True resistance genes harbored by rice cultivars on pathogenic variation of rice blast fungus re-isolated from panicles of resistant rice cultivars

摘要

This study was carried out to clarify the effect of true resistance genes in rice cultivars on the occurrence of pathogenic variants of the rice blast fungus, Pyricularia oryzae Cavara, during proliferation of avirulent isolates on panicles of the rice plants using isogenic lines of the Sasanishiki multiline. Re-isolates from blast-infected panicles of five rice cultivars and two lines which harbored different true resistance genes were tested to determine the Japanese races. The number of blast races and the rate of variation in the re-isolates from panicles of resistant rice cultivars were clearly higher than those from the susceptible rice. Pathogenic variation of the rice blast fungus was stimulated when the avirulent isolate proliferated on the panicles of resistant rice cultivars. Some pathogenic variants could break down the same true resistant genes in the re-isolates from the panicle of rice cultivars. However, many variants could break down other rice cultivars with different true resistance genes. From these results, the direction of pathogenicity of variants of this fungus was not influenced by the kind of true resistant genes in rice cultivars. The variants which could overcome the Pi-ta, Pi-i and Pi-z genes appeared frequently, whereas those overcoming the Pi-k and Pi-b genes were found less often. However, few or no variants could break down the Pi-k~m, Pi-ta~2 and Pi-z~t genes. Three types of avirulence genes in rice blast fungus are, therefore, considered likely, those which were highly unstable, moderately unstable or relatively stable.