Studies on host-specific AF-toxins produced by Alternaria alternata strawberry pathotype causing alternaria black spot of strawberry. 2. Role of toxins in pathogenesis.

Abstract

The role of AF-toxins in the interaction of A. alternata strawberry pathotype and its hosts, strawberry cv. Morioka-16 and Japanese pear cv. Nijisseiki, was investigated at the early stage of infection. Time course study of the production of AF-toxins from germinating spores elucidated that all three AF-toxins (I, II and III) were detectable early after inoculation and increased in quantity with incubation time. The leaves inoculated with pathogen were allowed to leak electrolytes by AF-toxin produced during the germination of spores. The electrolyte leakage was shown to have two peaks at different stages of the infection behaviors of the spores; the first was detected before the establishment of infection and the second was detected after the establishment of infection. The first peak suggests that the AF-toxins from germinating spores prior to fungal invasion could change the physiological potential related to the membrane system of the host cells to be desirable to the pathogen. The effect of AF-toxins on the establishment of infection by pathogenic spore was investigated by treating the leaves both the high molecular weight fraction “inducer” in germination fluids of spores and AF-toxins. The treatment with “inducer” rendered the susceptible leaves relatively resistant to the pathogen, while, the treatment with AF-toxins rendered the leaves susceptible even to non-pathogenic A. alternata. Concomitant treatment with the “inducer” and AF-toxins resulted in the suppression of the induction of host resistance. These data suggest that pathogenic spores of A. alternata strawberry pathotype produce AF-toxins during their germination, and thereby may suppress the resistance response through the disturbance of membrane integrity of the host cells, and consequently succeed in establishing infection.