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1999 Pacific Division Meeting Abstracts

June 15-16, 1999 - Riverside, California

Posted online May 20, 1999

Enhanced resistance against fungal diseases by expression of ATLP1 in transgenic potato plants. G. S. Ali (1), B. D. AMBRUZS (2), J. P. Hill (2), H. F. Schwartz (2), and A. S. N. Reddy (1). (1) Dept. of Biology, Colorado State University, Fort Collins, CO 80523; (2) Dept. of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, CO 80523. Publication no. P-1999-0001-PCA.

thaumatin-like protein 1 (ATLP1), a pathogenesis-related protein induced in response to pathogen attack, was constitutively expressed in Solanum tuberosum cv. Desiree. Elevated expression levels of the transgene were confirmed by northern and western blot analyses. Detached leaflets from four independent transgenic lines with high-level expression of ATLP1 were assayed for resistance against white mold (Sclerotinia sclerotiorum), early blight (Alternaria solani), and late blight (Phytophthora infestans). There were no significant differences among any of the lines when inoculated with S. sclerotiorum. The rate of lesion development was significantly reduced (p=0.05) in one of the transgenic lines, compared to the non-transgenic control line, when inoculated with A. solani. Non-specific resistance to P. infestans was evaluated by measuring sporulation indices (SI), infection efficiencies (IE), lesion sizes (LS) and sporulation capacities (SC). All these components of resistance were significantly reduced in the ATLP1, transgenic lines as compared to an antisense control line. These results demonstrate that the constitutive expression of a heterologous ATLP in potato confers enhanced resistance against the early and late blight pathogens.

Pasteuria penetrans
: an infective parasite on root-knot nematodes after twenty-one years in dry soil.
S. BEKAL, R. Mankau, and J. O. Becker. Department of Nematology, University of California, Riverside, CA 92521. Publication no. P-1999-0002-PCA.

Several characteristics of the endoparasite Pasteuria penetrans suggest its potential as a biological control agent against plant-parasitic nematodes. These attributes include high parasitic efficacy, host-specificity and the bacterial endospores' capability to tolerate extended periods of environmental extremes. The current study focused on its ability to survive and remain infective after long-term storage. A dried soil from Senegal containing spores of P. penetrans was stored in a closed can at room temperatures for 21 years. The soil was moistened and inoculated with second-stage juveniles of Meloidogyne incognita. Numerous spores of P. penetrans attached to the cuticle of the observed nematodes. Root-knot nematode-susceptible tomato seedlings were planted into the soil and were incubated in a growth chamber at 26C with a 14/8 day/night cycle. Observation of infested nematodes indicated that the life cycle of the bacterial parasite was completed within eight weeks and produced endospore-filled female nematodes.

First report of High Plains Disease in Washington corn (Zea mays).
E. M. BENTLEY (1) and K. C. Eastwell (2). (1) WSU-Cooperative Extension and (2) Dept Plant Pathology, Washington State University, Prosser, WA 99350. Publication no. P-1999-0003-PCA.

Sweet corn samples exhibiting "spot" type mosaic, yellow streaking and severe stunting were observed in three eastern Washington counties in 1998. Older leaves displayed extensive tip burn and severe scorch. Microscopic inspection revealed eriophyid mites within the leaf whorls. All fields were downwind and/or adjacent to dryland wheat. These symptoms and observations led to a preliminary diagnosis of High Plains Disease (HPD) caused by the High Plains Virus (HPV). Antigen trapped indirect enzyme-linked immunosorbant assays (ELISA) were conducted on composite samples. HPV was confirmed in sweet corn cultivars, 'Golden Jubilee', 'Peaches 'n Cream' and 'Lyric SE'. A fourth location (dent corn) was diagnosed by Stan Jensen (USDA-ARS, Department of Plant Pathology, University of Nebraska, Lincoln, NE 68583). This is the first report of High Plains Disease in Washington.

Epidemiology of Alternaria fruit rot of highbush blueberry in the Pacific Northwest.
P. R. BRISTOW and G. E. Windom. Washington State University, 7612 Pioneer Way E., Puyallup, WA 98371-4998. Publication no. P-1999-0004-PCA.

A Burkard volumetric spore trap was used to collect airborne spores in a commercial highbush blueberry (Vaccinium corymbosum L.) field from April through August in 1995 and 1996. Spores of the Alternaria spp. associated with fruit rot were first trapped in mid May. Their numbers remained low until early June, then rose. A few conidia were still detected in August. Spore catch was associated with periods of precipitation. To determine when infection occurred, potted blueberry plants were exposed to natural inoculum in the field for different periods between April and July 1996. Most berries became infected between mid-June and mid-July. The likelihood of flower infections leading to fruit rot was low because bloom occurred before many conidia were produced. Fungi were isolated from symptomless green and ripe berries of the six cultivars. For green berries, fungal recovery ranged from 5 to 17%, depending on the cultivar. Alternaria was obtained from 50 to 77% of the green berries yielding a fungus. For ripe berries, fungal recovery was 5 to 85%. The proportion represented by Alternaria, exceeded 93 and 70% for all six cultivars in 1995 and 1996, respectively. Protecting berries as they develop and ripen with effective fungicides may control this fruit disease.

