ANALYSIS OF VEGETATIVE COMPATIBILITY GROUPS IN NONPATHOGENIC POPULATIONS OF FUSARIUM-OXYSPORUM ISOLATED FROM SYMPTOMLESS TOMATO ROOTS

A collection of 471 isolates of Fusarium oxysporum was obtained from symptomless tomato roots of plants grown at eight locations in Louisiana. Isolates that produced nitrate metabolism mutants (317 isolates) were included in an analysis of vegetative compatibility groups. One hundred and eighty-six isolates were assigned to 48 vegetative compatibility groups (2015-2062) containing 2-15 members from one to three different collection sites. Even though each site contained sub-populations of F. oxysporum common to other sites, only about half of the vegetative compatibility groups (21 groups) contained isolates from more than one site. In addition, there were 131 isolates of F. oxysporum that formed single-member vegetative compatibility groups. This diverse population composition is probably the result of several factors, including breeding strategies for resistance to fusarium wilt and tomato production practices in Louisiana. Resistance genes and horticultural traits from several sources (Lycopersicon spp.) have been incorporated into commercial tomato cultivars currently being grown. In addition, the long growing season in Louisiana allows several tomato cultivars containing different genes for resistance to be grown consecutively in the same field in a single year. Furthermore, other vegetable crops may be alternated with the tomato crops. Thus, the selection pressures that influence the composition of the nonpathogenic populations of F. oxysporum are in a constant state of flux.