Outbreak of Phytophthora abietivora in a Québec Forest Nursery: Emergence of a New Phytophthora Tree Pathogen?

Tree nurseries play a key role in the Canadian economy and reforestation efforts, producing over 600 million seedlings annually. Despite rigorous management practices, nurseries are not exempt from pathogen outbreaks, which can be devastating on many levels. In October 2022, the public forest nursery of St-Modeste (Canada) noted an unusually high mortality rate among their 2-year-old balsam fir (Abies balsamea ) seedlings. Phytophthora abietivora, recently identified as responsible for the Phytophthora root rot (PRR) in Christmas tree plantations, was suspected to be the causative agent of the outbreak. The objectives of this study were to identify the pathogen(s) responsible for the outbreak in the nursery and determine its pathogenicity and transmissibility to other seedlings. After the isolation of the pathogen and molecular detection, it was confirmed that the epidemic was caused by P. abietivora. The pathogen was not only found on healthy-looking balsam seedlings, but also on many other tree species grown in the nursery showing no above-ground PRR symptoms, such as spruce seedlings. The strain isolated in the nursery proved to be highly infectious to Fraser fir seedlings, and results were exacerbated by artificial flooding of seedlings. More worryingly, the disease could be transmitted to susceptible recipient seedlings from asymptomatic donor seedlings. The pathogen could be detected in soil and roots from both donor and recipient seedlings. Together, these findings indicate the first report of an outbreak of P. abietivora on balsam fir seedlings under nursery conditions. Efforts must be increased to minimise economic losses and to manage future outbreaks better in order to protect Christmas trees and forests.