Factors affecting the effectiveness of fungicides in controlling interior needle blight syndrome on noble and grand fir Christmas trees.
G. A CHASTAGNER. Washington State University, Puyallup, WA 98390. Publication no. P-1999-0005-PCA.

Interior needle blight (INB) syndrome which is associated with an undescribed Mycosphaerella sp. can severely damage noble and grand fir Christmas tree in the Pacific Northwest. The effectiveness of fungicides in controlling INB was determined by exposing potted healthy grand fir seedlings that had been treated with different fungicides to infected trees in the field. The influence of application timing was examined by spraying noble fir trees with chlorothalonil at 4.9 g ai/l one or two times each spring from 1993 through 1996. INB was rated on a scale of 0 to 10 and ratings on the untreated grand fir seedlings average 6.6. Chlorothalonil at 2.5 and 4.9 g ai/l reduced disease ratings to 0.7 and 0.9. Benomyl at 1.2 g ai/l was partially effective, reducing ratings to 2.9. Applications of dodine, triadimefon, mancozeb, thiophanate-methyl, myclobutanil, and azoxystrobin were ineffective. Optimal disease control with chlorothalonil was obtained when the initial application was applied when new growth was 2 to 5 cm long followed by a second application about 4 weeks later.

Virus resistant potato germplasm.
D. L. CORSINI (1), J. J. Pavek (1), and C. R. Brown (2). (1) USDA-ARS, PO Box AA, Aberdeen, ID 83210; (2) USDA-ARS, Irrigated Ag R&E Center, 24106 N. Bunn Rd., Prosser, WA 99350-9687. Publication no. P-1999-0006-PCA.

The USDA potato breeding program for the Pacific Northwest has developed a collection of Solanum tuberosum genotypes that have combined resistance to potato viruses X (PVX), Y (PVY), and leafroll (PLRV). Resistance has been derived from S. andigena, S. acaule, S. demissum, and S. stoloniferum. Parental germplasm was obtained from the neotuberosum program, Cornell University, The Max Planck Institute, Germany, via the NRSP-6 Collection, and the former Polish Institute for Potato Research. A new source of PLRV resistance from S. chacoense was recently identified and is being introgressed into cultivated germplasm. Breeding selections A88597-7, A88617-6, and A88625-10 have shown no tuberborne infection with PVX, PVY, or PLRV in six years of continued field testing at Kimberly, ID, and at Prosser, WA, whereas Russet Burbank has averaged 68% PVX, 53% PVY, and 81% PLRV. Breeding to combine resistance to important viruses along with resistance to late blight into commercially accepted cultivars is in progress.

Verticillium wilt of peppermint: Interplay of varieties and fungal inoculum.
FRED CROWE (1,2), Robin Parks (1,2), and Neysa Farris (2). (1) Dept. Bot. & Pl. Pathology, Oregon St. Univ., Corvallis, OR 97331; (2) Central Oregon Agr. Res. Center, Madras, OR 97741. Publication no. P-1999-0007-PCA.

In peppermint, gamma-irradiated variants (GIVs) of the susceptible variety Black Mitcham (BM) were released as partially resistant to Verticillium dahliae (VD), based on differential wilt incidence and stand longevity, but without considering inoculum density changes. In our uniform, conidial root-dip greenhouse inoculations, BM and GIVs wilted equally. BM and five GIVs were planted into a replicated, randomized block field trial uniformly infested with VD. In yr 1, all varieties manifested similar, moderate wilt. In yr 1, plots were post-harvest propane flamed and were not tilled in the fall, practices expected to keep wilt from increasing. Wilt incidence did not increase in yr 2. Plots were not flamed and were tilled in yrs 2 and 3, practices expected to increase wilt. In yrs 3 and 4, wilt increased greatly in BM but only slightly in the GIVs, yield was reduced in BM vs the GIVs, and ID of VD from soil in BM plots was 3 to 5 times higher than in plots with GIVs. Differences in wilt and stand longevity between BM and GIVs may reflect changes in ID more than resistance to infection.

Control of Verticillium wilt of the Russet Burbank potato with corn and barley.
J. R. DAVIS (1), O. C. Huisman (2), D. O. Everson (3), L. H. Sorensen (1), and A. T. Schneider (1). (1) University of Idaho, Dept. of PSES, Aberdeen R&E Center, Aberdeen, ID 83210; (2) University of California, Dept. of Environmental Science, Policy, and Management, Berkeley, CA 94720-3112; (3) University of Idaho, Division of Statistics, Moscow, ID 83844-1104. Publication no. P-1999-0008-PCA.

