Implication of systemic induced resistance in the suppression of fusarium wilt of tomato by Pseudomonas fluorescens WCS417r and by nonpathogenic Fusarium oxysporum Fo47

Fluorescent pseudomonads and nonpathogenic Fusarium oxysporum have been sho wn to suppress fusarium wilts. This suppression has been related to both mi crobial antagonism and induced resistance. The aim of the present study was to assess the relative importance of syste mic induced resistance in the suppression of fusarium wilt of tomato in com mercial-like conditions by a reference strain of each type of microorganism (P. fluorescens WCS417r and nonpathogenic F. oxysporum Fo47). The spatial separation of the pathogen and the biocontrol strains excluded any possible microbial antagonism and implicated the involvement of the systemic induce d resistance; whereas the absence of any separation between these microorga nisms allowed the expression of both mechanisms. Since systemic induced res istance has often been associated with the synthesis of PR-proteins, their accumulation in tomato plants inoculated with WCS417r or with Fo47 was dete rmined. The analysis of the results indicates that the suppression of fusarium wilt by P. fluorescens WCS417r was ascribed to systemic induced resistance with out any detection of the PR-proteins tested (PR-1 and chitinases). In contr ast, the suppression achieved by nonpathogenic F. oxysporum Fo47 appeared t o be mainly ascribed to microbial antagonism but also to a lesser extent to systemic induced resistance. This induced resistance could be related to t he accumulation of PR-1 and chitinases. The possible relationship between the ability of Fo47 to suppress fusarium wilt more efficiently than WCS417r and its ability to show both mechanisms is discussed.