENSON (2). (1) Dept Entomology, Soils, and Plant Sciences, Clemson University, Clemson, SC 29634; (2) Dept Plant Pathology, North Carolina State University, Raleigh, NC 27695.
In May 2003, a statewide survey of North Carolina nurseries growing hybrid rhododendron and Pieris spp. was completed as part of a Pilot National Survey for Phytophthora ramorum. Individual samples of leaves and stems were assayed for the presence of P. ramorum by culture plating on PARP medium and by a nested PCR detection method. P. ramorum was not detected in any of the more than 300 plant samples collected. Two isolates recovered were identified as P. cactorum and 61 isolates as P. citricola based on morphological characteristics and RFLP-PCR with ITS primers ITS5 and ITS4 and restriction enzymes MspI and RsaI. A group of 38 isolates was identified as P. cambivora based on morphological characters and a second RFLP-PCR protocol that used primers ITS6 and ITS4 and restriction enzymes AluI and MspI. Isolates of P. cambivora tested were compatibility type A1 in matings with P. nicotianae A2. This is the first report of P. cambivora causing Phytophthora foliar and stem dieback on rhododendron and Pieris spp. in North America.
Recovery of Didymella bryoniae from melon crown debris. A. P. KEINATH. Clemson University, Coastal REC, Charleston, SC 29414-5329.
The crowns of melon (Cucumis melo ssp. melo) plants are susceptible to cankers caused by the fungal pathogen Didymella bryoniae. The length of time that D. bryoniae survived in cankers was determined in three 1-year studies from 2002 to 2005. Dried crowns with large cankers were buried 12.5 cm deep or placed on the soil surface in July 2002, Dec. 2003, and July 2004. Every 12 to 17 weeks, crowns were retrieved, washed, cut into pieces, and cultured. D. bryoniae was not recovered from crowns buried 35 and 45 weeks in 2003 and 2004 but was recovered from 4% of crowns buried 51 weeks in 2005. In contrast, D. bryoniae was recovered after 48, 45, and 51 weeks from 67%, 3%, and 26% of crowns on the surface in 2003, 2004, and 2005, respectively. In 2004, crowns also were placed on top of raised beds covered with white polyethylene mulch. Percentage of crowns with viable D. bryoniae did not change (P > 0.06) between 0 and 51 weeks (mean of 78%). Burying infested debris would reduce the time D. bryoniae survived after a cucurbit crop, but leaving mulched beds intact would promote survival of the pathogen.
Effects of zoxamide and dimethomorph on production and germination of zoospores of Phytophthora capsici. A. P. KEINATH and P. McLaughlin. Clemson University, Coastal REC, Charleston, SC 29414.
In South Carolina, Phytophthora blight causes losses over $250,000 annually to summer squash and pepper growers. Because mefenoxam-insensitive P. capsici occurs on some farms, other fungicides are needed. Effects of zoxamide and dimethomorph on production and germination of zoospores, critical steps in the disease cycle, were examined in two assays. V8 agar discs with actively growing mycelia were placed on water agar plus 0, 0.03, 0.31, or 3.1 mg/L zoxamide or 0, 0.01, 0.10, or 1.0 mg/L dimethomorph and zoospores were counted at 3 days. The percentage of encysted zoospores that germinated after 2 hours also was determined on V8 agar amended with the same concentrations of dimethomorph. EC(50) values for zoospore production by 14 isolates were <1.0 mg/L dimethomorph and <2.0 mg/L zoxamide. One isolate had an EC(50) >1.0 but <3.1 mg/L dimethomorph and another isolate had an EC(50) <0.03 mg/L zoxamide. EC(50) values for zoospore germination for 30 isolates were <0.10 mg/L dimethomorph. Both zoxamide and dimethomorph inhibit zoospore production, but P. capsici is more sensitive to dimethomorph than to zoxamide.
Managing Asian soybean rust in Georgia with fungicides. R. C. Kemerait (1), L. E. Sconyers (1), P. H. Jost (2), J. Kichler (3), and J. Clark (3). (1) Dept. of Plant Pathology, Coastal Plain Experiment Station; (2) Dept. of Crop and Soil Sciences, Statesboro; (3) Coop. Ext., Oglethorpe and Baxley, University of Georgia.