A 6-year field study to determine the effects of green manure crops on Verticillium wilt of potato was initiated in 1992 and concluded in 1997. This study showed Verticillium wilt, caused by Verticillium dahliae, to be controlled when potato crop was preceded by two consecutive years of green manures of either Jubilee sweet corn or Jubilee super-sweet corn. When potato was grown for two consecutive years (1994 and 1995), beneficial effects from green manures were minimal. Following green manure treatments of either sweet corn or barley in 1996, control of Verticillium wilt on Russet Burbank was restored in 1997 even though inoculum densities of V. dahliae had increased by 2-4 fold. With this control, yields were significantly increased by 22%, 31%, and 29% for total yields, U.S. 1's, and tubers >280 g, respectively.

Pathogenic Saprolegniaceae: Developmental pathways and physiological characteristics related to virulence.
J. DIEGUEZ-URIBEONDO (1), and K. Soderhall (2). (1) Dept. Plant Path., Univ. of California, Riverside, CA 92521; (2) Dept. Physiol. Mycol. Univ. of Uppsala, Sweden. Publication no. P-1999-0009-PCA.

are zoosporic fungi, some of which are pathogens that cause important losses of crop plants, e.g. peas, beets and radishes, or fishfarms, e.g., salmonids, crayfish, etc. The zoospore stage constitutes the main infective unit in these fungi. Two alternative pathways can occur upon encystment of the zoospores: cyst-germination or cyst-releasing a new zoospore. This last pathway appears to be common within the pathogenic species. We developed a protocol to study this pathway and we found that induction of germination can only be achieved during a limited period of time after encystment. We also found that the number of cyst-zoospore generations is limited and seems to depend on internal sources or pre-synthesized precursors in the spores. Finally, we studied the zoospore physiology of several strains of two animal pathogenic species and found that they exhibited different degrees of virulence at different temperatures. These differences appear to be related to adaptations of the pathogens to the habitat of their host.

The effect of carbon source on the growth of Phytophthora citrophthora and P. parasitica in synthetic media.
M. F. DIRAC (1) and J. A. Menge (2). (1,2) Dept. Plant Pathology, University of California, Riverside, CA 92521. Publication no. P-1999-0010-PCA.

In citrus growing regions with a Mediterranean climate, Phytophthora infection of feeder roots follows a distinct pattern. P. citrophthora can be isolated in the winter, occasionally in the summer after heavy rains. P. parasitica infection occurs in the summer and fall, but is absent in winter. The primary cause of this effect is not temperature alone, as summer soils are rarely too hot for P. citrophthora. The latter fungus can be isolated in mid-summer from alternative hosts planted in infected citrus groves. Citrus feeder roots are high in starches and low in sugars in the winter. The reverse is true in the summer. Both fungi can grow in synthetic media with either glucose or amylopectin as their sole carbon source at all temperatures at which they are normally viable, but they generally grow better with a combination of the two carbon sources. Thus seasonal infection is unlikely to be caused by this change in the form of stored carbon. Both fungi can also produce viable sporangia when grown in synthetic media using either amylopectin or glucose as the carbon source. A phytoalexin secreted by intact feeder roots is another possible mechanism which may be responsible for the seasonal infection of citrus by P. citrophthora in southern California as this effect was not observed on excised roots infected by zoospores in the laboratory.

Occurrence of mat-1 and mat-2 isolates of Tapesia yallundae, cause of eyespot of wheat in the U.S. Pacific Northwest.
G. W. DOUHAN and T. D. Murray. Dept. Plant Pathology, Washington State University, Pullman, WA 99164. Publication no. P-1999-0011-PCA.

Eyespot of wheat is caused by Pseudocercosporella herpotrichoides. The teleomorph of this fungus, Tapesia yallundae, has been found in the Northern and Southern hemispheres but has yet to be found in the U. S. The objective of this study was to determine if both mating types of the pathogen occur in the U.S. Pacific Northwest (PNW). In vitro crosses were made between PNW isolates of P. herpotrichoides and known mat-1 and mat-2 isolates from the U.K. Seven crosses out of 22 produced apothecia with viable ascospores. Two PNW isolates were mat-1 and five were mat-2. Subsequent crosses between PNW isolates of opposite mating type resulted in apothecia with viable ascospores. The isolates used in these crosses were collected over a wide geographical area of the PNW and included benomyl-resistant and -sensitive isolates. The fact that both mating types are present in the PNW and can successfully mate in the laboratory demonstrates the potential for the sexual cycle to occur in PNW wheat fields. The potential impact of sexual reproduction on the disease cycle and disease control is unknown.

Diversity of citrus tristeza virus in field isolates from Central Valley, California.
P. EVANS and J. A. Dodds. Dept. Plant Pathology, University of California, Riverside, CA 92521. Publication no. P-1999-0012-PCA.

