A current and large-scale portrait of Phytophthora root rot incidence, and its associated Phytophthora species in Québec Christmas tree plantations

Phytophthora root rot (PRR) is a serious disease affecting fir trees grown as Christmas trees in North America and Europe. In the province of Québec (Canada), based on scarce available data, current surveillance and diagnostic efforts have focused primarily on the emerging pathogen Phytophthora abietivora. The objectives of this study were to gather more data for both PRR incidence and Phytophthora diversity associated with PRR-symptomatic trees. Fifty-five recently planted fields were investigated for PRR incidence, which ranged from 0% to 52.6%. A GLMM modelling approach revealed field topography and seedling origin as significant contributors to PRR incidence. A total of 605 diseased seedlings from 102 fields were sampled for Phytophthora isolation. Diversity indices indicated that PRR-diseased tree communities were dominated by P. abietivora (84.1% of isolates). Further analysis revealed that planted seedlings originating from Canadian nurseries were more dominated by P. abietivora than seedlings from nurseries in the USA, which presented a wider diversity of potential pathogens. Our results suggest that P. abietivora is the major pathogen in the Québec Christmas tree industry. While this confirms the validity of currently deployed management efforts, it also underlines the importance for producers to refrain from using out-of-country seedling sources. This is the first study reporting the incidence of PRR in Christmas tree plantations in Canada, providing a general portrait of the Phytophthora diversity associated with the disease.