VEGETATIVE COMPATIBILITY GROUPS AND SUBGROUPS IN FUSARIUM-OXYSPORUM F SP RADICIS-LYCOPERSICI

Isolates of Fusarium oxysporum f. sp. radicis-lycopersici, the incitant of crown and root rot disease of tomatoes, were characterized by their ability to form heterokaryons with each other by using nitrate nonutilizing (nit) mutants. Isolates that formed mutual heterokaryons were placed in the same vegetative compatibility group (VCG). Seven VCGs were found among 218 isolates from Israel, Belgium, Canada, Greece, France, Italy, Japan, and the United States. Two of these VCGs could be divided into two subgroups each. The assignment of isolates to subgroups within a VCG was based on their capacity to form strong heterokaryons with isolates of one subgroup; whereas, their interactions with isolates of the other subgroup were weak and slow. At least one isolate was compatible with two VCGs. Isolates of F. o. radicis-lycopersici were not vegetatively compatible with isolates of F. o. lycopersici, melongenae, and melonis. In starch gel electrophoresis, the banding patterns of eight isozymes were identical for all 49 isolates of F. o. radicis-lycopersici and 19 isolates of two other formae speciales of F. oxysporum that were examined. One of the VCGs of F. o. radicis-lycopersici was characterized by a major esterase band that was present in 34 of 36 isolates tested.