Determination of apple scab races occurring in South African apple growing regions to underpin breeding for resistance

We sought to determine which races of apple scab (Venturia inaequalis) occur in the different apple growing regions of South Africa and also investigate the population genetic structure and the pathogenicity and virulence of the pathogen on different apple cultivars.

Apple differential cultivars with different resistance genes against apple scab have been imported. Apple scab samples were collected from four different apple growing regions. Single spored cultures were established and the differential cultivars were inoculated with mixed inoculum to determine the races Fingerprinting of apple scab isolates was undertaken with seven SSR markers to determine differences between isolates from the different apple growing regions.

Apple scab samples were collected from the Koue Bokkeveld, Elgin, and Lower and Upper Langkloof regions during the 2014/15 and 2015/16 growing seasons for race identification. Ten differential cultivars were inoculated with mixed inoculum and it was found that the Rvi1 and Rvi12 resistance genes are overcome by the South African isolates. The Rvi10 gene was overcome by the 2015/16 Koue Bokkeveld isolates. Apple scab isolates were genotyped in 2012/13 and 2013/14 with six and seven SSR markers. Genotyping results were the same for both growing seasons and indicated that minor population differences exist between the Ceres and Langkloof populations and moderate differences between the Elgin and other populations. Thus the Ceres and the Langkloof populations are more closely related to each other. Migration of apple scab isolates occurs between regions. Four different haplotypes were found in the fungal isolates for the barcoding sequence ITS and six for the ABC2 virulence factor.

Inoculation of differential apple cultivars with apple scab isolates from the four growing regions showed that the Rvi1 and Rvi12 resistant genes are ineffective in all four regions and the Rvi10 in the Koue Bokkeveld. Single spore isolates also showed differences in virulence and could only infect certain cultivars, consistent with research work reported in Europe. Collection of the apple scab samples from the four apple growing regions enabled a population genetic study which indicated differences in the apple scab fungal populations between the different climatic regions and also indicated that the fungus adapted to the regions over time. Differences in the ABC 2 gene region of the fungus could be associated with variation in virulence.