Viability staining of soybean suspension-cultured cells and a seedling stem cutting assay to evaluate phytotoxicity of Fusarium solani f. sp. glycines culture filtrates

The phytotoxicity of culture filtrates of Fusarium solani f. sp. glycines, the fungus causing sudden death syndrome (SDS) of soybean (Glycine max), wa s tested with a viability stain of soybean suspension-cultured cells and a stem cutting assay of soybean seedlings. Suspension-cultured cells from a S DS-susceptible soybean cultivar were exposed to cell-free culture filtrates of F. solani f. sp. glycines or other F. solani isolates for 2, 4, 6, and 8 days and then stained with 0.1% phenosafranin. The percentage of dead soy bean suspension-cultured cells was greater (P < 0.001) with filtrates prepa red from F. solani f. sp. glycines than from other F. solani isolates, and dead cells increased over time and with higher concentrations of culture fi ltrate. Cuttings of soybean seedlings with their stems immersed in culture filtrates of F. solani f. sp. glycines isolates developed SDS-like foliar s ymptoms, but not when immersed in filtrates of other isolates. There was a positive correlation (r = 0.94, P < 0.001) between soybean foliar symptom s everity and percentage of stained soybean suspension-cultured cells. Both m ethods were used to determine the phytotoxicity of fungal culture filtrates .