Population genetic structure of Plasmopara viticola in the Western Cape Province of South Africa

Plasmopara viticola populations in South Africa were studied for two consecutive grape growing seasons, in an organically managed and a conventional fungicide-sprayed vineyard. Three to four samplings in each season were genotyped with four microsatellite markers (GOB, CES, ISA and BER). Population differentiation (Fst) between the conventional fungicide-sprayed vineyard and organically managed vineyard was low (0.004 and 0.016) in both growing seasons, suggesting one metapopulation. However, differences in the relative contribution of the predominant and new genotypes to epidemics in the two vineyards suggested that fungicide applications may have selected for reduced pathogen diversity. In both years and vineyards, sexual (oosporic) reproduction and/or migration occurred throughout the year and contributed between 12 and 74% to the epidemic. Hardy-Weinberg analyses suggest that South African P. viticola populations are randomly mating. Epidemics in both years and vineyards were dominated by one or two genotypes that each contributed between 14 and 67% to the epidemic through asexual reproduction. The remaining genotypes showed low levels of asexual reproduction, with most genotypes never being able to reproduce asexually. However, for some genotypes asexual reproduction was important, as it enabled survival of the genotypes from one season to the next. In total, ten genotypes were able to survive asexually or vegetatively from one season to the next. The populations were further characterized by the presence of a high frequency of isolates that most likely have elevated ploidy levels.