Soybean trials were established in four counties in 2005 to assess fungicides for control of soybean rust (Phakopsora pachyrhizi). Fungicides included Headline SBR, 7.8 fl oz/A, Folicur, 4 fl oz/A, Stratego, 8 fl oz/A, Echo 720, 24 fl oz/A, Headline, 6.14-9 fl oz/A, Laredo, 6 fl oz/A, Quadris, 10.8 fl oz/A, and Folicur + Topsin 4.5F, 4 fl oz + 16 fl oz/A, Topguard, 7 fl oz/A, and Sparta, 4 fl oz/A. Rust was typically detected in nearby sentinel plots as plants in trials reached reproductive (R1-R2) growth stages. Application of triazole or strobilurin fungicides during R1-R2 growth stages followed by an application 2–3 weeks later resulted in decreased severity of rust and increased yield (5.1-18.18 bu/A for best treatments) compared to the untreated control, though differences were not always statistically significant. Though not as effective as 2 applications, a single application of Folicur or Headline SBR during R1-R2 stages, but not a single application during R3-R5 stages, improved disease control and yield over the control.
Managing mefenoxam-tolerant isolates of Phytophthora capsici on bell peppers. C. S. KOUSIK. Pepper Research Inc., Loxahatchee, FL. Present address: US Vegetable Laboratory, USDA-ARS, Charleston, SC.
Phytophthora capsici is an important pathogen of bell peppers in southern Florida. In a recent survey, over 51% of P. capsici isolates in Southeastern Florida were tolerant to mefenoxam (French et. al., 2005, Phytopathology 95:S31). Since 2003 we have been testing Ranman 400SC (cyazofamid) to manage P. capsici isolates that are tolerant to mefenoxam. In detached leaf experiments where mefenoxam-tolerant isolates were used as the inoculum, Ranman 400SC sprays at 2.75 fl oz/a provided about 70% disease reduction compared to nontreated or Ridomil Gold EC (1 pt/a) sprayed controls. Experiments also were conducted in commercial fields where mefenoxam-tolerant isolates were prevalent. Drenching field plots with Ranman 400SC, or combinations of Ranman plus Ridomil Gold Copper (2.5 lb/a) at transplanting, followed by sprays of the same treatments provided nearly 98% reduction compared to nontreated control against root and leaf infections. Root and collar infections also were significantly reduced by combinations of Omega 500F (fluazinam) drench (1.5 pt/a) followed by sprays of Ranman 400SC or Ranman+Ridomil Gold Bravo.
Epidemiology of powdery mildew on flowering dogwood in Tennessee. Y. H. LI (1), M. T. Windham (1), R. N. Trigiano (1), D. C. Fare (2), J. M. Spiers (3), and W. E. Copes (3). (1) Dept. of Entomology and Plant Pathology, University of Tennessee, Knoxville, TN; (2) USDA/ARS Floral & Nursery Plants Research Unit, McMinnville, TN; (3) USDA/ARS Southern Horticultural Research Unit, Poplarville, MS.
Powdery mildew, caused by Erysiphe pulchra (syn. Microsphaera pulchra) is an important disease on flowering dogwood (Cornus florida) in the Eastern United States. Temporal progress of powdery mildew on flowering dogwood cultivars with different levels of resistance was investigated in the field in 2004 and 2005. Disease onset was observed in late-May, followed by a rapid increase in disease severity until early- to mid-August. Thereafter, disease severity increased slowly. Using nonlinear regression analysis, disease progress curves were fitted to logistic models (R(^2) > 85%) and absolute rates and y(max) derived. Standard areas under disease progress curves (sAUDPC), absolute rates and y(max) were significantly different (P < 0.05) among dogwood cultivars in both years. Cultivar ‘Karen’s Appalachian Blush’ consistently expressed higher resistance with lower values of sAUDPC, y(max) and slower absolute rate in both years compared to a susceptible line of variety ‘Rubra’ and moderately susceptible cultivar ‘Cherokee Daybreak’.
Molecular and biochemical characterizations of elsinochrome toxins produced by Elsinoe fawcettii causing citrus scab. H.-L. LIAO and K.-R. Chung. Citrus Research & Education Center, and Dept. of Plant Pathology, University of Florida, Lake Alfred, FL 33850.
