Oomycete communities are influenced by land use and disease status in Christmas tree production in Southern Québec, Canada

Forests are threatened by many natural stressors intensified by climate change and anthropogenic activities, which tend to increase their susceptibility to pests and pathogens. Consequently, oomycete-related forest decline or dieback cases are increasing in natural, urban, and agricultural landscapes. It is in this context that Christmas tree growers from Southern Québec, Canada, are experiencing root rot problems, with reported incidences up to 25%. In a previous study, seven Phytophthora spp. were associated with this root rot problem, but the overall diversity of oomycetes has not yet been investigated. Hence, in this study, we use a metabarcoding approach to provide an overview of the diversity, richness, and composition of the oomycete community in fir plantations compared to surrounding natural forests. We showed that the P. cryptogea cluster, P. europaea cluster, P. sansomeana, and P. chlamydospora cluster were significantly more abundant in soils collected from plantations under diseased trees and confirmed that the P. europaea cluster (including P. abietivora) was most frequently associated with trees showing Phytophthora root rot-like symptoms. Finally, we report that land use (anthropogenic activities) shapes oomycete diversity, while plantations can act as a gateway for invading natural forests. In fact, the results presented here suggested that the P. europaea cluster might already have crossed this boundary and that other species might follow, advocating the importance of improved surveillance of oomycete diversity in various environments.