Sensitivity of non-exposed and exposed populations of Magnaporthe oryzae from rice to tricyclazole and azoxystrobin

Magnaporthe oryzae is the major pathogen of cultivated rice worldwide, which can cause substantial losses to rice production. Rice blast management is based predominantly on the application of fungicides, however, only little is known about responses of pathogen populations to the most widely used fungicides. In this work, the baseline sensitivity of the Italian M. oryzae population to tricyclazole and azoxystrobin in terms of mycelium growth was determined, and the possible adaptation of the pathogen population after several years of repeated exposure to fungicide treatments was evaluated. All the analyzed strains demonstrated higher sensitivity and variability to azoxystrobin (ED50 = 0.063 mg L−1) than to tricyclazole (ED50 = 99.289 mg L−1). After comparing two additional populations, collected from fields repeatedly treated with fungicides, to the baseline, no decrease in sensitivity towards these fungicides was observed and no resistant strains were detected. The shift of the pathogen sensitivity towards these fungicides has not occurred, although we observed slightly increased variance associated to ED50 of azoxystrobin. Therefore, both azoxystrobin and tricyclazole can be used to manage rice blast in Italy, but it will be important to continue monitoring M. oryzae population to early detect possible azoxystrobin resistance.