Colonization of Tomato Root by Pathogenic and Nonpathogenic Fusarium oxysporum Strains Inoculated Together and Separately into the Soil

In soil, fungal colonization of plant roots has been traditionally studiedby indirect methods such as microbial isolation that do not enabledirect observation of infection sites or of interactions between fungalpathogens and their antagonists. Confocal laser scanning microscopy wasused to visualize the colonization of tomato roots in heat-treated soiland to observe the interactions between a nonpathogenic strain, Fo47,and a pathogenic strain, Fol8, inoculated onto tomato roots in soil.When inoculated separately, both fungi colonized the entire rootsurface, with the exception of the apical zone. When both strains wereintroduced together, they both colonized the root surface and wereobserved at the same locations. When Fo47 was introduced at a higherconcentration than Fol8, it colonized much of the root surface, buthyphae of Fol8 could still be observed at the same location on theroot. There was no exclusion of the pathogenic strain by the presenceof the nonpathogenic strain. These results are not consistent with thehypothesis that specific infection sites exist on the root forFusarium oxysporum and instead support the hypothesis thatcompetition occurs for nutrients rather than for infectionsites.