First Report of Phytophthora Blight Caused by Phytophthora capsici on Snap Bean in New York.

Phytophthora capsici Leonian is an important pathogen of solanaceous and cucurbit crops. Phytophthora blight was first reported on snap bean (Phaseolus vulgaris L.) in Michigan in 2003 (2) and Connecticut in 2010 (3). This report documents the discovery of P. capsici on snap bean (cv. Bronco) grown in Riverhead, NY in September 2008 and August 2010 on snap bean (cv. Valentino) in Holley, NY, more than 690 km away. Disease was favored by frequent rainfall and prolonged wet periods with air temperatures of 24 to 29°C. Both locations were commercial fields previously planted to pepper or cucurbits affected by P. capsici. In Riverhead, infected pods had characteristic yeast-like growth of P. capsici, which were predominantly sporangia. In Holley, large water-soaked lesions were observed on snap bean foliage, and as the disease progressed, leaves became necrotic and detached from the plant. Reddish brown lesions were observed on stems in advance of white areas of sporulation. Infected pods displayed white mycelial growth, were shriveled, and desiccated. P. capsici was isolated from symptomatic tissues. Stems and pods were surface disinfested for 3 min in 0.525% sodium hypochlorite solution, rinsed for 3 min in sterile distilled water, transferred to PARPH (4) media, and incubated at 22°C. After 5 days, hyphae from colony margins were excised and transferred to 15% unclarified V8 agar media. Cultures consisting of white mycelia and ovoid papillate sporangia on long pedicels were identified as P. capsici. Sporangia were 25.0 to 70.0 × 10.0 to 22.5 µm (average 42.0 × 16.25 µm). Identity was further confirmed by PCR primers specific to P. capsici (1). DNA was extracted from mycelia produced on V8 agar and amplification with the species-specific primers resulted in a PCR product of the same size as that obtained from a known isolate of P. capsici. Pathogenicity of the isolate from Holley was determined by two methods on 50-day-old snap bean plants (cv. Valentino) grown in a greenhouse. In method one, four plants were inoculated with 1-cm-diameter mycelial plugs excised from 8-day-old cultures. A single plug was placed against the stem at the soil line. Four control plants were treated similarly with noncolonized agar plugs. In method two, entire plants were atomized with 10 ml of a zoospore suspension (2.6 × 105/ml). Control plants were atomized with sterile distilled water. All plants were placed in a growth chamber with continuous mist for 24 h at 24°C. After 24 h, plants were enclosed in plastic bags and placed in a greenhouse at 27°C. Stem lesions similar to those observed in affected fields were evident on plants treated with mycelia plugs 2 days after inoculation. Plants inoculated with the zoospore suspension developed stem lesions and desiccated pods. Control plants were asymptomatic. P. capsici was successfully recovered from infected plant tissue, fulfilling Koch's postulates. The Riverhead isolate was demonstrated as pathogenic on snap bean and cucumber by placing colonized plugs on pods and fruit that were subsequently incubated in moist chambers (24°C, 90 to 100% relative humidity). P. capsici was successfully recovered from symptomatic pods and fruit. To our knowledge, this is the first report of Phytophthora blight caused by P. capsici on snap bean in New York. References: (1) A. R. Dunn et al. Plant Dis. 94:1461, 2010. (2) A. J. Gevens et al. Plant Dis. 92:201, 2008. (3) J. A. LaMondia et al. Plant Dis. 94:134, 2010. (4) G. C. Papavisas et al. Phytopathology 71:129, 1981.

Population Structure and Resistance to Mefenoxam of Phytophthora capsici in New York State.

In 2006, 2007, and 2008, we sampled 257 isolates of Phytophthora capsici from vegetables at 22 sites in four regions of New York, to determine variation in mefenoxam resistance and population genetic structure. Isolates were assayed for mefenoxam resistance and genotyped for mating type and five microsatellite loci. We found mefenoxam-resistant isolates at a high frequency in the Capital District and Long Island, but none were found in western New York or central New York. Both A1 and A2 mating types were found at 12 of the 22 sites, and we detected 126 distinct multilocus genotypes, only nine of which were found at more than one site. Significant differentiation (FST) was found in more than 98% of the pairwise comparisons between sites; approximately 24 and 16% of the variation in the population was attributed to differences among regions and sites, respectively. These results indicate that P. capsici in New York is highly diverse, but gene flow among regions and fields is restricted. Therefore, each field needs to be considered an independent population, and efforts to prevent movement of inoculum among fields need to be further emphasized to prevent the spread of this pathogen.

