Widespread distribution of resistance to triazole fungicides in Brazilian populations of the wheat blast pathogen

Abstract

Fungicides have not been effective in controlling the wheat blast disease in Brazil. An earlier analysis of 179 isolates of Pyricularia oryzae Triticum lineage (PoTl) sampled from wheat fields across six populations in central‐southern Brazil during 2012 discovered a high level of resistance to strobilurin fungicides. Here we analysed azole resistance in the same strains based on EC50 measurements for tebuconazole and epoxiconazole. All six Brazilian populations of PoTl exhibited high resistance to both azoles, with in vitro EC50 values that were at least 35 to 50 times higher than the recommended field doses. We sequenced the CYP51A and CYP51B genes to determine if they were likely to play a role in the observed azole resistance. Although we found five distinct haplotypes in PoTl carrying four nonsynonymous substitutions in CYP51A, none of these substitutions were correlated with elevated EC50. CYP51B was sequenced for nine PoTl isolates, three each representing low, medium, and high tebuconazole EC50. Both PoTl CYP51A and CYP51B could complement yeast CYP51 function. All PoTl CYP51A‐expressing yeast transformants were less sensitive to triazoles than the PoTl CYP51B ones. Transformants expressing PoTl CYP51A haplotype H1 carrying the R158K substitution were not more resistant than those expressing PoTl CYP51A haplotype H5, which is synonymous to haplotype H6, found in triazole‐sensitive P. oryzae Oryza isolates from rice blast. Therefore, the reduced triazole sensitivity of wheat blast isolates compared to rice blast isolates appears to be associated with a non‐target‐site related resistance mechanism acquired after higher exposure to triazoles.

Taxonomic and pathogenic diversity of the blast pathogen populations infecting wheat and grasses in Minas Gerais

“…Similar to our study, PoT and non-PoT isolates have also been isolated from alternative hosts growing nearby wheat fields, further suggesting that cross-host infection is common in the field, and that the alternative host could serve as a source of inoculum for epidemics in wheat. However, it is possible that, at the time, the authors of those studies (Castroagudin et al 2017;Ceresini et al 2018Ceresini et al , 2019Dorigan et al 2019;Poloni et al 2021) have either failed to correctly identify the pathotypes occurring in the alternative hosts, for example: false positive for PoT infecting grass, or, they have indeed sampled the PoT strains from other hosts, particularly from signal grass, growing nearby wheat fields from other regions than Minas Gerais. The testing of those isolates using the C17 assays could be instructive to shed light on this issue.…”

“…The meta-analysis study showed no evidence of decline in fungicide efficacy over nine years (2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020) for any of the six fungicides evaluated (Ascari et al 2021). However, in vitro studies detected Brazilian blast population carrying mutation points that confer resistance to strobilurins and triazoles fungicides (Castroagudín et al 2015;Dorigan et al 2019;D' Ávila et al 2020;Poloni et al 2021). The efficacy of control of chemicals varied depending of the region, as wheat blast control and yield return were larger in subtropical climate than tropical conditions, differing by at least 18 percent points (40.6 to 58.9%) and 150.7 kg/ha (227.4 to 378.1 kg/ha), respectively.…”

Sequential post-heading applications for controlling wheat blast: a summary of fungicide performance

“…Additionally, the use of multi-site fungicides is important for managing fungicide resistance (FRAC-BR 2021). In fact, less sensitive PoT populations to QoIs and DMIs in Brazil have been reported, associated with mutations in cytochrome b gene (mainly G143A substitution) (Castroagudín et al 2015) or CYP51 gene (several mutation points) (Dorigan et al 2019;Poloni et al 2021). However, using a relatively long time series we did not find evidence of a decline in fungicide efficacy over the years for all commercial premixes amended with QoI as well as TEBU applied solo.…”

“…If that happens to be true and confirmed by field research, it is possible that the DMI component is responsible for the control. Further research in this area will be important to contribute to the debate around the risk associated with the use of strobilurins for wheat blast control (Castroagudín et al 2015;Oliveira et al 2015;Poloni et al 2021).…”

Droplet-based microfluidics platform for antifungal analysis against filamentous fungi

“…Several fungicides are used to control the disease, to increase productivity, and to improve the storage life of harvested crops. Intensive use of site-specific fungicides leads to the development of fungicide resistance in A. alternata 5 , 6 and in various other fungi 7 – 9 which necessitates the discovery of new antifungal agents. Discovery and development of new fungicides face great challenges including (a) screening of large libraries of antifungal candidates to evaluate their efficacy, phytotoxicity, and other possible effects, (b) high cost of product development driven by the extensive studies, and (c) complex in vitro and in vivo experiments 10 , 11 .…”