Abstract
We monitored the appearance of new races of the rice blast fungus Pyricularia oryzae Cavara in paddy fields after the release of new resistant rice cultivars having the new true resistance gene, noting specific sites in the plants. We looked for new races on blast-infected panicles of the newly developed ricecvs. Haenuki and Domannaka after inoculation with incompatible races using an injection method. Fifteen isolates were collected from blast lesions on naturally infected panicles of the susceptible rice cv. Sasanishiki at eight locations including Tsuruoka city in Yamagata Prefecture. After single conidiaisolation, subcultured isolates were identified as incompatible races on newly developed rice cultivars. Conidial suspensions (105 spores/ml) of each isolate were injected into flag leaf sheath at the boot stage of the two new rice cultivars grown in 1/5000 a Wagnerpot. Each incompatible race of this fungus could infect and form brownish lesions on more than 95% of the panicles of cvs. Haenuki and Domannaka. Furthermore, sporulation was abundant enough to reisolate the fungus on hull lesion. Races of the single conidia reisolates from hull lesions of the two new rice cultivars were differentiated using Japanese differential rice varieties. About 75% of these reisolates changed to the original pathogenicity. Many of the isolates could overcome resistance of rice cultivars with true resistance genes, Pi-ta, Pi-i, Pi-z, Pi-k and Pi-km, individually or in combination. About 75% of the variants appeared in this study could break down the resistance the newly rice cvs. Haenuki and Domannaka. From these results, we concluded that the panicle of the new resistant rice cultivars was an important sites for new races appearance of the rice blast fungus.