Managing Foliar Diseases of Processing Sweet Corn in New York with Strobilurin Fungicides.

Processing sweet corn (Zea mays) growers in New York are more concerned about the cost effectiveness of fungicide use against foliar fungal diseases (common rust and Northern corn leaf blight) and less about whether such sprays will reduce disease intensity. To address this concern, field trials were done in 2006 and 2007 with three processing sweet corn hybrids (Jubilee, Bold, and GH 9597) that differed in susceptibility to common rust and Northern corn leaf blight, and two strobilurin fungicides (azoxystrobin and pyraclostrobin). Single strobilurin applications were applied in response to foliar disease severity thresholds of 1, 10, and 20%. Single fungicide applications did reduce foliar disease severities. Applications were most cost effective when made in response to the 1 and 10% foliar severity thresholds, and generally only in the susceptible hybrid Bold. Spraying at the 20% severity threshold did reduce final foliar disease severity but was not cost effective. Azoxystrobin and pyraclostrobin were equally effective in disease management. The results suggest that a single application of a strobilurin fungicide against common rust and Northern corn leaf blight can be cost effective for New York processing sweet corn growers when such an application is made before foliar disease severity exceeds 20%.

First Report of Sclerotinia Stem Rot Caused by Sclerotinia sclerotiorum on Hibiscus trionum in New York.

Hibiscus trionum L. (Venice mallow) is an annual weed widely distributed in the United States. In September of 2008, Venice mallow plants with bleached stems and necrotic tissues were observed in a commercial field of cabbage (Brassica oleracea L. cv. Moreton) in Geneva, NY. White, cottony mycelium and dark sclerotia were readily found on the stems and in the stem pith. Cabbage plants in direct contact with diseased Venice mallow also displayed signs and symptoms of infection by Sclerotinia sclerotiorum (Lib.) de Bary. Sclerotia from within diseased Venice mallow stems were placed in 9-cm-diameter petri plates on potato dextrose agar amended with 0.1 g/liter each of chloramphenicol and streptomycin (ABPDA) and incubated at room temperature. In addition, diseased stem tissue was surface disinfested for 3 min in 0.525% sodium hypochlorite solution, rinsed for 3 min in sterile distilled water, and placed on ABPDA. After 5 days, hyphae from the colony margin were excised and transferred to potato dextrose agar (PDA) plates. Fungal cultures consisting of white mycelia and medium-sized (~4 mm), black, irregular sclerotia were consistently recovered and identified as S. sclerotiorum based on morphological characteristics (1). Pathogenicity of two isolates (one from a sclerotium and one from stem tissue) was determined by inoculating seven 43-day-old Venice mallow plants growing in greenhouse pots (65 mm in diameter). Mycelia plugs (7 mm in diameter) were excised from 2-day-old PDA cultures of each isolate and placed on the stems at the soil line. Seven control plants were inoculated with noncolonized PDA plugs. All plants were enclosed in plastic bags for 72 h and placed under shade in the greenhouse with temperatures from 20 to 38°C (average 27°C). Symptoms similar to those observed in the affected fields were evident within 2 days after inoculation, while control plants remained symptomless. S. sclerotiorum was successfully recovered from infected plant tissue, fulfilling Koch's postulates. The experiment was repeated with similar results. To our knowledge, this is the first report of Sclerotinia stem rot of Hibiscus trionum caused by S. sclerotiorum (2,3). References: (1) L. Buchwaldt. Sclerotinia White Mold. Page 43 in: Compendium of Brassica Diseases, 1st ed. S. R. Rimmer et al., eds. The American Phytopathological Society, St. Paul, MN, 2007. (2) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, MN, 1989. (3) C. Wehlburg et al. Index of Plant Diseases in Florida. Fla Dep. Agric. Consum. Serv. Bull. 11, 1975.

