Assessments and perspectives on stem rust resistance in South African malting barley

The production of malting barley under dryland and irrigated environments in South Africa (SA) has increased recently, making it the second most important winter crop after wheat. To assess the potential vulnerability of adapted barleys from SA to wheat stem rust (caused by Puccinia graminis f. sp. tritici, Pgt, race PTKST) and rye stem rust (Puccinia graminis f. sp. secalis, Pgs, isolate UVPgs1), 50 accessions were evaluated as seedlings and as adult plants. The same barley germplasm was tested against Pgt and Pgs cultures from the USA that have proven useful for detecting the stem rust resistance genes Rpg1 and the rpg4/Rpg5 complex. The possible presence of these resistance genes was assayed using molecular markers and sequence analysis. Barley accessions exhibited mostly resistant infection types (ITs) to the South African cultures, whilst field data revealed that several of the barley varieties displayed high severities in combination with more compatible host response types to race PTKST. The barley accessions exhibited mostly high ITs to cultures from the USA. None of the seedling IT data was sufficiently distinct to reliably postulate the presence of possible resistance genes. This result was corroborated from the molecular assays for Rpg1 and rpg4/Rpg5. Except for one line that tested heterozygous for the Rpg1 gene, none of the test entries contained this gene or a functional rpg4/Rpg5 complex due to the prevalence of mutations in the rpg4/Rpg5 + carriers. Susceptibility among barley varieties to wheat stem rust race PTKST under field conditions in the present study is of concern.