Unraveling the local origin of the wheat blast pathogen (Magnaporthe oryzae) in Central-Southern Brazil and evidence for the emergence of resistance to Strobilurin fungicides in the pathogen populations

Abstract

In this research proposal we will study the origins of the Ascomycetous fungus Magnaporthe oryzae (anamorphase Pyricularia oryzae), which has a worldwide distribution as a rice pathogen but in the last century emerged as an important pathogen adapted to wheat in southern Brazil and has spread to other countries that crop wheat in South America. Our main goal is to unravel the emergence of populations of M. oryzae adapted to wheat in Central-Southern Brazil. We aim to answer the following questions: From where have the wheat-infecting populations of M.oryzae originate? Our hypothesis is that the wheat-infecting populations of M. oryzae in Brazil had a "de novo" origin from endemic populations of the pathogen that infect other native Poaceae species (invasive to wheat areas) in the country. This question will be addressed by comparing sympatric populations of M. oryzae adapted to both wheat and to native Poaceae species looking for evidence of lack of genetic subdivision and specialization among these host populations of M. oryzae. We aim also to elucidate the population genetic structure of the wheat blast pathogen in Brazil on a large geographic scale. Our main goal is to answer the following questions: Are geographically distinct populations of M. oryzae from wheat genetically subdivided? How is gene and genetic diversity distributed among the regional populations of M. oryzae, 25 years after the first epidemics of wheat blast in Brazil? What is the predominant reproductive system of the wheat-infecting M. oryzae in the country? (i.e., is it essentially clonal, primarily sexual, or mixed?). To address these questions we will compare M. oryzae populations sampled from wheat fields in eight states in Central-west, Southeast and Southern Brazil to test the hypothesis that there is no subdivision between populations and that the pathogen has a mixed reproductive system. Using classical measures of population genetic parameters (such as the magnitude and extent of gene flow and the inference about the fungal predominant reproductive system) we intend to estimate the evolutionary potential of M. oryzae as an important wheat pathogen in Brazilian agroecosystems.Finally, as an applied aspect of this research, we will seek evidence that resistance to strobilurin fungicides has emerged in wheat infecting populations of M. oryzae. The emergence of resistance to strobilurin is considered a common phenomenon worldwide, but there is no information that is has already happened on Brazilian populations of wheat-infecting pathogens, including M. oryzae. We will test the hypothesis that the wheat-infecting populations of M oryzae from south-central Brazil remains sensitive to strobilurine fungicides or the alternative hypothesis that the use of the fungicide for several years has led to the emergence of fungicide resistance. Using a PCR-RFLP marker, we will investigate the occurrence of the G143A mutation in the cytochrome b gene conferring resistance to the QoI fungicide strobilurin. We will also determine whether the resistance has been selected multiple times, independently, as a result of the widespread use of strobilurin on wheat. It will be determined whether the G143A mutation at the cyt b is present in more than one genetic antecedent of the pathogen. (AU)