CTV isolates collected from the Central Valley were subcultured into seedlings at UC Riverside for future study and comparison. A host range, using 5 standard citrus varieties demonstrated a wide range of symptoms including stunting, seedling yellows, stem pitting and vein corking. When double-stranded RNA profiles were compared, most isolates had patterns typical of mild California strains, however several contained a low m.w. (0.5 × 10/6) band typically associated with more severe, stem pitting isolates. All isolates were readily amplified by RT-PCR and when the products were analyzed subsequently by Single-stranded Conformational Polymorphisms (SSCP) some conformational differences were found. RNase Protection Assays (RPA) detected many similarities as well as great heterogeneity between the isolates. RPA patterns appear to be able to predict biology in some cases. When CTV isolates were transmitted using the aphid vector Aphis gossypii, the probability of single aphid transmission ranged from 0.44% to 3.8%. This study indicates that while most field isolates of CTV in California's Central Valley are mild, some more severe aphid transmissible isolates do exist which may represent a threat to the citrus industry.

Identification of mating-regulated cDNAs in Phytophthora infestans.
A.-L. FABRITIUS and H. S. Judelson. Dept. Plant Pathology, University of California, Riverside, CA 92521, USA. Publication no. P-1999-0013-PCA.

Sexual development in the heterothallic oomycete, Phytophthora infestans, is a complex process involving the differentiation of male and female gametangia, meiosis, fertilization, and formation of a sexual spore (oospore). To identify genes that are differentially expressed during sexual development, cDNA libraries were constructed from mating and non-mating cultures, using a suppression subtractive hybridization technique. In preliminary screening, three genes that are up-regulated and two that are down-regulated during mating were identified; additional clones are currently being characterized. Genetic mapping of one cDNA suggested that the corresponding gene is linked to the mating type. Also, two cDNAs were cloned that were expressed in either A1 or A2 mating type strains used in this study, but were not present in their genomic DNA. The origin of these cDNAs may be extrachromosomal and is currently being investigated. Sequencing of differentially expressed cDNAs indicated that the product of one down-regulated cDNA is very similar to sorbitol dehydrogenases. All other cDNA clones may represent novel genes since no significant matches were found in databases. To study the structure and function of the corresponding genes, genomic clones were isolated from a bacterial artificial chromosome (BAC) library.

A resurgent incidence of pre- and postharvest tear stain of grapefruit and orange fruit caused by Colletotrichum gloeosporioides in California.
H. FÖRSTER (1), P. A. Mauk (2), J. A. Menge (1), and J. E. Adaskaveg (1). Dept. of Plant Pathology, (1) University of California, and (2) UCCE, Riverside, CA 92521. Publication no. P-1999-0014-PCA.

Tear stain of citrus fruit is a minor disease worldwide with only few reports since the first half of the century. The disease has been attributed to endemic populations of C. gloeosporioides. In 1997 and 1998, an epidemic developed in California citrus that caused significant postharvest crop losses from off-graded and decayed fruit. Fruit symptoms on Marsh White grapefruit and navel or Valencia oranges included: staining, pitting, and decay. Tear staining was the result of melanized appressoria developing on fruit along "drip-lines" from water/plant exudates. Because of recent changes in species concepts within Colletotrichum, morphological and molecular comparisons were made with isolates obtained from symptomatic fruit. These isolates matched reference collections of C. gloeosporioides from citrus and papaya in conidial morphology, temperature relationships, and in molecular comparisons using species-specific and random primers in PCR DNA amplifications. Fungicide trials indicated that a single (1000 g ai/ha) or two (500 g ai/ha) applications of benomyl, or a single application of azoxystrobin (200 g ai/ha) after fruit coloring significantly reduced the disease on grapefruit at harvest. Because benomyl is registered in Florida, a label amendment for California has been obtained for the 1998 and 1999 seasons.

Field evaluation of three biological seed treatments for control of Fusarium oxysporium f. sp. betae in sugar beet.
F. A. GRAY, D. W. Koch and J. W. Flake. Dept. of Plant Sciences, University Wyoming, Laramie, WY 82071. Publication no. P-1999-0015-PCA.

A sugar beet seed treatment trial was established in Worland, WY on April 24, 1998. Three biologicals, either alone or in combination with the standard Apron (metalaxyl) + Thiram (tetramethylthiuram disulfide) fungicide treatment were compared. The biologicals were Kodiak (Bacillus subtilis), Gustafson; Mycostop (Streptomyces griseovirdis), Kemira; and T­22 (Trichoderma harzianum), BioWorks, Inc. The test site had a known infestation of Fusarium oxysporium f. sp. betae. Symptoms of Fusarium Yellows (FY) were first noticed in mid­May and seedling stands were higher (P<0.05) where one or more biologicals were used, compared to the standard fungicide treatment or untreated check. The highest seedling stand was 5.4 plants/m row with the combination of all three biologicals compared to 3.1 plants/m row for the standard Apron + Thiram treatment. The percent plants of with FY in mid­July varied from 20 for Mycostop and Kodiak to 32 for Apron + Thiram and the untreated control. Sugar beet yields were highest (P<0.05) for Mycostop (50.2 Mg ha(^-1)) and lowest for Apron + Thiram (30.9 Mg ha(^-1)).

