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Abstract
The downy mildew pathogen Plasmopara viticola causes severe epidemics on cultivated grapes (Vitis vinifera and French-American hybrids) in Georgia and Florida, yet its epidemiology, mode of reproduction and dispersal in this area are largely unknown. Regular disease and pathogen monitoring in an experimental V. vinifera vineyard in the North Georgia Mountains across 3 years showed that the time of disease onset varied considerably among years and cultivars. Microscopic observation of oospores in infected leaves, season-long detection of new multilocus genotypes (MLG) in the field, and the fact that 59.9% of all sampled lesions harbored repeated (clonal) MLG suggest overwintering by oospores and a mixed mode of reproduction in the vineyard. Some repeated MLG were detected across years, suggesting that the pathogen may also be capable of asexual overwintering in this vineyard. At a larger geographic scale across five distinct grape-growing regions in Georgia and Florida, multilocus sequence analysis indicated the presence of three cryptic species of P. viticola, namely clades aestivalis (Pva), vinifera (Pvv), and vulpina (Pvu). Pva was the most prevalent and widely distributed cryptic species, whereas Pvv and Pvu were isolated exclusively from hybrid grapevines in the Coastal Plain; this expands the record of host range and geographic distribution of the three cryptic species in eastern North America. The spatial genetic structure of the most common cryptic species, Pva, was investigated in the five regions using microsatellite markers. The analysis revealed a complex population structure where some regional populations of P. viticola were genotypically highly diverse whereas others were largely clonal; some populations were strongly differentiated whereas others overlapped; and MLG were mostly unique to vineyards but there was also some evidence of limited long-distance migration and/or asexual overwintering. No population differentiation was observed based on host of origin (V. vinifera or hybrid grapes). These results may reflect the environmental diversity among the five wine-producing regions examined as well as the recent establishment of commercial vineyards in the area and their relatively isolated geography.