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, Califo rnia, New York, Oklahoma, South Carolina, Tennessee, and Washington in the United States as well as from Canada, Japan, and Sweden were exam ined for vegetative compatibility. A total of 110 isolates also were t ested for pathogenicity on spinach. A minimum of 23 vegetative compati bility groups (VCGs) were identified among the 439 isolates in the col lection. However, 216 (49%) of the isolates belonged to one of three v egetative compatibility groups (VCGs 1, 2, or 3). Of these 216 isolate s, 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 a nd 55 isolates vegetatively incompatible with VCGs 1, 2, and 3 were te sted for pathogenicity on the spinach cultivar Grandstand. Of the 55 i solates in VCGs 1, 2, and 3 tested, 53 were pathogenic on spinach seed lings, while the 55 remaining isolates were not pathogenic on spinach. Among the three VCGs of Fusarium oxysporum f. sp. spinaciae identifie d, two distinct virulence phenotypes were detected. Isolates in VCGs 1 and 3 were significantly more virulent on the cultivar Grandstand tha n isolates in VCG 2. The F. o. f. sp. spinaciae population was compose d of three VCGs that have a worldwide distribution, including the Unit ed States, Canada, Japan, and Sweden.