Screening winter wheat for resistance to speckled snow mold.
J. L. HENRIQUEZ and T. D. Murray. Dept. of Plant Pathology, Washington State University, Pullman, WA 99164. Publication no. P-1999-0016-PCA.

Host resistance is the primary control measure for speckled snow mold caused by Typhula spp. An effective and reliable test for resistance under controlled environment conditions is desired to improve cultivar development. The objective of this research was to determine the optimum environmental conditions to identify resistant lines. 'Daws' (susceptible) and 'Sprague' (resistant) winter wheats were grown at 15-20 C for 4 wk and then at 2 or 4 C for 1, 2 or 4 wk to cold-harden them. Plants were inoculated with sclerotia, covered with moist cotton and crushed ice and incubated in darkness at 0 C. Mycelium colonized all above ground tissues and abundant sclerotia were present after 105 days. Number of tillers per plant was a more useful indicator of recovery after snow mold than dry weight or number of plants. The greatest difference in recovery between cultivars occurred following 4 wk of cold hardening at 2 C, where Sprague and Daws had 10.5 and 0.5 tillers, respectively. No significant difference was detected when they were hardened at 4 C. Further research on plant age at cold hardening, inoculum density, and soil type are needed to optimize this screening procedure.

Evaluation of disease management strategies for control of Phytophthora crown rot of summer squash.
G. J. HOLMES (1), M. E. Lancaster (2), and P. B. Shoemaker (1). (1) Dept. Plant Pathology, NC State Univ., Raleigh, NC 27695; (2) N.C. Coop. Ext. Service, Hendersonville, NC 28792. Publication no. P-1999-0017-PCA.

Phytophthora crown rot (PCR) caused by Phytophthora capsici is one of the most destructive cucurbit diseases in North Carolina and the eastern United States. Statewide incidence and severity of the disease appears to have increased during the last 5 to 10 years. Both chemical and cultural disease management have been unsuccessful. In 1998, three trials were conducted to test the efficacy of several fungicides and a biological control agent. Fungicides included mefenoxam, dimethomorph, mancozeb, propamocarb hydrochloride, fosetyl-Al, and azoxystrobin. The biological control was a seed treatment of Gliocladium virens and Burkholderia cepacia. Disease pressure was high in all tests, but the time and type of symptom development varied. The range of plant mortality for the three trials was 61 to 100%, 22 to 75%, and 74 to 96%. While treatment effects were significant, none of the treatments provided commercially acceptable disease control. Mefenoxam treatments consistently yielded the highest level of disease control.

Synergy of soil fumigant combinations for control of fungal pathogens.
C. M. HUTCHINSON (1), M. E. McGiffen, Jr. (1), H. D. Ohr (2), J. J. Sims (2), and J. O. Becker (3). (1) Departments of Botany and Plant Sciences, (2) Plant Pathology, and (3) Nematology, University of California, Riverside, CA 92521. Publication no. P-1999-0018-PCA.

Efficacy of soil fumigation with combinations of methyl bromide (MB), methyl iodide (MI), and chloropicrin (PIC) against Fusarium oxysporum was determined. F. oxysporum was grown on autoclaved millet seed supplemented with 1/4 strength clarified V-8 broth. Individual soil samples containing F. oxysporum infested millet seed were treated for 48 hr with MB or MI at 0.0, 3.1, 6.3, 12.5, 25, 50, 75, 100, 200, 400, or 800 micro-moles in fumigation chambers. In the synergy experiments, MB and MI were combined with 17 percent PIC. Millet seed were plated on potato dextrose agar and the percent mortality was recorded after 48 hr. The relative potency of MB to MI was 1.4. MB/PIC and MI/PIC applied jointly were 2.2 and 2.8 times more efficacious, respectively, than when the compounds were applied individually. Combining MB and MI with chloropicrin resulted in a significant synergistic increase in activity of the fumigants against F. oxysporum.

Microbe-mediated germination of ascospores of Monosporascus cannonballus.
D. H. KIM, M. E. Stanghellini, and M. Waugh. Dept. of Plant Pathology, University of California, Riverside, CA 92521. Publication no. P-1999-0019-PCA.

Ascospores of Monosporascus cannonballus germinate in the rhizosphere of cantaloupe plants growing in field soil. However, little or no ascospore germination occurs in axenic culture or the rhizosphere of cantaloupe plants growing in field soil that was autoclaved prior to infestation with ascospores. The latter observation suggests that root exudates alone do not stimulate ascospore germination. Amending field soil with streptomycin (which inhibits gram negative microorganisms) did not suppress ascospore germination in the rhizosphere of cantaloupe plants. However, amending the soil with penicillin (which inhibits gram positive microorganisms) did suppress ascospore germination. These results indicate that the gram positive microorganisms may be involved, either directly or indirectly, in the induction of ascospore germination. Identification of the specific group(s) of gram positive microorganisms, as well as the mechanism (s), associated with the induction of ascospore germination is currently under investigation.

Temperature and water vapor pressure describe lesion expansion of powdery mildew on strawberry.
T. C. MILLER (1) and W. D. Gubler (1). (1) Dept. Plant Pathology, University of California, Davis, CA 95616. Publication no. P-1999-0020-PCA.

