Interactions Between Fusarium Oxysporum F. Sp. Lycopersici and Nonpathogenic Strains of Fusarium Oxysporum.

Race 1 isolates collected from tomato roots were used to develop an orange mutant of Fusarium oxysporum f. sp. lycopersici (FOL) for use in competitive infection tests against nonpathogenic F. oxysporum isolates. Eighty one color mutants were induced by exposure to ultraviolet light. The mutant ORA-22 retained pathogenicity and growth habit similar to the wild-type. Preliminary competitive infection tests indicated differences within the nonpathogenic population in the ability to suppress or enhance infection by FOL orange mutant ORA-22. Four vegetative compatibility groups (VCGs) were tested to determine if competitive infection ability was associated with VCG designation. Results from these tests revealed a wide range of effectiveness, ranging from no effect to suppression or enhancement of root infection by the pathogen, FOL. All members of a VCG showed the same reaction. The results suggest that competitive infection ability and VCG are closely related traits. Isozyme analysis by starch gel electrophoresis was used to estimate genetic diversity within and among the four VCGs. Principal components analysis of the results showed that each population was unique and clearly separated from the pathogenic controls. These results provide strong evidence that the genetic traits associated with competitive infection ability are confined to genetically isolated populations of nonpathogenic F. oxysporum as delimited by VCG.