Graduate Thesis Or Dissertation
 

Susceptibility of coast redwood seedlings to Phytophthora ramorum

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  • Coast redwood (Sequoia sempervirens) (D. Don) Endl. is a host for Phytophthora ramorum but it was unclear if the pathogen represents a significant disease risk to this tree species. In this study the susceptibility of coast redwood seedlings and the likelihood of sporulation on redwood were examined. Two methods were used to assess susceptibility. Seedling stems were artificially inoculated and pathogen colonization distance was measured in both main stems and side branches after 8 and 16 weeks. Pathogen colonization distance was usually minimal, rarely exceeding 15 mm from the inoculation site. In two seedlings the pathogen colonized tissue greater than 40 mm from the inoculation site. No seedling mortality was observed. However, small side branches (2.4 mm mean diameter) were often killed. A dose response experiment was conducted to determine the minimum zoospore concentration necessary to cause infection. Seedlings exposed to ≥ 5 x 10³ zoospores/mL became symptomatic. Nonetheless, symptom expression was variable unless seedlings were inoculated with > 1 x 10⁵ zoospores/mL. Sporulation was quantified by determining sporangia density (sporangia/cm²) on detached immature and mature foliage of redwood seedlings, and leaves of California bay (Umbellularia californica). In the first trial, significantly more sporangia/cm² were observed on California bay compared to redwood, but in the second trial, the results were reversed. Sporulation density on attached redwood foliage was also determined. Comparisons were again made between young and mature redwood foliage. In the attached branch experiment, sporangia density was higher than previously observed and no significant difference was found between sporangia/cm² necrotic tissue on young vs. mature redwood branches. Sporulation results and implications of these findings are discussed in relation to sporulation levels reported on other hosts. This study highlights the importance of determining both host susceptibility and the risk of P. ramorum spread from hosts to other uninfected plants.
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