Studies on host-selective infection mechanism of Pyricularia oryzae Cavara. (2). Production of susceptibility-inducing factor(s) from germinating spores and their phytotoxicity.

The factor(s) stimulating Pyricularia oryzae infection on rice leaves were detected by using a rice cv.Sekiguchi-asahi as a test plant.Factors were detected and partially purified from spore germination fluids of virulent P. oryzae isolates.Alternaria alternata spores, a non-pathogen of rice plants, formed numerous necrotic spots on rice leaves when inoculated with the factors, but not on several other non-host plants.The factors from P. oryzae were also found to be highly toxic to rice cv.Sekiguchi-asahi, and were extracted with ethyl acetate.The factors induced a characteristic necrosis on leaves of cv.Sekiguchi-asahi as those observed on inoculated leaves, but did not cause any symptom on leaves of cv.Shin 2. Furthermore, they inhibited strongly root growth of seedlings of cvs.Sekiguchi-asahi and Shin 2. However, phytotoxic metabolites such as tenuazonic acid, pyriculol and pyriculariol isolated from the culture filtrates of P. oryzae were not detected in the spore germination fluids.These results indicate that spores of P. oryzae release a new type of toxin(s) having susceptibility-inducing ability during germination.(