Citrus scab caused by the fungus Elsinoe fawcettii results in serious fruit blemishes and economic losses in Florida. The fungus produces elsinochrome phytotoxins, having structural similarities to cercosporin, a nonspecific light-activated perylenequinone toxin produced by many Cercospora spp. Elsinochromes and other perylenequinone toxins are grouped as photosensitizing compounds that can absorb light energy and produce reactive oxygen species such as singlet oxygen ((^1)O(2)) and superoxides which damage host cells. Assays using tobacco protoplasts and cell suspensions revealed that toxicity of elsinochromes is light-dependent and is inhibited by ((^1)O(2)) quenchers (bixin, dabco). Partial sequences similar to fungal polyketide synthase and calmodulin genes have recently been identified from E. fawcettii and will be molecularly and functionally characterized to determine if elsinochromes play any role in fungal pathogenesis and symptom development by analyzing mutants specifically defective in elsinochrome biosynthesis and regulation.
Monitoring abiotic and biotic stressors in greenhouse crops using color infrared imagery. C. R. LITTLE and K. R. Summy. Dept. Biology, The University of Texas - Pan American, Edinburg, TX 78541.
Abiotic and biotic stressors of greenhouse plants represent major constraints for optimum production and product quality. Changes in near-infrared (NIR) and red reflected wavelengths from stressed plant foliage (due to NaCl toxicity, N deficiency, mite damage (Tetranachus citri), sooty mold (Capnodium spp.), powdery mildew (Sphaerotheca fuliginea), or a foliar fungal leaf spot (Mycosphaerella citri) were tested independently and measured under greenhouse conditions using spectroradiometry and color infrared (CIR) imagery analysis. Significant (P < 0.05) changes were observed in the NIR and red wavelengths in most stressed plants as compared to control. Simple vegetative indices (NIR/red image ratios) were also significant and accentuated differences between healthy and stressed plants. These proven aerial remote sensing techniques should be readily adaptable to the production scale greenhouse environment.
Evaluation of phosphonate fungicides for control of foliar and tuber late blight of potato. H. MAYTON and W. E. Fry. Dept. Plant Pathology, Cornell University, Ithaca, NY 14853.
Most commercially cultivated potato varieties are susceptible to late blight, caused by Phytophthora infestans, therefore, an intensive fungicide schedule is needed to minimize loss of production. One potential method to reduce applications of conventional fungicides is to incorporate biopesticides into the potato production system. Four phosphonate (dipotassium phosphonate/dipotassium phosphate) biopesticides; Biophos, Fosphite, Prophyte, and Phostrol were evaluated in the field for efficacy against P. infestans. Phosphonates were assessed alone, in combination with, and in alternation with the conventional fungicide Bravo WS. Aliette (aluminum phosphonate) a conventional fungicide, was also evaluated. All biopesticide phosphonate treatments applied alone and in combination with Bravo WS suppressed foliar disease progress significantly when compared to the Aliette and control plots. Three soil drench applications of one phosphonate, Biophos, were particularly promising in terms of tuber blight suppression.
University of Florida certificate in Plant Pest Risk Assessment and Management. R. J. MCGOVERN (1) and N. C. Leppla (2). (1) Plant Medicine Program and Plant Pathology Department; (2) Entomology and Nematology Department, University of Florida, IFAS, CALS, Gainesville, FL 32611.
The University of Florida (UF) established a certificate in Plant Pest Risk Assessment and Management in 2006. This multidisciplinary certificate is coordinated by the UF Plant Medicine Program and applicable to graduate degrees (D.P.M., M.S., and Ph.D.) across departments and colleges. The objective of the certificate program is to provide opportunities for graduate students to develop the personal and professional skills required to effectively lead and conduct plant pest risk assessment and management on local, national, and international levels. Sixteen graduate credit hours are required for the certificate, including a capstone course (Principles of Plant Pest Risk Assessment and Management), a colloquium, elective courses (technical and policy) and internships. A wide variety of internships and careers are available with state, federal, and international plant health regulatory agencies.
