Characterization of Puccinia sorghi Isolates from South Africa

Common rust on maize, caused by Puccinia sorghi (Ps), is a regular occurring disease in South Africa. Knowledge of the race composition, effective resistance (Rp) genes, and genotypic variation among Ps isolates contributes to the genetic control of the disease and our understanding of pathogen diversity and evolution. Ninety-six single pustule isolates of Ps, including field isolates from the 2006, 2008, 2014, 2018, 2021, and 2022 production seasons, were studied. Isolates were phenotyped on 26 maize differential lines and genotyped with 11 microsatellite markers. A total of 35 Ps races were identified. The races BBBHC and BGBHC were the most common and constituted 16 isolates, each followed by the races MBGHC and JBNHC with 12 and 5 isolates, respectively. Nineteen races were represented by one, and eight races by two isolates. No avirulence was detected to Rp3-A, Rp8-A, and Rp8-B and no virulence to Rp7 and two lines with stacked gene combinations (Rp5G and Rp5CGJ). Lines with Rp1-E, Rp1-Kr4, RpFJ, RpCJ, RpDJ, RpCFJ, RpDGJ, and RpFGJ had a low susceptibility frequency (<10%). Microsatellite analysis revealed low genetic diversity among the Ps isolates, indicating a clonal population. STRUCTURE analysis of multilocus genotypes revealed a single genetic lineage. No correlation was found between the phenotypes and genotypes of the identified Ps races. The observed racial diversity emphasized the ability of Ps to evolve as well as the need to determine hybrid responses to assist in the control of the disease in areas considered as high risk in South Africa.