1986 Volume 52 Issue 3 Pages 428-436
Chemically characterized pure host-specific toxins produced by Alternaria alternata strawberry pathotype (AF-toxins) were examined for their biological activities. AF-toxin I induced even at concentrations as low as 3.2×10-8M leaf veinal necrosis on both leaves of susceptible cv. Morioka 16 of strawberry and cv. Nijisseiki of Japanese pear. AF-toxin II was found to cause veinal necrosis only on leaves of the susceptible Japanese pear cv. Nijisseiki with a minimum concentration of 2.6×10-8M. AF-toxin III was toxic to susceptible strawberry leaves at 3.1×10-7M, and also toxic to Nijisseiki pear leaves at considerable high concentrations more than 1.6×10-5M. At concentrations up to 10-4M, there was no effect observed in any of the experiments with the strawberry cv. Hoko-wase and Japanese pear cv. Chojuro, immune to the disease. We also examined the ability of AF-toxin II to protect strawberry leaf tissues from AF-toxin I action and fungal infection. Detached leaves were exposed to AF-toxin II for 18hr, and then treated with AF-toxin I. AF-toxin II gave a remarkable protection against AF-toxin I-induced veinal necrosis and potassium leakage. Minimum ratio of AF-toxin II: AF-toxin I giving 80% protection was approximately 10:1. Pre-treatment with AF-toxin II reduced significantly the number of black necrotic spots on inoculated susceptible cv. Morioka-16 of strawberry. The present experiment provided the first evidence of protecting plant from disease by means of counteracting toxin action by a chemical structurally related to the host-specific toxin.