Efficacy of Muscodor albus for the Control of Phytophthora Blight of Sweet Pepper and Butternut Squash.

The efficacy of Muscodor albus, a potential soil biofumigant, to control root and stem rot by Phytophthora capsici, was examined in a greenhouse study. P. capsici-infested potting mix was treated with three rates of M. albus, mefenoxam (Ridomil Gold EC, Syngenta Crop Protection, Inc.), or nothing. Seedlings of five sweet pepper cultivars and one butternut squash cultivar were transplanted into the treated potting mix. After 7 days, the plants were rated on a scale of 0 (healthy) to 5 (dead). The experiment was conducted three times and there was a significant interaction between pepper cultivar and soil treatment. Treatment with the highest rate of M. albus resulted in a slight but significant reduction in disease severity on Alliance, Aristotle, Paladin, and Revolution pepper compared with the pathogen-only control, while no significant decreases in disease severity were observed with butternut squash or the highly susceptible pepper cv. Red Knight. Of the four less-susceptible pepper cultivars, Paladin (the most tolerant cultivar) was the only one on which M. albus, as applied in this study, reduced disease severity to commercially acceptable levels.

Identification and Characterization of Russet on Snap Beans Caused by Plectosporium tabacinum.

Russet symptoms of unknown etiology are sporadically observed on snap bean (Phaseolus vulgaris) pods in New York and Maryland. Symptoms can render the whole crop unmarketable, and seem to appear when heavy rainfall occurs around harvest time. In 2000 and 2004, a microorganism not previously encountered was isolated from russet lesions on snap bean pods from commercial fields in Maryland and New York. Typical russet symptoms were produced on snap bean pods of cv. Brio after inoculation with spores of the isolates. Koch's postulates were also fulfilled. The organism was identified as Plectosporium tabacinum (Van Beyma) M.E. Palm, W. Gams & Nirenberg. A continuous 48-h leaf wetness duration at 23 to 27°C was essential for rapid symptom development. Large (11 cm long on average) snap bean pods were more susceptible to disease than smaller (pin) pods in cvs. Brio and Gold Mine. Light mechanical damage to the pods did not enhance infection. Four isolates of P. tabacinum (three from snap bean pods, one from zucchini) were inoculated onto large pods of the snap bean cvs. Brio, Gold Mine, and Hercules. All four isolates induced a russet on the pods, but the severity was significantly lower with the zucchini isolate.

Assessment of Phenotypic Variability in Erwinia stewartii Based on Metabolic Profiles.

One hundred twenty-four bacterial isolates originating from sweet corn or corn flea beetles in the northeastern, midwestern, and mid-Atlantic United States were verified as Erwinia stewartii (Pantoea stewartii subsp. stewartii) and characterized phenotypically by their respiratory response to 91 carbon sources. The unweighted pair group method of averages (UPGMA) was used to construct a dendrogram that revealed homogeneous metabolic profiles at 93% similarity. Two-thirds of the isolates formed 18 separate groups, each sharing the same metabolic profile. One-third of the isolates had distinct metabolic profiles. Most groups shared either isolation source, geographical location, and/or year of isolation. Members of some groups persisted through time and had been isolated from diverse geographical locations. Four representative strains of the proposed Pantoea stewartii subsp. indologenes were also characterized; their metabolic profiles were most similar to those of Erwinia herbicola (Pantoea agglomerans).

Transmission of Alternaria brassicicola to Cabbage by Flea Beetles (Phyllotreta cruciferae).

