Abstract
Resistant strains of rice blast fungus against Kasugamycin (KSM) increased population density in fields after successive use of KSM, but their density became lower year by year after a ceasing from KSM application.
Then the behavior of sensitive and resistant strains in an infection process to rice seedlings was investigated for elucidating the cause of the decline of population density of resistant strain. Each lesion formed by a spray inoculation with a mixture of equal numbers of sensitive and resistant spores was found to consist of either the sensitive or the resistant strain alone, and the lesions colonized by resistant strains were less than those by sensitive strains. Therefore a resistant strain will have generally more inferior competitive ability in respect to infection than that of sensitive strain. There were no significant differences in the number of lesions per leaf between sensitive and resistant strains when spore suspension of each strain was separately inoculated to rice seedlings and they were kept in an incubation chamber for 24 hours, but percent infections by resistant strains were lower than that by sensitive strains when the incubation periods were 10 or 12 hours. And also the rates of appressorium formation of resistant spores after 8 hours were lower than that of sensitive spores. It seemed that the time necessary for appressorium formation and colonization by sensitive strains are shorter than that by resistant strains, and those time lag will cause inferiority of resistant strain in an infection process to rice seedlings.
