Factors Affecting the Racial Distribution of Pyricularia oryzae

II Comparison of Multiplication of Races C-1 and C-3 on Rice Varieties with Kanto 51-type Resistance

Abstract

Races C-1 and C-3 of Pyricularia oryzae were compared for their multiplying potential, i.e., time needed for fungus infection, number of susceptible lesions formed, sporulation in lesions, lesion size, and racial competition on two Kanto 51-type rice varieties, Tatsumimochi and Fukei 69, which are compatible to both races. Race C-3 produced fewer spores and smaller lesions than race C-1 on Tatsumimochi, but on Fukei 69 there was no difference between the two races in lesion size or number of spores produced. Tatsumimochi appears to have a race-specific factor that inhibits lesion expansion of race C-3. When the two varieties were inoculated with a mixture of spores of the two races, race C-3 was reisolated less frequently than race C-1 on Tatsumimochi, but on Fukei 69 the two races were reisolated with about an equal frequency. Race C-3 requires slightly shorter time for infection than race C-1 on both varieties. Apparently even in varieties that have no major genes for resistance against the prevalent races, there may be some race-specific factors that influence the distribution of the races. Antagonism and competition among races may also play a role in racial distribution.