In 1995 and 1996, flea beetles (Phyllotreta cruciferae) were observed in the field feeding on cabbage plants that were infected with Alternaria brassicicola. Flea beetles were captured in glass vials, etherized, and placed on agar media for isolation of A. brassicicola. In 1995, A. brassicicola was isolated from 13 out of 69 (18.8%) flea beetles in the first test and 38 out of 132 (28.8%) in the second test. In 1996, flea beetles were collected nine times during the growing season, and the isolation frequency increased from 0 to 77% as the crop approached maturity. In another study, flea beetles were collected from a field of A. brassicicola-infected cabbage, enclosed in plastic bags containing potted healthy cabbage plants, and then placed on a shaded greenhouse bench for 6 days. Alternaria leaf spot developed on plants that were infested with the contaminated flea beetles. Feces obtained from flea beetles that fed on cabbage infected with A. brassicicola contained intact and broken conidia of A. brassicicola and undigested pieces of cabbage leaf. The conidia were viable after passing through the flea beetles, as evidenced by their germination on the glass slides used for collecting the feces. Conidia of A. brassicicola were observed by scanning electron microscopy on all parts of flea beetle bodies, including wings, mouthparts, antennae, and legs.

Survival of Colletotrichum coccodes in Infected Tomato Tissue and in Soil.

Studies were initiated in 1988 and 1991 to assess long-term survival ability of Colletotrichum coccodes. Sclerotia and infected tomato fruit skin tissue were enclosed in nylon pouches and placed on the soil surface (0 cm) or buried 10 and 20 cm deep in fields located in Geneva, New York. Over time, the greatest decline in recovery of C. coccodes from tomato skin and decrease in viability of sclerotia were from samples placed on the soil surface. In the 1988 study, after 8 years in the field, 0, 90, and 88% of the sclerotia were viable, and C. coccodes was isolated from 0, 54, and 86% of the tomato skin tissues at the 0-, 10-, and 20-cm soil depths, respectively. In the 1991 study, after 5 years in the field, C. coccodes was isolated from 22, 35, and 37% of the tomato skin tissues, and 55, 91, and 92% of the sclerotia were viable at the 0-, 10-, and 20-cm soil depths, respectively. It is apparent that lengthy crop rotations are required to significantly decrease viable inoculum of C. coccodes. In a separate study, C. coccodes overwintered in naturally infected tomato roots in commercial fields and was consistently isolated from roots in the fall and the following spring. Fields sampled in the fall yielded similar numbers of plants with infected roots the following spring.

Thlaspi arvense, a New Host for Alternaria brassicicola.

A leaf spot was observed on cruciferous weeds growing in a cabbage field located in Geneva, NY, on 1 August 1996. The leaf spots on the weeds were dark gray to black in color and varied in size from pinpoints to 1 mm in diameter. The cabbage (Brassica oleracea L. var. capitata L.) was infected with Alternaria brassicicola (Schwein.) Wiltshire, the cause of Alternaria leaf spot. The weeds were identified as Thlaspi arvense L., a winter annual commonly referred to as field pennycress, stinkweed, or fanweed depending on geographic location. Isolations from the diseased weed tissue yielded A. brassicicola (2). The numerous conidia occurred in chains of 10 or more, ranged in size from 14 to 53 µm in length, were 5 to 18 µm wide, contained from 1 to 6 transverse septa with rare longitudinal septa, and were olivaceous in color. An apical beak was absent. On potato dextrose agar (PDA) the colony was dark olive-green to black in color and velvety. Seed was collected from the T. arvense plannts in the spring of 1997. One hundred seeds were placed in petri plates containing PDA amended with 0.01% of chloramphenicol and streptomycin sulfate. A. brassicicola was not isolated from the seeds. A different area of the field was planted to cabbage in 1997 and the cruciferous weeds were allowed to grow. The 1997 population of T. arvense consisted of plants from the previous season that flowered early and plants from seeds that germinated late in the season but did not flower. A. brassicicola was isolated from nonflowering weeds in September and from flowering weeds in October. Nonflowering plants were removed from the field in November, planted in pots, and placed in the greenhouse to induce flowering. Identity of both plant populations was confirmed as T. arvense (Warren Lamboy, Cornell University, Geneva, NY). Pathogencity of A. brassicicola isolates from T. arvense was demonstrated on cabbage and T. arvense by following Koch's postulates. Conidia (105) from a 5-day-old culture isolated from T. arvense grown on PDA were atomized onto field pennycress and cabbage plants with a Preval sprayer. The plants were enclosed in plastic bags and put under lathe shading in the greenhouse. The pathogen was reisolated from symptomatic tissue of both plants after 5 days. This weed could serve as a potential source of A. brassicicola inoculum because it is not controlled by herbicides used in crucifer production systems. Alternaria raphani has been reported on T. arvense in Canada (1). This is believed to be the first report of A. brassicicola on T. arvense. References: (1) K. Mortensen et al. Can. Plant Dis. Surv. 73:129, 1993. (2) P. Neergaard. 1945. Danish Species of Alternaria and Stemphylium. Oxford University Press, London. pp. 137-138.

