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Morphological, Pathological, and Genetic Differentiation of Didymella bryoniae and Phoma spp. Isolated from Cucurbits. A. P. Keinath, Department of Plant Pathology and Physiology, Clemson University, Coastal Research and Education Center, Charleston, SC 29414; M. W. Farnham(2), and T. A. Zitter(3). (2)USDA-ARS, U.S. Vegetable Laboratory, Charleston, SC 29414; (3)Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 85:364-369. Accepted for publication 1 December 1994. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-364.

Didymella bryoniae (anamorph Phoma cucurbitacearum), which causes gummy stem blight of cucurbits, occurs throughout the eastern United States. Other Phoma spp., such as P. exigua, also have been reported to cause symptoms of gummy stem blight. Twenty-seven isolates provisionally identified as D. bryoniae or Phoma spp. were obtained from diseased watermelon, cantaloupe, cucumber, pumpkin, and squash grown in South Carolina, New York, and Florida. D. bryoniae was clearly distinguished from Phoma after 7 days of growth on quarter-strength potato-dextrose agar at 24 C with a 12-h photoperiod. D. bryoniae produced white aerial mycelium, olivaceous green substrate mycelium, and few pycnidia; Phoma produced sparse aerial mycelium and numerous pycnidia, sometimes in concentric zones. The percent monoseptate conidia for D. bryoniae isolates ranged from 0 to 18%, whereas no Phoma isolate produced any septate conidia. Seventeen of 19 D. bryoniae isolates were pathogenic on watermelon cv. Charleston Gray and cantaloupe cv. Classic; all eight isolates of Phoma and two isolates of D. bryoniae were nonpathogenic. Genomic DNA was extracted from all 27 isolates described above plus two additional isolates of D. bryoniae from New York and one from Florida. DNA was amplified using PCR primed with random oligonucleotide decamers. RAPD amplification patterns clearly differentiated D. bryoniae from Phoma. Each of five primers used produced two to four amplified fragments that were unique either to all D. bryoniae or to all Phoma isolates. Thirteen additional fragments were present in all D. bryoniae isolates except two of the three isolates from New York.

Additional keywords: Ascochyta cucumis.