Fitness Studies Between Field Isolates of Magnaporthe oryzae Resistant and Sensitive to Scytalone Dehydratase Inhibitors of Melanin Biosynthesis (Mbi-Ds) and Decline of Frequency of Resistant Isolates at a Rice Field

Abstract

Fitness of isolates of Magnaporthe oryzae resistant to scytalone dehydratase inhibitors of melanin biosynthesis (MBI-Ds) was studied in comparison with sensitive isolates. We found no significant differences in pathogenicity between sensitive and resistant isolates. In competition studies, sensitive isolates became dominant after four to six successive inoculations from the diseased rice seedlings infected by both sensitive and resistant isolates, irrespective of the initial ratio of these isolates. No revertants were detected after ten successive inoculations from diseased rice seedlings. Besides, at one rice paddy field, the frequency of resistant isolates declined dramatically after stopping the MBI-Ds applications. These results support our hypothesis that resistant isolates have lower fitness than sensitive isolates.