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.

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.

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.