Can solarization match methyl bromide fumigation in sites colonized by fungi? R. MCSORLEY (1), K.-H. Wang (1), and S. K. Saha (2). (1) Dept. Entomology and Nematology, (2) Horticultural Sciences Dept., University of Florida, Gainesville, FL 32611.
Two field tests comparing soil solarization and methyl bromide fumigation were affected by extreme weather events. In fall 2003 in Martin Co., both solarization and methyl bromide reduced weeds prior to planting snapdragon (Antirrhinum majus). Heavy rains washed soil from an untreated area into the plots, resulting in plant mortality from soilborne fungi. Losses following solarization (35.5%) were similar to methyl bromide (48.3%), but less (P < 0.05) than an untreated control (67.3%). In fall 2004 in Marion Co., two solarization treatments were compared to methyl bromide fumigation. Two hurricanes passed over the site shortly after planting of pepper (Capsicum annuum) seedlings, and an outbreak of infection by Pythium spp. followed. One month after the second hurricane, plant mortality was lower (P < 0.05) in both solarization treatments (10.3%-12.4%) than methyl bromide (18.8%), and remained so until the end of the experiment. Results of both experiments suggest that solarization may offer some advantage over methyl bromide when pathogens are introduced into treated sites.
Leaf blight in dogwood (Cornus spp.) caused by Phytophthora sp. MARGARET T. MMBAGA and Frank Mrema. Tennessee State University, Otis Floyd Research Center, McMinnville, TN 37110.
Foliage blight caused by Phytophthora parasitica was first reported in container-grown flowering dogwood in Florida in 1987, but Phytophthora leaf blight in dogwood has not previously been reported in Tennessee. In 2002 leaf blight was observed on 7- 8-year-old kousa dogwood (Cornus kousa) and Phytophthora sp. was isolated. The affected plants develop the disease every year and the infection starts from the middle of the plants and not the lower leaves. Thus the perpetuation of the disease appeared to be through infected stem. The infection was restricted to one side of the plant facing away from direct sunlight. Another unidentified Phytophthora species was isolated from the stem of unhealthy flowering dogwood and produced foliage blight that started as leaf blight, die-back and killed 3- 4-month-old seedlings within 12 days. Pathogenicity test on older seedlings (2- 3-year-old) resulted in slower disease progress that consisted of leaf blight and die-back. Identification of the Phytophthora species using morphological features and DNA sequence analysis has indicated that more than one species of Phytophthora may be associated with dogwood.
Epiphytic bacteria from dogwood leaves show potential in powdery mildew bio-control. F. A. MREMA, M. T. Mmbaga, and A. Shi. Tennessee State University, Otis L. Floyd Nursery Research Center, McMinnville, TN 37110.
Fifteen bacterial isolates were isolated from dogwood leaves in the wild and evaluated for biological control of powdery mildew in dogwood seedlings in green house environment. Treatments with fungicide thiophanate methyl (Cleary’s 3336 F®) and sterile water were used for comparison. Two isolates of unidentified bacteria showed high potential in controlling powdery mildew and reduced disease severity to 23.3% and 27.7% as compared to 70% from the water treated control and 19.5% from fungicide treatment. Other four isolates also showed some potential in reducing powdery mildew severity. The bacterium Curtobacterium sp. reduced disease severity to 29.1%; Arthrobacterium sp. (30.6%), Bradyrhizobium sp. (30.5%) and Bacillus sp. (33.3%). These epiphytic bacteria may have provided a buffer against powdery mildew in the wild environment where powdery mildew occurrence was insignificant. Some of the bacteria could be used as bio-control agents against powdery mildew in nursery or landscape plants.
Evaluating GAFP as a genetic determinant for disease tolerance in woody plants. A. K. NAGEL (1), K. D. Cox (1), D. R. Layne (2), and G. Schnabel (1). (1) Dept. Entomology, Soils, and Plant Sciences; (2) Dept. Horticulture, Clemson University, Clemson, SC 29634.
