Abstract
Alternaria kikuchiana TANAKA is specifically pathogenic to a few Japanese pear varieties such as Nijusseiki and Hakata-Ao causing the black spot disease, whereas it is weakly-or non-pathogenic to most of the Japanese pear varieties including Chojuro and Kikusui. It has already been reported that the fungus produces a toxin which is selectively toxic to the susceptible variety, the chemical nature of which is yet unknown.
The present paper deals with the identification of two toxic substances isolated from the mycelial mat and culture filtrate of the fungus and their toxicity to both susceptible and resistant varieties.
After the fungus was cultured on potato medium containing 2% sucrose for 18 days at 25-28°C, the dried mycelium was extracted with ether to give two compounds, which were also obtained from the cultural filtrate by chloroform extraction. By chemical and phyeical properties and by comparison with the authentic specimens kindly furnished by Dr. H. Raistrick, the compounds were identified as alternariol and its monomethyl ether. While the former was found inactive to the young leaves of Japanese pear, the latter caused round necrotic lesions to the young leaves of both the resistant and susceptible varieties. The acid (VIII) and phenol (IX) obtained from alternariol dimethyl ether by alkaline hydrolysis showed activity to the both varieties. These results indicate that both substances are not responsible for the selective toxicity of the fungus to the susceptible variety.
