Classification ofFusarium oxysporumf. sp.asparagiinto Vegetatively Compatible Groups

Abstract

Ninety-seven strains of Fusarium oxysporum were isolated from asparagus and other crops in Michigan or obtained from collections in the U.S., Europe, and Taiwan. Pathogenicity tests on asparagus (Asparagus officinalis L. 'U.C. 157') seedlings revealed that 85 strains (87%) caused root lesions, including strains of F. oxysporum f. sp. cepae, F. oxysporum f. sp. gladioli, and F. oxysporum f. sp. apii race 1. Vegetative compatibility between strains was demonstrated with heterokaryons produced between complementary nitrate-nonutilizing (nit) mutants. Nit mutants were placed into one of three phenotypic classes (nit1, nit3, or nitM) by their ability to utilize various nitrogen sources. Mutants with nitM phenotype were recovered from 59% (57 of 97) of the strains, and each one was paired against a mutant with a nit1 or nit3 phenotype from each of the other strains. Twenty-seven strains of F. o. asparagi and one nonpathogenic strain were placed into eight vegetatively compatible groups (VCGs). Thirty-four virulent strains each belonged to a unique VCG. The remaining 24 virulent strains and 11 strains from other formae speciales (that were nonpathogenic on asparagus) were not vegetatively compatible with any other stains, but nitM phenotypes were not recovered from these strains. The largest VCG (1001WE) contained a total of seven strains from Taiwan, Washington state, and three counties in Michigan. Seven VCGs (1002WE-1008WE) contained two to six strains of F. o. asparagi from Michigan. No pattern was observed between VCG in F. o. asparagi and locality. Most heterokaryons formed between vegetatively compatible strains were slow to develop and lacked robust growth. These findings indicate that strains of F. o. asparagi belong to a minimum of 43 VCGs. These data are in contrast to what has been observed in several other formae speciales of F. oxysporum