Studies on host-specific AF-toxins produced by Alternaria alternata strawberry pathotype causing alternaria black spot of strawberry. 1. Production of host-specific toxins and their biological activities.

The causal pathogen of Alternaria black spot of strawberry was identified as a distinct pathotype of Alternaria alternata (Fries) Keissler. Only one strawberry cultivar Morioka-16 was susceptible to the pathogen. Several cultivars of Japanese pear including Nijisseiki which were susceptible to A. alternata Japanese pear pathotype were also diseased by this pathogen. This fungus produced at least three species of host-specific toxins (named AF-toxin I, II and III, respectively). AF-toxin I and II were considered the major toxins. AF-toxin I induced necrosis and potassium leakage on the susceptible cvs. Morioka-16 and Nijisseiki. AF-toxin II induced only potassium leakage from cv. Morioka-16 but did not induce necrosis. AF-toxin II as well as AF-toxin I caused necrosis and potassium leakege from the cv. Nijisseiki.