Disease Progress of Black Dot on Tomato Roots and Reduction in Incidence with Foliar Applied Fungicides.

Progression of black dot caused by Colletotrichum coccodes was determined at regular intervals on roots of processing tomatoes growing in a naturally infested field. In 1993 and 1994, C. coccodes was first isolated from tomato roots 30 and 37 days after transplanting, respectively, which corresponded to the opening of flowers in the first flower clusters. Black dot incidence increased rapidly from the time when large green fruit were present to production of mature red fruit. In both years, C. coccodes was isolated from the roots of 97% of the plants at the postharvest sampling date (162 days after transplanting). Areas under the incidence disease progress curves were not significantly different in the 2 years of study (1993 = 83.5, 1994 = 86.9). Root decay was severe at the postharvest sampling, and sclerotia of C. coccodes were abundant on the roots. C. coccodes was isolated from 38 and 44% of the root segments in 1993 and 1994, respectively. Areas under the disease progress curves for infected root segments were not significantly different in the 2 years of study (1993 = 25.8, 1994 = 33.9). In a separate study, chlorothalonil (2.5 kg/ha) or mancozeb (1.68 kg/ha) was applied at 7-, 10-, or 14-day intervals to tomato plants. Recovery of C. coccodes from root segments at harvest (113 to 118 days after transplanting) was significantly reduced in the chlorothalonil 7- or 10-day and mancozeb 10-day interval treatments in both years. However, the percentage of plants with black dot was not consistently reduced by fungicide applications.

An Assessment of Fungicide Benefits for the Control of Fungal Diseases of Processing Tomatoes in New York and New Jersey.

Concurrent studies on the benefits of fungicide use for control of fungal diseases of processing tomatoes were conducted in New York and New Jersey in 1993 and 1994. Fungicides (chlorothalonil at 2.5 kg/ha or mancozeb at 1.68 kg/ha) were applied at 7-, 10-, or 14-day intervals to processing tomatoes for control of anthracnose caused by Colletotrichum coccodes, early blight caused by Alternaria solani, and Septoria leaf spot caused by Septoria lycopersici. The New Jersey trial included an additional treatment using the disease-warning system TOM-CAST. All fungicide treatments significantly reduced foliar disease severity (in New York) and anthracnose incidence (New York and New Jersey) in the 2 years of study. Yield of usable fruit was significantly increased by all fungicide treatments with the exception of the TOM-CAST treatment using the cultivar Brigade in 1994 in New Jersey. In New York, usable yield and financial benefit were consistently the highest in plots treated with chlorothalonil on a 7-day interval. In New Jersey, the highest usable yields and the greatest financial benefits occurred in the chlorothalonil 7- and 10-day interval treatments in 1993. At both locations, the yield and financial benefit associated with the fungicide treatments was primarily due to suppression of anthracnose and other fruit rots. Suppression of foliar diseases was less important.

First Report of a Leaf Spot Caused by Cladosporium oxysporum on Greenhouse Tomato.

