VEGETATIVE COMPATIBILITY, PATHOGENICITY, AND VIRULENCE DIVERSITY OF FUSARIUM-OXYSPORUM RECOVERED FROM SPINACH

Four hundred thirty-nine isolates of Fusarium oxysporum recovered from symptomatic spinach seedlings and mature plants from Arkansas, California, New York, Oklahoma, South Carolina, Tennessee, and Washington in the United States as well as from Canada, Japan, and Sweden were examined for vegetative compatibility. A total of 110 isolates also were tested for pathogenicity on spinach. A minimum of 23 vegetative compatibility groups (VCGs) were identified among the 439 isolates in the collection. However, 216 (49%) of the isolates belonged to one of three vegetative compatibility groups (VCGs 1, 2, or 3). Of these 216 isolates, 125 (58%) belonged to VCG 1, 58 (27%) to VCG 2, and 33 (15%) to VCG 3. Fifty-five geographically diverse isolates from VCGs 1, 2, and 3 and 55 isolates vegetatively incompatible with VCGs 1, 2, and 3 were tested for pathogenicity on the spinach cultivar Grandstand. Of the 55 isolates in VCGs 1, 2, and 3 tested, 53 were pathogenic on spinach seedlings, while the 55 remaining isolates were not pathogenic on spinach. Among the three VCGs of Fusarium oxysporum f. sp. spinaciae identified, two distinct virulence phenotypes were detected. Isolates in VCGs 1 and 3 were significantly more virulent on the cultivar Grandstand than isolates in VCG 2. The F. o. f. sp. spinaciae population was composed of three VCGs that have a worldwide distribution, including the United States, Canada, Japan, and Sweden.