The sustainability of peach production in the Southeast is threatened by a soil-borne fungus, Armillaria tabescens, causing Armillaria root rot (ARR). Rootstocks resistant to ARR are not available, but we are currently working on making existing rootstocks more tolerant to fungal diseases. The Gastrodianin anti-fungal protein (GAFP) allows the medicinal orchid Gastrodia elata to metabolize the hyphae of Armillaria mellea in the cortical cells of the root. GAFP expressing tobacco lines revealed enhanced disease tolerance to various pathogens but the protein has not been investigated in woody plants. Transgenic plum lines were created previously expressing the GAFP protein. Our objective is to assay the transgenic plum lines against the stone fruit pathogens Phytophthora spp., Verticillium dahliae, Sclerotium rolfsii, and Phymatotrichum omnivorum as well as two species of nematode, Meloidogyne incognita and Criconemella xenoplax. This will help to elucidate the spectrum of disease tolerance GAFP may provide.
New host and location reports for Tomato spotted wilt virus (TSWV) and Iris yellow spot virus (IYSV). C. NISCHWITZ, R. D. Gitaitis, A. S. Csinos, and S. W. Mullis. University of Georgia, Coastal Plain Experiment Station, Dept. of Plant Pathology, P.O. Box 748, Tifton, GA 31794.
Tospoviruses in the family Bunyaviridae are very destructive plant pathogens vectored by thrips. TSWV has a very large host range with over 1000 hosts reported worldwide. In contrast, IYSV is an emerging tospovirus, which primarily infects species in the family Liliaceae. In 2005, a survey of symptomless soybeans (Glycine max), a crop commonly double-cropped or rotated with onion and peanut, respectively, was conducted in Georgia. Combined results from DAS-ELISA and RT-PCR were that 6.6% of the soybean plants tested were infected with TSWV. Also in 2005, leeks (Allium porrum) grown near the Georgia - Florida border displaying symptoms of tip dieback (straw bleaching) and necrotic lesions tested positive for TSWV. In 2003, short-day onions (Allium cepa) grown in Peru with necrotic lesions and tip dieback symptoms tested positive with ELISA for IYSV. In 2005, similar appearing onions tested positive for IYSV with both DAS-ELISA and RT-PCR. Soybeans and leeks could potentially serve as green bridges for TSWV to infect peanuts, tobacco and onions.
Incidence of damping-off in soils solarized and planted single or multiple years in South Carolina. S. M. C. NJOROGE (1), J. E. Toler (2), and A. P. Keinath (1). Clemson Univ., (1) Coastal REC, Charleston, SC 29414; (2) Dept. Appl. Econ. & Stat., Clemson, SC 29634.
It is not known if microbial populations and disease development in soils planted immediately after solarization differ from those in soils left fallow and planted after solarization in 2 or 3 successive years. Plots were solarized 10 weeks in 2003 only; 2004 only; 2003 and 2004; 2003, 2004, and 2005; or not solarized. Each year cucumber and broccoli were planted 1 month after solarization ended, or plots were left fallow. Soil was sampled 0 to 10 cm deep for Pythium, Pseudomonas and Rhizoctonia solani at monthly intervals from the start of solarization until 1 month after planting. No differences in microbial populations were found for solarization treatments applied in the current year irrespective of whether the soil was previously solarized, planted, or fallowed. In 2004, damping-off incidence for both crops was low in soil solarized 2 years and planted only in year 2. In 2005, cucumber damping-off was significantly lower in soils planted only after 3 years of solarization than in soil solarized and planted all 3 years.
Single- and multiple-year effects of soil solarization on population density of soilborne microorganisms in South Carolina. S. M. C. NJOROGE (1), J. E. Toler (2), and A. P. Keinath (1). Clemson Univ., (1) Coastal REC, Charleston, SC 29414; (2) Dept. of Appl. Econ. & Stat., Clemson, SC 29634.
Soil solarization can reduce population density of soilborne pathogens, but it is not known how long this reduction persists. A 3-year study was initiated in the summer of 2003 to evaluate 5 soil solarization treatments. Plots were solarized 10 weeks in 2003 only; 2004 only; 2003 and 2004; 2003, 2004, and 2005; or not solarized. Soil was sampled 0 to 10 cm deep at monthly intervals from the start of solarization until 2 months after solarization. Soil dilutions were plated on P(5)ARP to enumerate Pythium and on S1 for flourescent Pseudomonas. Rhizoctonia solani was quantified by placing soil organic matter on EPN(2). In all years, microbial counts were significantly lower throughout the solarization period in solarized soils than in nonsolarized soils. Following solarization, Pythium significantly increased in soils solarized during 1 year only but not in soils solarized during 2 or 3 years; however, R. solani did not significantly increase in solarized soils.
