Determination of apple scab races occurring in South African apple orchards

Abstract: Apple scab, caused by the fungus Venturia inaequalis, is one of the most important apple diseases in terms of economic crop losses, in South Africa and world-wide. To date, 17 different apple scab races have previously been identified. Each of these races has overcome major host resistance genes in the apple plant. The ARC apple breeding programme in South Africa currently makes use of only a few major resistance genes for breeding against apple scab, and it is not known which apple scab races occur in the country. The aim of this study was therefore to identify the apple scab races occurring in the South African apple growing regions. Apple scab isolates were collected from two Western Cape apple growing regions namely Elgin and Ceres. Differential apple cultivars with known major resistance genes were imported from Switzerland for race identification. Hardened-off plants of the differential cultivars Gala (none), Golden Delicious (RVi1), M. floribunda (Rvi7), 9AR2T196 (RVi5), TSR34R18 (RVi2), A723 (RVi10), Hansen’s baccata#2 (RVi12), Dülmener Rosen (RVi14), GMAL 2473 (RVi15), G711 (Geneva x Braeburn) (RVi3), Priscilla (RVi6), were spray inoculated in the greenhouse with spore inoculum mixtures from different orchards per region and incubated at 18°C at 100% relative humidity. The plants were evaluated at 14 and 21 days after inoculation. Isolations were made from plants that were susceptible and showed sporulation. Results showed apple scab symptoms on the plant leaves, which ranged from chlorosis and necrosis to sporulation. Venturia inaequalis races 1 and 12 were found to already occur, and the RVi7 resistance from M. floribunda still has not been overcome, in South Africa.