Changes in Fungicide Resistance Frequency and Population Structure of Pyricularia oryzae after Discontinuance of MBI-D Fungicides

Abstract

The changes in fungicide resistance frequency and population structure of the rice blast fungus Pyricularia oryzae were monitored after the discontinuance of melanin biosynthesis inhibitor targeting scytalone dehydratase (MBI-D) fungicides use in Saga Prefecture, Japan. After discontinuance in 2003, the frequency of resistant isolates decreased from 71.8% in 2002 to 25% in 2003, and became undetectable in 2007. The initial marked decrease was due to a decline of isolates possessing the predominant haplotype, although the haplotypic diversity among resistant isolates remained high from 2003 to 2005. These results revealed that resistant isolates were less fit in comparison with sensitive isolates in the absence of MBI-D fungicide pressure under field conditions. Pairwise FST values indicated that the change in population structure after MBI-D discontinuance was explainable by a rapid change in the proportions of resistant and sensitive subpopulations. Depending upon the existence of fitness cost and rapid changes in population structure, it may be possible to reintroduce MBI-D fungicides in areas where resistance has already developed, although we speculate that fitness cost related to MBI-D resistance may be small based on our present results and previous findings.

Action mechanism of the novel rice blast fungicide tolprocarb distinct from that of conventional melanin biosynthesis inhibitors

“…However, evolving resistance to some cMBIs that inhibit dehydration steps has become a problem in rice blast control as well as other plant diseases. 20) Tolprocarb, which is being developed by Mitsui Chemicals Agro, Inc., showed a high controlling effect on rice blast by both nursery box application and broadcasting on paddy rice (data not shown), and its practical value has been verified. Therefore, in considering the possibility of cross-resistance, clarifying tolprocarb's target site becomes of paramount importance.…”

“…However, developing resistance to some cMBIs that inhibit dehydration steps has been a problem in controlling rice blast and other plant diseases. 20) For effective disease control, fungicides having different modes of action are required.…”

Synthesis and activities of tolprocarb derivatives against <i>Pyricularia oryzae</i>: relationships among the activities for polyketide synthase, melanin biosynthesis, and rice blast

“…However, the evolution of resistance to conventional MBIs that inhibit dehydratase has become a problem for the control of rice blast. 16) Tolprocarb developed by Mitsui Chemicals Agro, Inc., showed a high controlling effect on rice blast by both nursery-box application and broadcasting on paddy rice. 19) In our previous paper, 20) we demonstrated that tolprocarb inhibited PKS activity with an IC 50 =0.03 µM.…”

“…[13][14][15] However, the development of resistance to conventional MBIs that inhibit dehydratase has posed a problem for controlling rice blast and other plant diseases. 16) Therefore, fungicides with different modes of action are required to combat the development of resistance to the currently used fungicides.…”

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

“…Suzuki et al monitored the emergence frequency of resistant isolates in fields in the Kyushu area after ceasing MBI-D applications and showed that the frequency had declined. 6) This indicates that after several years with no applications, the resistant populations decreased. Thus, MBI-Ds may again provide adequate disease control.…”