Sporulation of Alternaria alternata, the cause of brown spot of tangerine as affected by lesion age and fungicide application. R. F. Reis (1), A. de Goes (1), S. N. MONDAL (2), and L. W. Timmer (2). (1) São Paulo State University, Jaboticabal, SP 14884, Brazil; (2) University of Florida, Citrus Research and Education Center, Dept. of Plant Pathology, Lake Alfred, FL 33850.
Alternaria alternata, the cause of brown spot of tangerine, produces necrotic lesions on young leaves, twigs and fruit. Inoculum from these lesions plays an important role in overwintering of the pathogen. The effects of lesion age and fungicide application on sporulation of the pathogen were investigated. Sporulation did not occur until 10 days after symptom development on leaves and remained high for the next 20 to 40 days. It was highest on twigs 30 days after lesion formation and minimal sporulation occurred on fruit lesion 70 days after symptom development. Inoculum production on leaf lesions was far higher than on twigs and fruit. A single application of a Q(o)I or copper fungicide was effective in reducing sporulation on leaf lesions for 14 to 21 days. Additional applications did not appear to reduce inoculum further. Fungicide applications are effective in reducing foliage infection and also reduce inoculum development in the grove.
Fungi and bacteria associated with watermelon vine decline in Florida. P. D. ROBERTS (1), S. Adkins (2), and B. D. Bruton (3). (1) University of Florida, 2686 SR 29 N, Immokalee, FL 34142; (2) USDA-ARS, Ft. Pierce, FL 34945; (3) Lane, OK 74555.
Since 2003, a watermelon vine decline of unknown etiology has caused widespread crop loss in South Florida. As fruit approach maturity, the foliage turns yellow, then scorched and brown, followed by collapse of the entire vine. A uniform tan to light brown discoloration of the xylem was observed. Although there are no external symptoms, fruit frequently exhibited greasy-brown blotches in the rind. Standard isolation techniques yielded numerous fungi and bacteria from crown, root, and fruit tissue. The fungi most frequently isolated were: Fusarium oxysporum, F. semitectum, Plectosporium tabacinum, Rhizoctonia solani, Pythium spp. and Didymella bryoniae. P. tabacinum was often isolated from the vascular tissue 10–30 cm above the crown. Race 0, 1, and 2 of Fusarium oxysporum f. sp. niveum are often isolated from symptomatic plants, but vine decline symptoms are distinct from Fusarium wilt. Some bacterial strains caused tissue maceration and/or necrosis or brown blotches similar to field symptoms on fruit. The role of fungi, bacteria, viruses and other factors continue to be investigated.
Asian soybean rust: Syngenta spore trap project. J. C. Rupe (1), M. D. Wiglesworth (2), S. Smith (1), and S. Wickizer (1). (1) University of Arkansas, Fayetteville, AR; (2) Syngenta Crop Protection, Greensboro, NC.
Early detection of Asian soybean rust (ASBR) (Phakopsora pachyrhizi) is essential for optimum control with fungicides. One method of early detection is the use of sentinel plots, soybean plots planted before the main crop. Another method used in Paraguay is spore trapping, where ASBR-like spores can be detected up to 10 days before disease appears. To test this method in the US, 96 spore traps were placed in 13 states from WI to GA in the summer of 2005. Spores were caught on glass slides coated with petroleum jelly and exposed in the field for one week. The slides were sent to the University of Arkansas. Spores were identified as ASBR-like spores based on size, shape, pigmentation, and surface ornamentation from pictures and killed urediniospores of P. pachyrhizi. ASBR-like spores were found in each of the 13 states and in 66 of the 96 spore traps. ASBR did not develop at most sites. However, at the 12 sites in GA and AL where the disease did develop, ASBR-like spores were found 5 to 77 days before disease was found at the site (average 30 days), except at one site in GA where disease developed 31 days before spores were found.
