Characterization of the brown leaf spots pathosystem in Brazilian pecan orchards: pathogen morphology and molecular identification

Due to the increase in pecan nuts demand, plantation areas are expected to expand around the world and more frequent epidemics caused by fungal pathogens may occur in orchards and nurseries.  Ragnhildiana diffusa is a pathogenic fungus reported as causing brown leaf spots in pecan in Mexico, South Africa, and the U.S.A. The scarcity of comprehensive information in symptoms on the host and morphology of the fungus lead this disease to be initially incorrectly identified in Brazil. In this study, we employed different approaches to characterize the pathogen morphology and pathogenicity and to molecularly identify the organism causing brown leaf spots in 10 different orchards in southern Brazil. A phylogenetic analysis based on the ITS and the LSU gene sequences confirmed  R. diffusa  as the causal pathogen of the disease in all orchards. Inoculation tests on healthy leaflets confirmed that all sampled isolates were pathogenic, although some variation in their virulence was observed. Variation in the morphology of the asexual stage was observed among and within isolates. The accurate and prompt identification of the disease may assist controlling further spread of the pathogen into orchards and nurseries still free of the disease in South America.