Oomycete communities in lowland tropical forest soils vary in species abundance and comprise saprophytes and pathogens of seeds and seedlings of multiple plant species.

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The soils in lowland tropics are teeming with microbial life, which can impact plant community structure and diversity through plant-soil feedbacks. While bacteria and fungi have been the focus of most studies in the tropics, oomycetes may have an outsized effect on seed and seedling health and survival, given their affinity for moister, warmer environments. We assessed the diversity and pathogenicity of oomycete species present in a lowland tropical forest in Panama. We used a culture-dependent leaf-baiting assay and culture-independent soil DNA metabarcoding methods to quantify zoospore abundance and species diversity. A subset of the isolates from the baiting assay were used to evaluate pathogenicity and symptom severity on seedlings of three tree species. Oomycetes were ubiquitous and common members of the soil microbial community in lowland tropical forests, and zoospore abundance was far greater compared to similar studies from temperate and mediterranean forests. The various oomycete species also varied in the ability to infect host plants. Species of Pythium were more virulent, while species of Phytopythium caused less severe symptoms but were more diverse and commonly isolated from the soil. Finally, we found that individual hosts accumulated a distinct oomycete community and was the only factor that had an effect on community structure. Collectively, these findings demonstrate that oomycetes are ubiquitous, host-generalist pathogens and saprophytes, that can impact seed and seedling survival in lowland tropical forests.