Characterizing Douglas-fir tissue colonization by the "sudden oak death" pathogen, Phytophthora ramorum

Kathleen Mary McKeever

In 2001, Koch's Postulates confirmed Douglas-fir as a host for P. ramorum. Naturallyinfected saplings have been observed in California forests, and studies on artificially inoculated Douglas-fir stems and shoots have established susceptibility parameters. In 2009, P. ramorum stem cankers were recently observed on 8-year old plantation grown Douglas-fir in Great Britain. These findings substantiate the importance of research to characterize the behavior of P. ramorum in Douglas-fir and assess the potential risk that this pathogen poses to Douglas-fir ecosystems. Goals of this research included determining which tissues are colonized by P. ramorum, whether woody tissues can support sporulation, the likelihood of stem infections occurring through intact bark, and pathogen viability in foliage. Methods included isolation, ELISA, and histology of infected stem tissues; RT-PCR and isolation to determine colonization of foliage, and baiting studies to determine the ability of bark to reduce colonization of Rhododendron leaves by the pathogen. ELISA showed that pathogen proteins were detectable in the phloem, cambium, and superficial xylem, with infrequent detection in asymptomatic tissues. The pathogen was not able to be isolated from non-discolored tissues. ELISA and isolation results were highly positively correlated (r2 =0.62, p=0.78), and histological observations supported results from these techniques. Bark reduced infection on leaf baits by up to 83%. Pathogen DNA was detectable in foliage using qPCR methods, but the pathogen could not be isolated. Evidence of sporulation in stem tissues was not observed. The pathogen may infect into the shallow xylem tissues of wounded Douglas-fir stems, but spore formation is not apparent. The lack of isolation from non-discolored tissues suggests that proteins detected by ELISA may be elicitins secreted in advance of hyphae. The inability to isolate from needles may indicate chemical inhibitors that render the pathogen non-viable subsequent to initial infection. The ability of Douglas-fir bark to suppress infection of leaf baits suggests that bark is inhibitory to P. ramorum and that stem infection through intact bark may be limited. Characterizing P. ramorum colonization of Douglas-fir is important for understanding host susceptibility and pathogen behavior to enhance our ability to assess risk and thwart the spread of this exotic pathogen.

Characterizing Douglas-fir tissue colonization by the "sudden oak death" pathogen, Phytophthora ramorum

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