In early August 2004, pumpkin and zucchini squash (Cucurbita pepo) plants grown in conventional and organic commercial operations in Orange and Dutchess counties, respectively, showed spindle-shaped lesions on vegetative tissues and silver russeting and spots on fruit, typical of Plectosporium blight. Approximately 20% of pumpkin fruit were affected at this early time in yield development, while the zucchini planting had been abandoned due to disease. Symptomatic pieces of stem, petioles, and main leaf veins were excised, surface disinfected with 0.5% sodium hypochlorite, placed on one-quarter-strength potato dextrose agar, and incubated at 21°C with a 12-h photoperiod. Pale pink colonies with pinkish, hyaline, aerial mycelium developed from the tissues. When examined microscopically, simple and branched conidiophores with apical phialides were observed, as well as non- and one-septate ellipsoidal to slightly curved conidia that measured 7.5 to 13.0 × 2.5 to 3.3 μm. The fungus fits the description of Plectosporium tabacinum (van Beyma) M.E. Palm, W. Gams, & H.I. Nirenberg (synonyms Microdochium tabacinum (von Arx, 1984) and Fusarium tabacinum (Gams & Gerlagh, 1968) (1). Pathogenicity was tested on 10 seedlings each of pumpkin, zucchini, gourd (C. pepo), winter squash (C. moschata), and cucumber (Cucumis sativa). Plants were spray inoculated at the three true-leaf stage with a spore suspension at 104 conidia/ml in water with 1% gelatin. Plants were held overnight in a moist chamber and then transplanted into 12-cm-diameter pots and kept in the greenhouse for the rest of the experiment. P. tabacinum was reisolated from all inoculated plants which completes Koch's postulates. Symptoms were noted 3 days after inoculation on pumpkin, zucchini, and gourd, with typical spindle-shaped lesions on the main stem, petioles, and main leaf veins (2). Symptoms developed after 1 week on winter squash, and lesions were mostly concentrated on the older portion of the stem with occasional lesions on the petiole and main leaf veins. Symptoms on cucumber, however, did not develop until 2 weeks after inoculation and appeared as an inconspicuous line of coalesced lesions on the ridges of the main stem only. These symptoms could easily be misidentified as physical abrasions from handling or from wind scarring. These results confirm the high susceptibility of C. pepo species, and indicate that other cucurbits are susceptible, albeit at a lower level. To our knowledge, this is the first report of P. tabacinum in New York. A voucher specimen has been deposited in the Cornell Plant Pathology Herbarium (Accession No. CUP 67504).
References: (1) M. E. Palm et al. Mycologia 87:397, 1995; (2) T. A. Zitter. Microdochium blight. Page 28 in: Compendium of Cucurbit Diseases. T. A. Zitter, D. L. Hopkins, and C. E. Thomas, eds. The American Phytopathological Society, St. Paul, MN, 1996.
