Characterization of <i>Puccinia sorghi</i> Isolates from South Africa

Common rust on maize, caused by <i>Puccinia sorghi</i> (<i>Ps</i>), is a regular occurring disease in South Africa. Knowledge of the race composition, effective resistance (<i>Rp</i>) genes, and genotypic variation among <i>Ps</i> isolates contributes to the genetic control of the disease and our understanding of pathogen diversity and evolution. Ninety-six single pustule isolates of <i>Ps</i>, 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 <i>Ps</i> 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 <i>Rp3-A</i>, <i>Rp8-A</i>, and <i>Rp8-B</i> and no virulence to <i>Rp7</i> and two lines with stacked gene combinations (<i>Rp5G</i> and <i>Rp5CGJ</i>). Lines with <i>Rp1-E</i>, <i>Rp1-Kr4</i>, <i>RpFJ</i>, <i>RpCJ</i>, <i>RpDJ</i>, <i>RpCFJ</i>, <i>RpDGJ</i>, and <i>RpFGJ</i> had a low susceptibility frequency (<10%). Microsatellite analysis revealed low genetic diversity among the <i>Ps</i> 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 <i>Ps</i> races. The observed racial diversity emphasized the ability of <i>Ps</i> 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.