Knowledge of pathogen growth response to variables of weather will improve application of measures to control Sphaerotheca macularis f. sp. fragariae in California strawberry fruit production fields. To define rate of lesion growth, this obligate pathogen-host disease interaction was subjected to controlled environments of constant temperatures and constant relative humidities spanning the ranges of these variables observed in their natural environment. Temperature and water vapor pressure offered the most predictive value in mildew pathogen growth response, followed closely by water vapor pressure deficit. Relative humidity did not correlate with pathogen growth, although a trend toward better growth at higher relative humidities was observed at most temperatures. Sporulation was observed at all temperatures (5-30 degrees Celsius) and relative humidities (32-100 percent) tested, but was delayed when outside optimal conditions. Disease progress curves constructed from commercial field site sampling is compared with results from controlled experiments.

Occurrence of bacterial spot of plum in Idaho and Oregon.
S. K. MOHAN and V. P. Bijman. University of Idaho, Research and Extension Center, Parma, ID 83660. Publication no. P-1999-0021-PCA.

Symptoms of leaf spots, fruit spots and twig cankers were observed in Japanese plum (Prunus salicina 'Friar') orchards in Southwest Idaho and Eastern Oregon. Leaf spots were angular and water-soaked, later becoming necrotic and brown. Spots on immature fruit began as water-soaked, circular areas that later turned black, became sunken, and developed cracks. Twig infections started as greasy, dark green, elongated areas that progressed to sunken cankers. Microscopic examination revealed abundant bacterial streaming from the symptomatic tissues. Isolations on nutrient agar consistently yielded lemon-yellow, translucent, mucoid colonies. Based on results of standard bacteriological tests, Biolog microbial identification system, cellular fatty acid profile and pathogenicity, the bacterium was identified as Xanthomonas arboricola pv. pruni. Frequent wetting of tree canopies during irrigation by under-tree sprinklers apparently facilitated infection and disease development. The disease was not observed in other cultivars of Japanese or European plums growing in the vicinity. This is the first report of this pathogen from the Pacific Northwest.

Epidemiological evidence for spread of Eutypa dieback by asexual means.
G. MUNKVOLD (1) and G. Hughes (2). (1) Dept. of Plant Pathology, Iowa State Univ., Ames, 50011; (2) Inst. of Ecology and Res. Mngmnt., Univ. of Edinburgh, EH9 3JG, Scotland UK. Publication no. P-1999-0022-PCA.

Ascospores are believed to be the sole means of dispersal for Eutypa lata, but Eutypa dieback is severe in some California vineyards in areas too arid for ascospore production. Explanations include long-distance ascospore dispersal from coastal areas and local ascospore production in irrigated Central Valley orchards and vineyards. Eutypa-affected vines were surveyed for 3 years in 8 locations and analyzed for nonrandom patterns suggestive of contagion. Methods evaluating spatial dependence of disease incidence among 9-vine quadrats (geostatistics, spatial autocorrelation) indicated nonrandom patterns only in vineyards with perithecia. However, analyses evaluating within-quadrat spatial dependence (beta-binomial and LNB distributions) indicated nonrandom patterns in all vineyards and stronger aggregation in vineyards lacking perithecia. These results are consistent with two modes of disease spread operating at different scales: ascospore dispersal over longer distances and disease spread by asexual means (possibly pycnidiospores) over very short distances.

Fungicides for the control of Botrytis (Botrytis cinerea) and Anthracnose (Colletotrichum acutatum) fruit rot of strawberry.
A. O. PAULUS (1), M. Vilchez (1) and K. Larson (2). (1) Plant Pathology Department, University of California, Riverside, CA 92521; (2) Pomology Department, University of California, Davis, CA 95616. Publication no. P-1999-0023-PCA.

Fungicide trials were established in Orange County, CA on the strawberry variety Camarosa during the period from 1997-99. In the 1997 trial, cyprodinil/fluidioxonil and benomyl/thiram alternated with iprodione provided the highest yields in this Botrytis trial. Postharvest fruit evaluations showed less Botrytis infected fruit in the cyprodinil/fluidioxonil and fenhexamid treatments after storage for six days. In 1998 a Botrytis trial was established in a commercial strawberry field. There were no significant differences between cyprodinil/fluidioxonil, benomyl/thiram alternated with iprodione or captan. The 1999 trial was for the control of Botrytis and anthracnose fruit rot. Cyprodinil/fluidioxonil alternated with fenhexamid and cyprodinil/fluidioxonil treated alone provided significantly higher yields than any other treatments in the test. The lowest yield of Botrytis infected fruit was obtained with applications of cyprodinil/fluidioxonil and cyprodinil/fluidioxonil alternated with fenhexamid, fenhexamid and benomyl/thiram alternated with iprodione. Cyprodinil/fluidioxonil, cyprodinil/fluidioxonil alternated with fenhexamid and azoxystrobin gave significantly less anthracnose fruit rot than any of the other fungicides.

