Higher fitness and competitive advantage of Pyricularia oryzae Triticum lineage resistant to QoI fungicides

Abstract

Background: Quinone outside inhibitor (QoI) fungicides have not been effective in controlling the wheat blast disease [Pyriculariaoryzae Triticum lineage(PoTl)] in Brazil. Thefirst report of resistance of PoTl to QoIs in this country occurred in 2015. This study aimedto test hypotheses about the changes infitness parameters and competitive advantageof the QoI-resistant (R) PoTl isolate groupcompared to the sensitive (S) isolate group. Mycelial growth on PDA medium andin vivoconidial production, incubation periodand disease severity were analyzed asfitness parameters. The competitive ability wasmeasured on wheat leaves and heads inocu-lated with mixtures of R:S isolates at the following proportions: 0S:100R, 20S:80R, 50S:50R, 80S:20R, 100S:0R, and 0S:0R.Results: The QoI-R isolate group had significantly higherfitness than the sensitive isolate group, considering bothin vitroandin vivoparameters. The highestin vivoconidial production on wheat leaves and the highest leaf and head disease severity weredetected when resistant strains were predominant in the isolate's mixtures (20S:80R or 0S:100R proportions), in the absence offungicide pressure. Conidia harvested from wheat blast lesions on leaves inoculated with 20S:80R and 0S:100R mixtures wereresistant to QoIsin vitroassays based on discriminatory doses of the fungicide.Conclusion: Therefore, QoI resistance facilitated a higherfitness and a competitive advantage in PoTl, which contrasts with theevolutionary theory that associates afitness cost to fungicide resistance. We discuss the evolutionary and ecological implica-tions of the higherfitness as found in the fungicide-resistant adapted populations of the wheat blast pathogen.