Mode of Occurrence of Kasugamycin Resistant Rice Blast Fungus

Abstract

Kasugamycin sensitivity was evaluated of 207 isolates of Pyricularia oryzae, derived from 3 fields where the antibiotic had given poor blast control and also from 3 fields where the antibiotic still remained effective. From each of the former 3 fields, resistant isolates were found together with sensitive ones, but all isolates from the latter 3 fields were sensitive to the antibiotic. Isolates from each lesion on rice plants were either all resistant or all sensitive, and in no case resistant and sensitive isolates were found together in the same lesion. The frequency distribution of kasugamycin sensitivity among the 207 isolates, as evaluated by agar diffusion method, indicated the clear distinction between resistant and sensitive strains, and no intermediate resistant strains were detected. With regard to mycelial growth on agar plates, sporulation in vitro, number and length of lesions on inoculated leaves, no significant difference was found between sensitive group and resistant group. Frequency of the resistant clones among conidia from stock cultures did not vary within the range of selection pressure from 100 to 500μg/ml kasugamycin, but varied with strains, and with culture condition. A single spore culture just before the selection test led to a low frequency of resistant conidia, suggesting that the resistant clones might have occurred at least partly, by spontaneous mutation during stock culture.