Secondary metabolites released into the rhizosphere by Fusarium oxysporum and Fusarium spp. as underestimated component of nonspecific replant disease

A study was performed to investigate the role of fungal metabolites released into the rhizosphere of replanted orchards as a potential biotic component of tree growth decline. The phytotoxicity of the gamma ray-sterilized crude culture filtrates of sixteen fungal species originating from replanted apple orchards was tested in a bioassay. Low molecular weight compounds released by Fusarium spp. were analyzed. The fungal culture filtrates affected seedling growth and health with an activity that varied from growth inhibition to promotion. Three out of the six species of Fusarium tested produced species-specific mycotoxins such as equisetin and enniatin B and D (< 1 mu g ml(-1) and < 6 mu g ml(-1), respectively) associated with root-tip necrosis, whereas fusaric acid (80-230 mu g ml(-1)) was associated with asymptomatic plant growth inhibition. These findings were consistent with those obtained using pure compounds. Moreover, methoxyconidiol, paecilaminol, integrastatin B and other biologically active compounds, whose fungal origin and phytotoxicity have not yet been reported, were found. in all fungal filtrates. Findings suggest that i) phytopathogenicity of soil borne fungi can be expressed regardless of root infection; ii) a synergistic interaction between co-occurring mycotoxins and other biologically active compounds may explain plant growth inhibition. Iii) fungal metabolites released into soil may represent an underestimated component of nonspecific replant disease.