VEGETATIVE COMPATIBILITY GROUPINGS IN A LOCAL-POPULATION OF FUSARIUM-OXYSPORUM

One hundred isolates of Fusarium oxysporum collected from soil in the San Joaquin Valley in 1988 were tested for their ability to form intra- and inter-isolate heterokaryons. Five thousand and fifty isolate combinations were tested for vegetative compatibility, using complementary nitrate nonutilizing mutants. Ninety-eight combinations (2%) produced weakly positive reactions, 248 (5%) produced strong reactions, and 4704 combinations (93%) were negative for heterokaryon formation. This results suggests that anastomosis occurs infrequently among isolates in this population. Isolates were assigned to vegetative compatibility groups based on strong reactions. By this procedure 100 isolates were placed in 29 vegetative compatibility groups. At least one isolate from each of the 29 vegetative compatibility groups was tested and found to be vegetatively incompatible with the muskmelon wilt pathogen, F. oxysporum f. sp. melonis, which occurs in the same field. Each of these isolates also was avirulent to muskmelon in greenhouse pathogenicity tests. Forty isolates of F. oxysporum were collected in 1989 and 32 (80%) were compatible with isolates collected in 1988. The remaining eight isolates represented eight different vegetative compatibility groups.