Resistance to Fungicides in Pyricularia oryzae

Abstract

Mutants resistant to fungicides were obtained by selecting a large number of conidia of rice blast fungus, Pyricularia oryzae, with respective fungicides. The mutation of resistance to kasugamycin and to organophosphorus thiolate (PTL) fungicides seemed to be spontaneous in most cases. In 1971, kasugamycin-resistant strains were found in the field and investigated. Identity of the kasugamycin resistance found in the field and that selected in the laboratory remained to be solved. In the investigation with laboratory-derived mutants, cross resistance was found among PTL fungicides and between PTL and isoprothiolane. Negatively correlated cross resistance was found between PTL and certain phosphoramidate (PA) compounds and between isoprothiolane and PA. This negative correlation was not found in most of the PTL-resistant field-isolates, and therefore the resistance in the field seemed different from that obtained in the laboratory in most cases. Fungal metabolism of PTL fungicides and a PA was investigated to elucidate resistance mechanism. Detoxifying metabolism of PTL as well as PA was lower in laboratory-derived PTL-resistant mutant and this may explain the higher sensitivity to PA, but the mechanism of resistance to TPL remains to be solved in this type of mutants. A higher rate of PTL metabolism was found in most of the PTL-resistant field-isolates and this may be a mechanism of the resistance found in the field.