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It has been well documented that blast fungus (Pyricularia oryzae Cavara) loses its ability of lesion formation on rice plant during serial subculture on PSA1). It was anticipated that the accumulation of many observations on such pathogenicity-lost (avirulent) isolates may provide valuable informations on the elucidation of aggressiveness and pathogenicity of P. oryzae. Therefore, in this study, it was investigated whether or not these avirulent isolates possess the ability of the appressorium formation, insertion of infection hypha and/or the development of hyphae from firstly invaded cells. Further, the effect of thermal and cycloheximide (CY) treatments before inoculation on the behaviors of avirulent blast fungus was investigated. The abstract was partly repored elsewhere2). The rice cultivars, Shin 2, Aichi-asahi and Kanto 51, and fresh onion were used. Each cultivar was raised as described previously3). Virulent isolate Hoku 1 (race 007) and avirulent isolates Naga 69-73 and Ken 62-78 of P. oryzae was used. All isolates were grown on oat meal medium, and their conidia were obtained as described previously3). Penetration and hyphal growth of P. oryzae in leaf-sheaths treated with heat and CY were determined by Takahashi's method4). As shown in Table 1, over 90% of conidia of two avirulent isolates (Naga 69-73, Ken 62-78) germinated well and formed appressorium on the sheath tissues of the cv. Shin 2 24hr after inoculation. Although there was difference in rate of infection hypha formation between virulent and avirulent isolate, appressoria of avirulent isolates inserted infection hyphae into the sheath cells. The growth of infection hyphae of avirulent isolates was very poor into the rice cells. When these two isolates were inoculated to the nine differential cultivars, no lesion formation was observed on the rice leaves of the all cultivars. Then, in order to elucidate such invading ability of avirulent isolates

Effect of thermal and cycloheximide treatments on infection behaviors of avirulent Pyricularia oryzae Cavara in rice leaf-sheath.