First Report of Plectosporium Blight on Pumpkin Caused by Plectosporium tabacinum in Alabama.

In October of 2001, Plectosporium tabacinum (van Beyma) M.E. Palm, W. Gams, & H.I. Nirenberg (formerly known as Microdochium tabacinum (von Arx, 1984) and Fusarium tabacinum (Gams & Gerlagh, 1968)) was observed in field plantings of pumpkin (Cucurbita pepo L.) in Cullman and Jackson counties in north Alabama. Symptoms were white or tan, spindle-shaped lesions on the stems and leaf petioles and slightly raised, corky, white or light brown lesions on pumpkin fruit and fruit stems. Pumpkin symptoms were identical to a previous description of P. tabacinum (published as M. tabacinum) on pumpkin, zucchini, and yellow summer squash (1). Disease severity ranged from less than 10% stem tissue damage on pumpkins in Cullman County to 40 to 45% stem tissue damage on pumpkins in Jackson County. A field section of pumpkins in Jackson County sprayed with azoxystrobin (Quadris 2.08F, 0.20 kg a.i./ha) alternated weekly with chlorothalonil (Bravo Ultrex, 2.44 kg a.i./ha) beginning at vine-run had stem damage of approximately 5% compared to approximately 45% stem damage on pumpkins in an unsprayed field section. A 50% reduction in marketable fruit due to P. tabacinum was observed in the unsprayed field section compared to the section sprayed weekly with fungicides. When thin slices of lesions were taken from stem and fruit surfaces using a scalpel and examined microscopically, one- or two-celled, hyaline, bilaterally symmetric spores, 7.0 to 8.5 × 2.8 to 3.0 μm were observed. The ends of the spores were slightly narrowed and rounded. Spore characteristics were identical to previous descriptions of P. tabacinum produced in culture and on diseased pumpkins and squash (2,3). Surface-sterilized tissue from fruit lesion margins was plated on potato dextrose agar and incubated under light (Sylvania Gro-Lux, 40w) with a light/dark cycle of 12 h at 23°C. After 10 days, spores were observed that were similar to those from fruit except they were multiguttulate and had a phialide arrangement. The fungal mycelium was pale pink to pale orange and closely appressed to the agar. Fungal characteristics in culture agree with a previous description of P. tabacinum in culture (2). To our knowledge, this is the first report of P. tabacinum in Alabama. References: (1) S. C. Bost and C. A. Mullins. Plant Dis. 76:861, 1992. (2) M. E. Palm et al. Mycologia 87(3):397, 1995; (3) 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.