In a commercial greenhouse in upstate New York, dark brown, angular lesions were first observed in April on lower, older leaves of 4-month-old tomato plants (Lycopersicon esculentum Mill. 'Jumbo'). Chlorosis frequently developed around the lesions. Removal of the infected leaves reduced the rate of epidemic development. However, by July, lesions were present throughout the plant canopy, up to 2 m. The irregularly shaped lesions varied in size from 1 to 5 mm, frequently with tan-colored centers initially. Conidia developed in the center of the lesions, primarily on the outer, or adaxial side of the leaf, but were infrequent on the abax-ial surface. This contrasts with Cladosporium leaf mold caused by Fulvia fulva, in which the conidia develop as a velvety brown patch in lesions on the abaxial, or underside of the leaf, accompanied by chlorosis on the upper side of the leaf (1). The conidia ranged in shape from oval or li-moniform (5 to 6 µm in diameter) to cylindrical (5 to 6 µm wide, 7 to 20 µm long). The fungus was identified as Cladosporium oxysporum Berk. & M. A. Curtis (3) by C. J. K. Wang and J. M. McKemy of the State University of New York College of Environmental Science and Forestry in Syracuse, NY. Koch's postulates were fulfilled with a tuft of mycelium and conidia grown on potato dextrose agar as inoculum on fully expanded leaves of 5-week-old tomato plants, cv. Jumbo. Two weeks later, characteristic sporulating lesions developed on inoculated plants, about 1 cm from the inoculation site. Within 3 weeks, in the research greenhouse, the disease spread to healthy tomato plants in the vicinity, confirming the highly infectious nature observed in the commercial greenhouse. The fungus was reisolated from inoculated leaves and also from the adjacent naturally infected plants. C. oxysporum was previously reported as the causal agent of a leaf spot disease of pepper (2) and also a storage disease of ripe tomato fruit (4). To the best of our knowledge this is the first report of C. oxysporum causing disease on tomato foliage. References: (1) E. E. Chamberlain. N.Z. J. Agric. 45:136, 1932. (2) A. M. Hammouda. Plant Dis. 76:536, 1992. (3) J. M. McKemy and G. Morgan-Jones. Mycotaxon 41:397, 1991. (4) S. Singh et al. Indian J. Microbiol. 23:133, 1983.

Influence of Tillage Practices on Anthracnose Development and Distribution in Dry Bean Fields.

Three tillage practices-chiseling, rototilling, and moldboard plowing-were evaluated in 1993 and 1994 to determine their impact on initial disease development, distribution, and progression over time in a field of the susceptible kidney bean cultivar Horizon. The tillage treatments were administered in the spring in a field infested in 1992 with the bean anthracnose pathogen, Colletotrichum lindemuthianum race ß. Initial disease incidence was highest in the chiseled plots, where more bean debris was left on the surface than in the other treatments. Significantly higher final disease incidence and area under the disease progress curve (AUDPC) occurred in the chiseled plots than in the rototilled and moldboard plowed plots. There was a significant correlation (r = 0.75) between the percentage of debris left on the surface and subsequent disease incidence on pods in the field. Anthracnose incidence or severity in the field was highly correlated with disease incidence on harvested pods (r values ranged between 0.87 and 0.98). Results from the ordinary runs analysis showed that anthracnose occurred randomly within the field early in the season, indicating that initial inoculum was from bean debris within the field. Later in the season, plant-to-plant spread resulted in a more clustered distribution of diseased plants.

Identification of Erwinia stewartii by a ligase chain reaction assay.

A PCR-coupled ligase chain reaction (LCR) assay was developed to distinguish the plant pathogenic bacterium Erwinia stewartii from other erwiniae. This new technique allows discrimination to the species level on the basis of a single-base-pair difference in the 16S rRNA gene which is unique to E. stewartii. Portions of the 16S rRNA genes of E. stewartii and the closely related Erwinia herbicola were sequenced. From comparison of the two 16S rRNA gene regions, two primer pairs were constructed such that only E. stewartii DNA gave a product in the LCR assay. The ligated product was separated from the radioactively labelled primers by denaturing polyacrylamide gel electrophoresis and visualized by autoradiography. Twenty-four different Erwinia species and strains were tested by PCR-coupled LCR to verify the specificity of the assay, and only E. stewartii strains gave a positive reaction. In addition, infected and healthy plant material was also assayed. E. stewartii was detected in infected plant material, even when large populations of epiphytic bacteria were present. No enrichment was necessary for detection of the pathogen in corn leaves. This assay has potential as a diagnostic technique for the detection of E. stewartii in infected plant and vector material.