Stem susceptibility of six eastern Canadian tree species to Phytophthora ramorum

Phytophthora ramorum (Pr) is an emerging pathogen that causes diseases known as sudden oak death, ramorum leaf blight and ramorum shoot dieback. Even though Pr has been reported to naturally infect around 120 species, it has not been detected in the wild in eastern North America. However, there is real concern that Pr could be introduced and spread into this area. To better estimate this risk, seedlings of the following six eastern Canadian forest species were stem-inoculated with Pr: Abies balsamea (Ab), Acer saccharum (As), Betula alleghaniensis (Ba), Fraxinus americana (Fa), Larix laricina (LI), and Quercus rubra (Qr). Bark necrosis, colonization by Pr as well as host defense reactions were evaluated. Two months after inoculation, nearly 25% of LI and Ab seedlings died. Necrotic areas on the bark were larger in LI, Ab, and Qr than in Fa, As, and Ba. Chlamydospores were observed close to the inoculation point in the phloem of LI, Ab, and Qr. Pr hyphae were abundant in the phloem and cambium but also in the xylem of the two coniferous species where the colonization is facilitated mainly through the invasion of ray cells. In broadleaf species, hyphae were observed in a few xylem vessels and fibers close to the inoculation point, except for Qr where Pr was abundant in xylem vessels and still present up to 5 cm above the inoculation wound. However, among the six species, Qr was the only one where defense reactions were clearly apparent, especially when the inoculation occurred later in the growing season. Overall, Ab, LI, and to a certain extent Qr appeared susceptible to Pr and they could be killed should Pr be introduced during conditions conducive to disease development.