Phytotoxicity of Fusarium solani culture filtrates from soybeans and other hosts assayed by stem cuttings

Fusarium solani infects roots of a number of different plant species and some strains produce phytotoxins. F. solani f. sp. glycines, the causal organism of sudden death syndrome (SDS) of soybean (Glycine max), colonises soybean roots and produces toxin(s) that are translocated to leaves and cause intervienal chlorosis and necrosis. Several experiments evaluated the phytotoxicity of cell-free culture filtrates of F. solani f. sp. glycines by immersing cuttings of soybean seedlings into filtrates to determine what in vitro growth conditions alter the phytotoxicity, and to determine the specificity of toxicity of F. solani f. sp. glycines and other F. solani on cuttings of soybean and other legume species. Foliar disease severity ratings of soybean cuttings in cell-free culture filtrates diluted 25- and 50-fold were higher than when diluted 100-fold or more. Cell-free culture filtrates originating from cultures grown at 15, 20, and 25°C caused greater (P = 0.05) foliar disease severity ratings on cutting than when the fungus was grown at 30°C. Cell-free culture filtrates of F. solani isolates from cucumber (Cucumis sativus) and pumpkin (Cucurbita pepo) did not cause symptoms on soybean cuttings whereas filtrates of F. solani isolates obtained from other hosts caused some leaf chlorosis and/or necrotic spots. F. solani f. sp. glycines inoculated on eight legume species caused symptoms such as leaf chlorosis, defoliation, wilt or death on most of the species, but azuki bean (Vigna angularis), common pea (Pisum sativum) and vetch (Vicia sativa) were symptomless. Cell-free culture filtrates of F. solani f. sp. glycines caused foliar symptoms on all cuttings of legume species except for mung bean (Vigna radiata), although none of the symptoms matched the SDS symptoms observed on soybean foliage.