Implementation of precision agriculture technology for disease management.
G. PICCINNI and C. M. Rush. Texas A&M University, TAES Amarillo/Bushland. Publication no. P-1999-0024-PCA.

Good irrigation scheduling is among the most important factors for producing high yielding crops. In an effort to reduce costs associated with irrigation, studies are being conducted to automate irrigation systems using infrared thermometers (IRT) or other remote sensing devices. Preliminary studies have shown the efficacy of such devices for scheduling irrigation and variable rate irrigation technology is now available for spot irrigating only the areas under water stress. However, because plant infected by soilborne pathogens exhibits symptoms similar to those of plants under water stress, IRT based irrigation scheduling may not work in pathogen infested soils. In order to verify if IRT or other remote sensing instrumentation can differentiate between stress caused by soilborne pathogens or water, a series of studies are being conducted under a center pivot irrigation system and in greenhouse to evaluate the effectiveness of different type of remote sensing instrumentation. If the effectiveness of these instrumentation holds true, diseased area can be mapped and less water can be applied with the use of variable rate water application technology.

Genetic diversity among mycelial compatibility groups of Sclerotium rolfsii and Sclerotium delphinii.
Z. K. PUNJA and L. J. Sun. Dept. of Biological Sciences, Simon Fraser University, Burnaby, B.C., Canada V5A 1S6. Publication no. P-1999-0025-PCA.

The extent of genetic diversity among 128 isolates of S. rolfsii and 15 isolates of S. delphinii was determined by random amplified polymorphic DNA (RAPD) analysis. Isolates of S. rolfsii originated from 36 hosts and 23 geographic regions worldwide and those of S. delphinii were from 5 hosts and 5 geographic regions. In S. rolfsii, 68 mycelial compatibility groups (MCG) were present, and in S. delphinii there were 5 MCG. Several MCG were present in a given geographic region, and similar MCG were distributed over wide regions. There was no relationship between MCG and host of origin of the isolate. Isolates within an MCG were genetically diverse, as indicated by different RAPD patterns using 5 primers. However, these isolates were clustered together in UPGMA analysis, suggesting greater genetic similarity. Isolates from widely different geographic regions had different RAPD patterns and lower coefficients of similarity. Isolates with the same RAPD pattern always belonged to the same MCG; these isolates were considered to be clonally derived. Within S. delphinii, the extent of genetic diversity was lower than in S. rolfsii.

Survey of field soils for biological suppression of Meloidogyne incognita.
A. Pyrowolakis (1), A. WESTPHAL (2), R. A. Sikora (1), and J. O. Becker (2). (1) Institute of Plant Pathology, University of Bonn, 53115 Bonn, Germany; (2) Dept. of Nematology, University of California, Riverside, CA 92521. Publication no. P-1999-0026-PCA.

In a 10-wk greenhouse trial, a Heterodera schachtii-suppressive soil or its methyl iodide (MI)-fumigated equivalent were infested with Meloidogyne incognita. After infestation with nematode eggs or juveniles (J2) and planting to tomato, Lycopersicon esculentum, population densities remained lower in the untreated than in the fumigated portions of the soil. In a similar trial, M. incognita population densities were suppressed under tomato in 3 of 12 soils from California fields. Nine weeks after inoculation with eggs of M. incognita, galling rates, numbers of egg masses per root system, and numbers of eggs/g soil were significantly higher in the MI-fumigated than in untreated portions of the 3 soils. When 2,000 J2/100 cc soil were added to the untreated and MI-fumigated portions of the 12 soils, in two soils the number of J2 recovered after 3 days were not significantly different in untreated and MI-fumigated portions. In 4 untreated soils J2 recovery was more than 40% lower compared to the MI-fumigated portion of the equivalent soil.

The role of the Spitzenkörper in fungal growth and morphogenesis.
M. RIQUELME (1), G. Gierz (2), and S. Bartnicki-Garcia (1). (1) Depts. of Plant Pathology and (2) Mathematics, Univ. of California, Riverside, CA 92521. Publication no. P-1999-0027-PCA.

The Spitzenkörper (Spk) is a dynamic body at the apex of elongating hyphae of higher fungi. It contains vesicles, cytoskeletal elements and amorphous material. By video-enhanced phase-contrast microscopy we studied positional changes of the Spk in growing hyphae of Neurospora crassa. We found that Spk trajectory, hyphal morphology and growth direction were closely correlated. We examined hyphae of the N. crassa ropy mutants ro-1 and ro-3, with mutations affecting microtubular-associated motor proteins. We found a strong correlation between presence, size and behavior of the Spk and growth rate. With our computer program ("Fungus Simulator") based on the hyphoid model for fungal morphogenesis, we duplicated the directional changes and the morphology of wild type and ropy hyphae, supporting the idea that hyphal morphology is controlled by the position of a Spk functioning as a vesicle supply center. Our observations suggest a role of the microtubules in maintaining the trajectory of the Spk and, hence, the direction of growth and the morphology of the hyphae.

