PRODUCTION OF A HOST-SPECIFIC TOXIN BY GERMINATING SPORES OF ALTERNARIA-TENUISSIMA CAUSING LEAF-SPOT OF PIGEON PEA

Production of a host-specific toxin by Alternaria tenussima, the cause of pigeon pea leaf spot, was investigated in spore-germination fluids (SGF). The SGF selectively induced necrosis on pigeon pea leaves in a detached leaf assay. Necrotic lesions were observed when a toxin from SGF was applied onto detached young leaves of the pigeon pea cultivar Bahar at a concentration as low as 5 ng/ml. The resistant line Tanzania and nonhosts tolerated at least 20,00. times higher concentration of the toxin. The differential activity of the toxin on hosts and nonhosts of the fungus, as well as on susceptible and resistant cultivars or lines, suggested host-specific property of the toxin. At a concentration of 10 ng/ml, the toxin induced susceptibility of pigeon pea leaves to a non-pathogenic isolate of Alternaria alternata. The toxin possibly plays a role as a disease determinant of A. tenuissima, because the toxin was released from germinating spores as early as 3 h of incubation and the amount detected within 9 h was about 6 times of the concentration required for necrotic toxicity.