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Genetic Variation Among Fusarium oxysporum Isolates from Sugar Beet as Determined by Vegetative Compatibility

One hundred sixty Fusarium oxysporum isolates were collected over a 3-year period (1992 to 1994) from diseased sugar beet and pigweed plants from seven counties in Texas. Disease symptoms on sugar beet included root-tip-rot symptoms with wilting and vascular necrosis, and wilting and vascular necrosis only. Pathogenicity testing on sugar beets indicated that 132 isolates of the 160 recovered were pathogenic and were considered to be F. oxysporum f. sp. betae (FOB). Of the 132 isolates of FOB, 28 were initially chosen as testers and paired in all possible combinations to estimate the number of vegetative compatibility groups (VCGs) present. Once VCGs were determined from the 28 isolates, a nitrate nonutilizing mutant (nit) 1 or nit 3 from each of the remaining isolates was paired against a Nit M from each of the established VCGs. Thirty-three isolates obtained from other sugar-beet-growing states also were tested for vegetative compatibility. A total of 95 of the 132 isolates of FOB from Texas were assigned to one of seven VCGs identified. Sixty-three isolates were assigned to VCG 1, with VCGs 2 through 7 containing 6, 16, 2, 2, 2, and 4 isolates, respectively. VCGs 1, 3, and 6 were recovered from both sugar beet and pigweed. Two additional isolates collected from Texas in 1987 also belonged to VCG 1. A number of the isolates collected from Texas could not be assigned to any of the seven established VCGs. These included two single-member Nit Ms, 11 self-incompatible isolates, and 24 of unknown affiliation. None of the isolates from any one state were compatible with those from any other state. Results suggest that substantial variation exists among sugar beet isolates of FOB from the U.S., and that these populations of F. oxysporum are apparently distinct and endemic to their respective areas.