Environmental conditions conducive to the occurrence of blackmold (Alternaria alternata) on processing tomatoes.
J. M. SAKAMOTO (1), M. D. Cahn (2), E. M. Miyao (3), and R. M. Davis (1). (1) Dept. of Plant Pathology, University of California, Davis, CA 95616; (2) U.C. Cooperative Extension, Sutter & Yuba Co., 142-A Garden Highway, Yuba City, CA 95991; (3) U.C. Cooperative Extension, Yolo & Solano Co., 70 Cottonwood Street, Woodland, CA 95695. Publication no. P-1999-0028-PCA.

Blackmold on processing tomatoes in Central California causes significant economic losses. To maximize efficiency of fungicide application, it is important to determine the environmental conditions conducive for infection. Relative humidity (RH), hours of free moisture, and temperature were studied in growth chamber experiments. RH greater than 90% increased the mean infection rates. Increasing periods of free moisture on fruit resulted in increased infection. Results of temperature experiments will be presented. Weather stations were used to collect field data on relative humidity, temperature and hours of fruit wetness. Susceptibility to disease increased with fruit maturity.

Development and symptom expression of rust caused by Tranzschelia discolor in shoots and leaves of cling peach.
A. SOTO-ESTRADA, C. D. Eddleman, and J. E. Adaskaveg. Plant Pathology Dept., University of California, Riverside, CA 92521. Publication no. P-1999-0029-PCA.

Peach rust is caused by the macrocyclic, heteroecious fungus T. discolor. In California, in the absence of the alternate host, the fungus survives host dormancy as mycelium only in infections of current year's shoots. Uredinia are produced in the spring resulting in twig cankers on 5-10 month old shoots and they are the primary source of inoculum in the disease cycle. In greenhouse studies, cankers developed on Lovell peach shoots 4-5 weeks after urediniospore inoculation and a 24-48 hr wetness period at 20°C. Histological studies revealed that mycelium of the fungus developed sub-epidermally in the cortex of the shoot but did not extend into the cambium or xylem tissue. A wound periderm that formed in either the cortex or primary phloem limited shoot colonization by the fungus. No urediniospores were produced in 2-year-old branches or cankers and the stems recovered from rust infections during secondary growth. On leaves, the fungus penetrated through stomata and developed as intercellular mycelium in the spongy mesophyll and later the palisade parenchyma but did not grow into vascular tissue. This resulted in the angular, chlorotic leaf symptoms that develop 8-10 days after inoculation.

Detection of Erwinia amylovora on leaves in a blighted apple orchard.
S. V. THOMSON and S. Ockey. Department of Biology, Utah State University, Logan, Utah, USA 84322-5305. Publication no. P-1999-0030-PCA.

The spatial and temporal distribution of Erwinia amylovora on 2556 leaves in a mixed cultivar apple orchard in Utah was monitored during the 1997-98 seasons. Imprints of apparently healthy leaves from shoots within 0.3 m of shoot blight and greater than 0.3m or from healthy trees were made on CCT differential media. Populations of E. amylovora were detected on usually less than 25% of the leaves near infections but once on 90% of the leaves during a rainstorm. The highest incidence was detected on leaves during or soon after rain storms but the populations declined rapidly during dry days. The bacteria were detected on only 2 of 732 leaves taken from a distance greater than 0.3m. This is surprising because 26% of the "check" leaves were taken from trees that had blight or from trees where blight was present in adjacent trees. We propose that E. amylovora does not spread very far from oozing fire blight infections and that it does not survive more than a few hours on leaves. Prompt removal of infections and spot treatments of bactericides on adjacent foliage may help reduce the reoccurrence of fire blight in orchards.

Enhanced detection of Agrobacterium tumefaciens in soil.
TJ White and Larry W. Moore. Oregon State University, Department of Botany and Plant Pathology, Corvallis, OR 97331. Publication no. P-1999-0031-PCA.

The presence of pathogenic strains of Agrobacterium tumefaciens, the causal agent of crown gall, within nursery soil is suspected to serve as a source of inoculum to wounded tissues of planting stock. However, most standard detection methods are not sensitive enough to detect the presence of low and/or unculturable populations of the pathogen. We have developed an efficient method to identify the presence of tumor-inducing gene sequences from A. tumefaciens in soil. Five soils, received from growers in Washington and Oregon and representing four different soil types, were analyzed in this trial. Soils were collected from growing sites which had previously demonstrated crown gall disease on planted stock. Soils were treated to both direct lysis of bacteria within the soil followed by isolation and purification of bacterial DNA and to isolation of the bacterial fraction from a soil slurry. The bacterial fraction was then plated to media and subjected to colony hybridization to T-DNA sequences. A portion of the bacterial fraction was also lysed to recover bacterial DNA. Purified DNA samples were subjected to PCR using two primer sets specific for the ipt gene of A. tumefaciens. Control experiments included duplicate extractions of soils spiked with a marked strain of pathogenic A. tumefaciens.