Studies on host-selective infection mechanism of Pyricularia oryzae Cavara. (3). Light-dependency of leaf necrosis formation by toxin(s) from germinating spores.

In 1965, Sekiguchi and Furuta1) reported about a mutant rice plant, cv. Sekiguchi-asahi originated from cv. Asahi, showing unique response to Cochliobolus miyabeanus or chemicals. When spores of Pyricularia oryzae were inoculated to rice cv. Sekiguchi-asahi, large orange to orange brown lesion, which was named •gSekiguchi lesion•h by Kiyosawa, was formed on rice leaves2,3). Further, in 1983, Marchetti et al.4) also reported the occurrence of the •gSekiguchi lesion•h in American rice lines. Recently, we reported that such highly sensitive response of cv. Sekiguchi-asahi was useful for detection of disease determinants in rice blast3). In fact, Arase et al.5) demonstrated that P. oryzae produced toxin(s) which has necrosis-inducing activity during spore germination by using this rice cultivar. When these toxins were applied to the wounded sites on the rice leaves and were kept under natural diffuse-light condition in the laboratory, •gSekiguchi lesion•h-like necrosis was formed. Such biological activities of the toxin(s), however, were not recognized in non-host plants (i.e. cucumber, pear, ragi and tomato) of P. oryzae. These results suggest strongly that the toxin(s) is playing an important role in infection