Sentinel plot and commercial field monitoring of Asian soybean rust in Georgia in 2005. L. E. SCONYERS (1), R. C. Kemerait (1), D. V. Phillips (2), and P. H. Jost (3). Dept. of Plant Pathology, University of Georgia, (1) Tifton, GA 31793, (2) Griffin, GA 30223; (3) Dept. of Crop and Soil Science, University of Georgia, Statesboro, GA 30460.
In 2005, 3 clover, 5 kudzu and 17 soybean sentinel plots were established for monitoring Asian soybean rust Phakopsora pachyrhizi (SBR) development. For each 15.2 m(^2) soybean sentinel plot, maturity groups II, III and IV were planted on one planting date, ranging from the first to the third week of April. On a weekly basis, SBR severity (low, moderate or heavy), incidence (number of leaflets infected out of 100 leaflets collected at random within the plot), and growth stage were recorded for soybean sentinels. Of the 25 sentinel plots, SBR developed in 1 kudzu plot and 13 soybean plots, while no SBR developed in clover plots. SBR initially developed in discrete foci within soybean sentinel plots and was typically identified first in the lower canopy after bloom at about R4 to R5 reproductive stages. By 3 Nov, SBR was confirmed in 33 counties. Observations from this season will be used to construct disease forecast models as well as provide information for management strategies for 2006.
Characterization of watermelon (Citrullus lanatus var. (citroides) germplasm for resistance to root-knot nematodes. J. A. THIES and A. Levi. U.S. Vegetable Laboratory, USDA, ARS, Charleston, SC 29414.
Root-knot nematodes (Meloidogyne incognita, M. arenaria, and M. javanica) cause extensive damage to watermelon and resistance to these pests has not been identified in any watermelon cultivar. In greenhouse tests, we evaluated 265 U.S. Plant Introductions (PIs) for nematode resistance (based on root galling and nematode reproduction), and identified twenty-two PIs of Citrullus lanatus var. citroides as moderately resistant to M. arenaria race 1. In this study, these 22 PIs exhibited low to moderate resistance to M. incognita race 3, M. javanica, and M. arenaria race 2. Three watermelon cultivars (C. lanatus var. lanatus), three C. colocynthis PIs, and four C. lanatus var. citroides PIs, all previously shown to be susceptible to M. arenaria race 1, also were susceptible to M. incognita race 3, M. javanica, and M. arenaria race 2. The C. lanatus var. citroides PIs that are most resistant to all three Meloidogyne spp. should be useful sources of resistance for developing root-knot nematode resistant watermelon cultivars.
Root‑knot nematode resistance in African pearl millets. P. Timper and J. P. WILSON. USDA‑ARS, Tifton, GA 31793.
Resistance to Meloidogyne incognita in pearl millet reduces nematode populations that can damage crops grown in rotations. Pearl millets from Africa were evaluated as sources of resistance. Seventeen pearl millets were evaluated as bulk (S0) populations. All African varieties expressed some level of resistance. P3Kollo was among the least resistant, Zongo and Gwagwa were intermediate, and SoSat‑C88 was among the most resistant. Thirty selfed (S1) progenies from SoSat‑C88, Gwagwa, Zongo, and P3Kollo were evaluated for heterogeneity of resistance and reactions were verified in 13 S2 progeny. In S1 evaluations, variety was heterogeneous for resistance. Patterns of apparent segregation of resistance varied among the four varieties. Discreet resistant and susceptible phenotypes were identified in Zongo. We estimate two dominant genes for resistance segregated in this variety. Reproduction of M. incognita on S2 progeny tended to confirm the results from S1 progeny inoculations. Heritability of nematode reproduction by parent‑offspring regression was 0.54. Realized heritability by divergent selection was 0.87.
Variables affecting linear gall growth in fusiform rust-infected loblolly and slash pines. C. H. WALKINSHAW. USDA Forest Service, 2500 Shreveport Highway, Pineville, LA 71360.
Vertical length of fusiform rust galls in stem tissues of loblolly and slash pine is an indicator of the extent and severity of the host-parasite interaction. The objective of this study is to determine the effect of a number of observed variables on the length of galls in rust-infected seedlings and saplings both in the greenhouse and in field plantings. These variables were: pine family, growth of infected pine families, source of basidiospore inoculum in the greenhouse, and density of inoculum. Within a six-month period after inoculation, the majority of galls grew 5 to 9 mm in length in the greenhouse and 2 to 8 cm in field plantings. Frequency distribution graphs of greenhouse inoculated loblolly and slash are normal for gall lengths except for bimodal distributions for select pine families. Regression analysis indicated a significant relationship between length of galls and the variables of pine family and source of inoculum. Means for length of galls in greenhouse inoculations had low standard deviations.
