Occurrence and pathogenicity of Puccinia coronata var avenae f. sp. avenae on oat in South Africa

Crown rust, caused by Puccinia coronata Corda var. avenae f. sp. avenae (Urban & Markovd) (Pca), can negatively impact oat (Avena sativa L.) production in South Africa and little is known about pathogenic variability and cultivar response. Pca infected leaves were obtained from oat trial plots, commercial fields and wild oat (Avena spp.) during the 2016 and 2017 growing seasons for race analysis. Uncharacterised collections from 1998 to 2005 were also included. Single pustule isolates were derived from field collections and used for race typing on 16 international differential lines. Six races were identified from 88 isolates pathotyped with race JBLL originating from wild oat samples, SGLL, SQPL and SSTL from commercial oat samples and SBLL+ from both wild and commercial oat samples. Isolates collected in 1998 typed as races SBLL and SBLL+ and the 2005 isolates as race SQPL. Race SBLL was not detected in the 2016 and 2017 samples with SBLL+ being dominant during 2016-17. This is the first report of South African races with virulence for the resistance genes Pc38, Pc58 and Pc59. Seedling infection types of 70 additional oat lines and cultivars to the described races showed incompatible phenotypes for 27 entries. Field data revealed moderate to high levels of adult plant resistance for a few cultivars to races SBLL+ and SSTL. Genetic analysis of 31 isolates, including two USA control isolates, revealed three genetic lineages. No apparent correlation was observed among year of collection, geographic origin, phenotype and genotype of isolates except for race JBLL that originated from wild oat collections in the Western Cape.