Population Genetic Diversity and Structure of Venturia carpophila on Peach and Almond in the United States.

Scab of peach and almond is caused by Venturia carpophila, but little is known regarding the population genetics of the pathogen, which aids in understanding the epidemiology of the disease and guiding its management. Isolates (735) of V. carpophila were sampled from peach and almond orchards in the Eastern United States and California, respectively. The aims were to determine population genetic diversity and structure of V. carpophila, compare populations of the pathogen from each host, and determine mating type ratios. Measures of gene and genotypic diversity indicated greater diversity among peach as compared with almond populations of the pathogen. Mating types were consistently at equilibrium (clone corrected) for populations from peach, but none of the populations from almond were at equilibrium. The MAT1-1-1 idiomorph was at low frequency or absent from almond populations. All populations on both peach and almond exhibited linkage disequilibrium. Analysis of molecular variance showed structure (71.8% of variance at the scale of the tree and 25.3% at the scale of the geographic region or crop). A discriminant analysis of principal components showed that the peach and almond populations clustered as independent groups. The results suggest that on peach in the Southeastern United States, V. carpophila is a sexually reproducing pathogen, but on almond in California, it is likely asexual. The confounded factors of geographic location and crop host may have both contributed to population differentiation. Although more research is needed, these results have implications for our understanding of the disease epidemiology and management in the two regions.