Sensitivity to fungicides in Pyricularia populations from wheat and signal grass in Minas Gerais

Wheat blast, caused by Pyricularia oryzae Triticum pathotype, is a damaging disease of wheat in the Brazilian Cerrado. Fungicides play an important role in management, but their efficacy has been variable. Failures in disease control have been associated with the presence of fungicide resistant populations. To determine if these resistance patterns are also present in Pyricularia populations infecting wheat and signal grass from MG state, we characterized phenotypically and genotypically the resistance of the blast pathogen against seven fungicides belonging to three chemical groups (DMI, QoI and SDHI). The in vitro sensitivity was assessed in 64 isolates obtained from the Triângulo Mineiro and Sul de Minas. For DMIs, the EC 50 was estimated for all isolates. For QoIs and SDHIs, the EC 50 was obtained for 13 isolates prior to determining a discriminatory dose. Control efficacy of each fungicide was also evaluated in vivo upon inoculation of wheat heads with a resistant (R) or a sensitive (S) PoT isolate (determined in vitro) to each fungicide. The presence of mutations in the genes encoding the target proteins of the different groups of fungicides was also evaluated. For the DMIs (TEBU - tebuconazole and EPOX - epoxiconazole), despite showing relatively high EC 50 values for some isolates, the control efficacy on wheat heads was high. We found no association between mutations in the cyp51A region and resistance to DMI. For QoIs, azoxystrobin (AZOX) did not inhibit the in vitro germination of PoT even with an extra dose of 100 µg/ml, and was poorly controlled in vivo (~ 20%). For pyraclostrobin (PYRA), despite the presence of isolates with high leves of spore germination in vitro using a discriminatory dose of 1.3 µg/ml, the control efficacy was also high. The G143A substitution was present in all PoT cytB sequences, even in those of the PYRA sensitive isolates. The non-PoT isolates did not present mutations in the cytB hot spots. For SDHIs, the most fungitoxic in in vitro and in vivo experiments was benzovindiflupyr, followed by bixafen and fluxapyroxad. Molecular analysis of the sdh subunits showed no relationship between mutations and sensitivity to SDHI fungicides. For the seven fungicides tested in this work, the PoT isolates were statistically less sensitive than the isolates from signal grass. Our results demonstrate that PoT populations can be controlled in vivo with at least one fungicide of each of the three chemical groups evaluated. Contrary to previous statements, determining EC 50 values in in vitro experiments was not sufficient to infer loss of efficacy in in vivo experiments. Benzovindiflupyr was the most effective active ingredient evaluated in this work. Conversely, azoxystrobin should not be considered for controlling wheat blast. Keywords: Wheat blast. Fungicide resistance. Epidemiology. Magnaporthe oryzae. Pyricularia oryzae.