First Report of Blight of Common Bean Caused by Phytophthora capsici in Connecticut

Phytophthora capsici Leonion was first identified on pepper (Capsicum annuum L.) and is widespread on solanaceous and cucurbitaceous crops. It was first documented on Phaseolus lunatus L. in Delaware in 2002 (1), followed by reports on snap beans (Phaseolus vulgaris L.) in Michigan in 2003 (2), and on Long Island, NY in 2008 ( http://vegetablemdonline.ppath.cornell.edu/ NewsArticles/Bean_phytoJune09.html ). In 2009, we observed snap and wax beans in commercial production with water-soaked lesions on foliage, stems, and pods. Twelve to sixteen hectares were affected in the flood plain of the Connecticut River in central Connecticut. Weather conditions had been warm and very wet. Lesions displayed white mycelia and sporangia. P. capsici was isolated from surface-sterilized tissue on potato dextrose agar (PDA) and malt extract agar. Hyphal tips were subcultured onto V8 media for further analysis. To confirm Koch's postulates, two isolates were tested for pathogenicity against bean (cv. Valentino) and pepper (cv. Cayenne) by placing colonized PDA plugs or PDA alone next to the crown or in stem branches. Symptoms similar to those observed in the field on bean and pepper developed on inoculated plants and the pathogen was reisolated. Controls did not develop disease. Sporangia of P. capsici growing on V8 medium were ellipsoid, ovoid, pyriform, but occasionally irregular, papillate, and 54.0 ± 5.7 × 31.1 ± 4.7 μm (n = 31) with a length/width (L/W) ratio of 1.8 ± 0.3. The papillae were 5.4 ± 0.9 μm (n = 31) and the pedicels were 24.5 ± 12.6 × 3.0 ± 1.0 μm. Sporangia collected from bean plants were smaller with longer pedicels; the sporangia were 44.9 ± 9.1 × 26.0 ± 2.8 μm with a L/W ratio of 1.7 ± 0.2; papillae were 4.6 ± 1.0 μm; and the pedicels were 49 ± 20.0 × 2.8 ± 0.9 μm (n = 20). To confirm the identity of our isolate genetically, DNA was extracted from one P. capsici isolate and the nuclear ribosomal internal transcribed spacer (ITS) region was amplified and sequenced (GenBank Accession No. GU011684). The ITS sequence was identical to sequences of P. capsisci in GenBank and confirmed our identification of this new isolate as P. capsici. To our knowledge, this is the first report of P. capsici infecting Phaseolus vulgaris in Connecticut and New England. References: (1) C. R. Davidson et al. Plant Dis. 85:886, 2002. (2) A. J. Gevens et al. Plant Dis. 92:201, 2008.