Standing genetic variation facilitates climatic adaptation in the invasive grapevine pathogen Plasmopara viticola under demographic bottlenecks

Invasive pathogens often experience demographic bottlenecks during colonization, yet how they rapidly adapt remains unclear. Plasmopara viticola, the causal agent of grapevine downy mildew, provides a model to address this question. Using β-tubulin sequences and whole-genome data, we reconstructed its invasion history and adaptation in China. Our analyses support a North American ancestral origin of Chinese P. viticola, while demographic modeling and ABBA-BABA tests indicate historical connections between Chinese and European-related lineages, suggesting a complex invasion history. Chinese populations showed reduced genetic diversity and regional bottlenecks, with the low-latitude population showing the most pronounced reduction in effective population size. Despite these constraints, candidate adaptive variants were frequently retained as polymorphisms in other populations and occurred at relatively high frequencies, supporting an important role for selection on pre-existing genetic variation during climatic adaptation. These findings highlight how invasive pathogens can retain adaptive potential despite severe founder effects.