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.

Triazole fungicides have not been effective for managing the wheat blast disease in Brazil. A broad analysis across six geographical populations of Pyricularia graminis-tritici in central-southern Brazil indicated a high level of resistance to triazole fungicides. Since P. graminis-tritici is also associated with others poaceous species, here, we analyzed whether triazole-resistant isolates of the blast pathogen could be recovered from other poaceous hosts that are invasive of sprayed wheat fields. In addition to P. graminis-tritici (Pygt), we also evaluated the levels of sensitivity of three other grass-associated blast pathogens, which included P. grisea (Pg), P. pennisetigena (Pp), and P. urashimae (Pu). Resistance to the triazole fungicides tebuconazole and epoxiconazole was assessed phenotypically based on EC50 values and molecularly by analysis of the presence of mutations in the CYP51A gene, which encodes for the target enzyme 14-alpha-demethylase. We detected triazole-resistant Pyricularia spp. (Pg, Pp, Pu and Pygt) that is associated with Avena sativa, Cenchrus echinatus, Chloris distichophylla, Cynodon sp., Digitaria horizontalis, D. sanguinalis, Panicum maximum or Urochloa spp. The major outcome from our study was the evidence that invasive poaceous species from wheat fields could be an important source of triazole resistant fungal inoculum for the initial phases of the wheat blast epidemics.

The fungus Rhizoctonia solani anastomosis group (AG)-1 IA emerged in the early 1990s as an important pathogen causing foliar blight and collar rot on pastures of the genus Urochloa (signalgrass) in South America. We tested the hypothesis that this pathogen emerged following a host shift or jump as a result of geographical overlapping of host species. The genetic structure of host and regional populations of R. solani AG-1 IA infecting signalgrass, rice, and soybean in Colombia and Brazil was analyzed using nine microsatellite loci in 350 isolates to measure population differentiation and infer the pathogen reproductive system. Phylogeographical analyses based on the microsatellite loci and on three DNA sequence loci were used to infer historical migration patterns and test hypotheses about the origin of the current pathogen populations. Cross pathogenicity assays were conducted to measure the degree of host specialization in populations sampled from different hosts. The combined analyses indicate that the pathogen populations currently infecting Urochloa in Colombia and Brazil most likely originated from a population that originally infected rice. R. solani AG-1 IA populations infecting Urochloa exhibit a mixed reproductive system including both sexual reproduction and long-distance dispersal of adapted clones, most likely on infected seed. The pathogen population on Urochloa has a genetic structure consistent with a high evolutionary potential and showed evidence for host specialization.