Survey of the Phytophthora species diversity reveals P. europaea-like isolates as Phytophthora root rot pathogens in Québec Christmas tree plantations

Christmas trees are an economically and culturally important ornamental plant in North America. Many microorganisms are pathogens of firs cultivated as Christmas trees. Among those, Phytophthora causes millions of dollars in damage to plantations every year. In Canada, it is still not known which species or group of species is responsible for Phytophthora root rot (PRR) of the cultivated Abies species. Between 2019 and 2021, soil and root samples were collected from 40 Christmas tree plantations in Québec province. We used soil baiting and direct root isolation to assess the diversity of culturable Phytophthora spp. The obtained isolates were identified with a multi-locus sequencing approach and we used the sequencing data to place them along the Phytophthora phylogeny. A total of 44 isolates from 6 different Phytophthora species were identified, one of which was fitting the undescribed P. sp kelmania. There was also a group of 10 isolates which could not be assigned to a known Phytophthora species based on DNA sequences. Among the known species, a Phytophthora europaea-like was the prevalent isolated species associated with PRR symptomatic trees. The fulfillment of Koch’s postulates confirmed the pathogenicity of P. europaea-like on both Fraser and balsam fir seedlings. Our studies provide a first snapshot of the Phytophthora diversity in Québec’s Christmas tree productions and describe multiple first associations between Phytophthora species and Abies balsamea and A. fraseri.