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.

Management of melanin biosynthesis dehydratase inhibitor (MBI-D)-resistance in <i>Pyricularia oryzae</i> using a non-MBI-D fungicidal application program for nursery boxes and a diclocymet and ferimzone mixture for field foliar applications

“…Suzuki et al (2010) monitored the emergence frequency of resistant isolates from fields in Kyushu after MBI-D applications were stopped and showed that the frequency declined. 6) Kimura and Fujimoto (2015) reported that the emergence frequency of resistant isolates decreased rapidly without the presence of MBI-Ds in laboratory and field tests. 7) Kimura and Fujimoto (2017) also reported that the resistant SH activity was lower than the sensitive SH activity.…”

“…6) Kimura and Fujimoto (2015) reported that the emergence frequency of resistant isolates decreased rapidly without the presence of MBI-Ds in laboratory and field tests. 7) Kimura and Fujimoto (2017) also reported that the resistant SH activity was lower than the sensitive SH activity. 8) Previous reports suggested that P. oryzae isolates resistant to MBI-Ds paid a fitness penalty to acquire the resistance.…”

“…Thereafter, the frequency of MBI-D-resistant isolates declined annually, and by 2004, the frequency was approximately 1%. 7) Pyricularia oryzae isolates were collected from panicle blast lesions in a field using a previously published method. 7) For the Ehime trial, the chemicals used are shown in Table 1 and were purchased from an agricultural cooperative.…”

“…7) Pyricularia oryzae isolates were collected from panicle blast lesions in a field using a previously published method. 7) For the Ehime trial, the chemicals used are shown in Table 1 and were purchased from an agricultural cooperative. All granule products were applied in nursery boxes at planting time.…”

MBI-D resistance management of <i>Pyricularia oryzae</i> using an application program incorporating benomyl

“…7) These previous reports suggested that MBI-D-resistant isolates paid a fitness penalty for acquiring their resistance and that sensitive isolates would be dominant without MBI-D pressure. However, the re-use of MBI-Ds has not been implemented in the area where resistant isolates were widely found.…”