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    Author(s): E. Fichtner; D. Rizzo; S. Lynch; D. Rizzo; G. Buckles; J. Parke
    Date: 2009
    Source: In: Goheen, E.M.; Frankel, S.J., tech. coords. Proceedings of the fourth meeting of the International Union of Forest Research Organizations (IUFRO) Working Party S07.02.09: Phytophthoras in forests and natural ecosystems. Gen. Tech. Rep. PSW-GTR-221. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station: 142-150
    Publication Series: General Technical Report (GTR)
    Station: Pacific Southwest Research Station
    PDF: View PDF  (150.0 KB)

    Description

    Sudden oak death manifests as non-lethal foliar lesions on bay laurel (Umbellularia californica), which support sporulation and survival of Phytophthora ramorum in forest ecosystems. The pathogen survives the dry summers in a proportion of attached bay leaves, but the propagules responsible for survival are unknown. This study focuses on summer pathogen survival associated with bay laurel in redwood-tanoak and mixed-evergreen forests with specific objectives including:i) detection of P. ramorum in leaf litter and soils throu ghout the annual disease cycle, ii) quantification of chlamydospores on and within attached symptomatic leaves, and in fresh and aged litter, iii) determination of chlamydospore germination, and, iv) assessment of pathogen survival within litter and canopy leaves, addressing the location of viable inoculum within foliar tissues. Ten trees were tagged for repetitive sampling in four redwood-tanoak and four mixed-evergreen forests. Sampling was conducted at four times between May 2006 and September 2007 to target different points during the disease cycle. To determine pathogen presence in leaf litter and soil, three soil samples and 20 symptomatic litter leaves were collected and independently bulked from each tree. Samples were then baited for P. ramorum with rhododendron leaves. Chlamydospore populations on surfaces of attached leaves, and fresh and aged litter were determined by scrubbing individual leaves with a moistened toothbrush and filtering the resulting suspension through 35μM nylon mesh. Chlamydospores were then counted under a dissecting microscope and a subsample of chlamydospores was placed on selective medium to observe germination potential. To evaluate chlamydospore production within tissue, leaves were cleared with KOH and then observed with light microscopy. Pathogen survival and colonization was determined by subdividing symptomatic tissue from each leaf for detection by PCR, culture, and microscopy. Chlamydospore populations on attached leaf surfaces were higher in redwood-tanoak than in mixed-evergreen forests, but chlamydospore germination was never observed. Pathogen recovery was highest in the late spring, but declined by the end of the summer survival period. Isolation recovery was higher in attached leaves than in freshly cast leaves, but was rare from aged leaf litter tissues. High pathogen detection frequency was achieved by PCR, even when recovery in culture was not possible. Pathogen detection by PCR and isolation did not correspond with the heightened chlamydospore production observed at some sites. Though bay laurel supports chlamydospore production, the quantity of chlamydospores produced per leaf varies between sites and the lack of germination of these survival propagules contributes to the mystery of their potential role in the epidemiology of sudden oak death.

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    Citation

    Fichtner, E.; Rizzo, D.; Lynch, S.; Rizzo, D.; Buckles, G.; Parke, J. 2009. Survival and chlamydospore production of Phytophthora ramorum in California bay laurel leaves. In: Goheen, E.M.; Frankel, S.J., tech. coords. Proceedings of the fourth meeting of the International Union of Forest Research Organizations (IUFRO) Working Party S07.02.09: Phytophthoras in forests and natural ecosystems. Gen. Tech. Rep. PSW-GTR-221. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station: 142-150.

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