M. T. McGrath, Department of Plant Pathology and Plant-Microbe Biology, Cornell University, Long Island Horticultural Research and Extension Center, Riverhead, NY 11901; and
J. Strauss and
H. R. Dillard, Department of Plant Pathology and Plant-Microbe Biology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456
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.