Pathogenicity of two soil-borne fungi from commercial snapdragon range in Florida. K.-H. WANG (1), E. Malek (2), and R. McSorley (1). (1) Entomology and Nematology Dept.; (2) Doctor of Plant Medicine Program, University of Florida, Gainesville, FL 32611.
Fusarium oxysporum (F) and Pythium aphanidermatum (P) were frequently isolated from commercial snapdragons (Antirrhinum majus) exhibiting severe stem rot and dieback in Stuart, Florida in 2003. Koch’s postulates were used to test the pathogenicity of these fungi in two greenhouse trials. Snapdragon seedlings were inoculated with F, P, F+P, or were treated with culture media of each fungal treatment. A negative control was also included. In Experiment I, treatments with Pythium (P and P+F) reduced (P < 0.05) plant height, shoot and root weights, and number of flowers as compared to the controls with and without fungal culture media. Slight reduction of plant growth by F was not different from the control with its medium, potato dextrose broth (PDB). A root damage index confirmed that the root damage was due to the presence of P. When volume of the PDB that carried F was reduced in Experiment II, pathogenicity of F was weaker than P. Although stem rot was not observed in greenhouse trials, consistent dieback and root damage from P was observed in both experiments. Pathogenicity of an Pythium aphanidermatum isolate on snapdragon was confirmed in this study.
Microtiter assay shows effectiveness of a natural fungicide for control of Colletotrichum spp. DAVID E. WEDGE (1) and Barbara J. Smith (2). (1) USDA-ARS, Natural Products Utilization Research Unit, University, MS 38677; (2) USDA-ARS Small Fruit Research Station, Poplarville, MS 39470.
Anthracnose diseases of strawberry are serious problems for fruit and plant production worldwide. New approaches to anthracnose disease control are necessary as the effectiveness and availability of commercial fungicides decreases. A micro-dilution assay was used to evaluate sensitivity profiles of Colletotrichum spp. isolates from strawberry. Sixteen agrochemicals were tested for in vitro activity against eight C. acutatum isolates, two C. fragariae isolates, and two C. gloeosporioides isolates. The most effective fungicides were azoxystrobin, cyprodinil, and chlorothalonil and provided nearly 100% growth inhibition of all ten Colletotrichum sp. isolates at 48 hr. An acaricide, quinomethionate, and the fungicides, captan and thiram, also provided near 100% growth inhibition. All 12 Colletotrichum isolates were insensitive to vinclozolin and iprodione. In addition the eight C. acutatum isolates were insensitive to benomyl and thiabendazole.
Development of a bioassay to quantify fungicide residues on peanut foliage. J. E. WOODWARD and T. B. Brenneman. Dept. of Plant Pathology, Univ. of Georgia, Tifton, GA 31793.
Fungicides used to manage soilborne diseases are generally applied to peanut (Arachis hypogaea) foliage; therefore, understanding fungicide deposition and retention is important in maximizing disease control. A bioassay using Sclerotium rolfsii was developed to quantify fungicide residues on peanut stems and leaflets. Tissues from the upper, middle and lower canopy were treated by dipping them in serial dilutions of azoxystrobin, flutolanil or tebuconazole. Terminal stems and leaflets were more susceptible to colonization than were basal stems and leaflets. Label rates (1.0x) of each fungicide in 187 liters of water ha(^-1) prevented lesion development for tebuconazole and flutolanil, but not azoxystrobin. The 0.01x rate of tebuconazole, flutolanil, and azoxystrobin resulted in 100, 93 and 89% control on leaflets, and 96, 100 and 78% control on stems, respectively in terminal tissues. The % control on basal tissues was 94, 92, and 84 for leaflets and 100, 100, and 97 for stems, respectively. Results from the two assays were correlated (R(^2) = 0.71, P < 0.01). Regression analysis of rates 1.0 to 0.001x will be used to quantify fungicide residues in other studies.
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