MORPHOLOGICAL, PATHOLOGICAL, AND GENETIC DIFFERENTIATION OF DIDYMELLA-BRYONIAE AND PHOMA SPP ISOLATED FROM CUCURBITS

Didymella bryoniae (anamorph Phoma cucurbitacearum), which causes gumm y stem blight of cucurbits, occurs throughout the eastern United State s. Other Phoma spp., such as P. exigua, also have been reported to cau se symptoms of gummy stem blight. Twenty-seven isolates provisionally identified as D. bryoniae or Phoma spp. were obtained from diseased wa termelon, cantaloupe, cucumber, pumpkin, and squash grown in South Car olina, New York, and Florida. D. bryoniae was clearly distinguished fr om Phoma after 7 days of growth on quarter-strength potato-dextrose ag ar 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 c oncentric zones. The percent monoseptate conidia for D. bryoniae isola tes ranged from 0 to 18%, whereas no Phoma isolate produced any septat e conidia. Seventeen of 19 D. bryoniae isolates were pathogenic on wat ermelon cv. Charleston Gray and cantaloupe cv. Classic; all eight isol ates of Phoma and two isolates of D, bryoniae were nonpathogenic. Geno mic DNA was extracted from all 27 isolates described above plus two ad ditional isolates of D. bryoniae from New York and one from Florida. D NA 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 fragme nts that were unique either to all D. bryoniae or to all Phoma isolate s. Thirteen additional fragments were present in all D. bryoniae isola tes except two of the three isolates from New York.