1986 Volume 52 Issue 1 Pages 39-46
A series of experiments were conducted to clarify the reason why the invading hyphae of Diaporthe citri (Faw.) Wolf is arrested and killed in melanose spot. After a melanose spot on citrus leaf was formed, pycnospore suspension of D. citri with 2% fructose was inoculated onto the melanose spot. The pycnospore germination was markedly inhibited and the germination rate was only 7%. On the healthy part of the leaf, the germination rate was 98%. The extract of melanose spot inhibited the pycnospore germination of D. citri. An antifungal substance was isolated from the extract of melanose spots by the chromatography on silica gel G layer. It was then crystallized from acetone-water, and needle-like crystal was obtained and named inhibitor D. Inhibitor D was not detected on the chromatogram of the extract of uninfected leaves and peels of citrus plant. It means that inhibitor D was produced in the plant post infectionally, and may be a phytoalexin. The extract of uninfected leaf also inhibited the pycnospore germination of D. citri. It seems that the inhibitory effect of the extract of uninfected leaf is caused by the action of pre-inhibitins such as citrinol, naringin and hesperidin in the uninfected tissues of citrus plant. These results explain the reason why the invading hyphae of D. citri into the citrus leaf and peel was arrested and then killed; namely, it is due to the antifungal action of pre-inhibitins and the phytoalexin-like substance. Therefore, this phenomenon seems to be a chemical defense reaction in citrus plant against invasion of pathogenic fungus. The phytoalexinlike substance was also detected from scab spot, canker spot, star melanose and oleocellosis on citrus fruits.