Toxicity of fungal endophyte secondary metabolites to plant parasitic nematodes and soil-borne plant pathogenic fungi

Fungi isolated from the cortical tissue of surface sterilized tomato roots collected from field plots produced secondary metabolites in nutrition broth that were highly toxic to Meloidogyne incognita. Especially strains of Fusarium oxysporum were highly active with 13 of 15 strains producing culture filtrates toxic to nematodes. The mechanism of action of the toxic metabolites produced by the non-pathogenic F. oxysporum strain 162 with proven biological control of M. incognita in pot experiments was investigated. These metabolites reduced M. incognita mobility within 10 min of exposure. After 60 min, 98% of juveniles were inactivated. Juveniles were initially inactivated within a few minutes of exposure, but with exposure of 5 h 50% of the juveniles were dead and 24 h exposure resulted in 100% mortality. In a bioassay with lettuce seedlings metabolite concentrations >100 mg/l reduced the number of M. incognita juveniles on the roots comparing to the water control. The F. oxysporum toxins were highly effective towards sedentary parasites and less effective towards migratory endoparasites. Non-parasitic nematodes were not influenced at all. Metabolites of strain 162 also reduced significantly the growth of Phytophthora cactorum, Pythium ultimum and Rhizoctonia solani in vitro. Secondary metabolites of endophytic fungi on plant-parasitic nematodes and soil-borne fungi should be considered for control of plant parasitic nematodes and plant pathogenic fungi. The results also show the need for proper selection of target nematodes in in vitro bioassays.