Laboratory diagnosis of Phytophthora ramorum from field samplesAuthor(s): Cheryl Blomquist; Thomas L. Kubisiak
Source: American Phytopathological Society online symposium on Sudden Oak Death. http://www.apsnet.org/online/SOD/Papers/blomquist_kubisiak/default.htm. [Date accessed:] June 26, 2003.
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A plant disease should never be diagnosed on the basis of a single test. Using as much information as possible leads to the most informed diagnosis. The species of host plant, its symptoms, the location of the plant, the status of the county or state (known infested versus not infested with the pathogen), the culture results, and the results of DNA tests should all be used to make the diagnosis. In the case of Sudden Oak Death (SOD), caused by Phytophthora ramorum (Pr) (Phylum Oomycota), different kinds of proof are required depending upon whether a sample comes from an infested county versus a county or state not yet known to be infested. In an infested county, recognized host plants with characteristic symptoms and an unequivocal DNA test result are adequate to confirm the presence of Pr. To confirm Pr in a previously uninfested county or state, or infecting a new host species, the pathogen must be grown in culture and identified using morphological characteristics and DNA sequence analysis. To be unequivocally confirmed, the sequence of the intergenic transcribed spacer (ITS) region of the ribosomal DNA of the suspect organism must exactly match that of Pr.
Symptoms associated with Pr infection look different in nurseries than in the wildlands of California. Nursery infections are characterized by large necrotic spots on rhododendron (Rhododendron spp.), typical of infection by many Phytophthora spp. (Figure 1). Dieback symptoms on Viburnum spp. also occur; however, to date, Pr has been found only on Viburnum in Europe. In the wildlands of California, symptoms include leaf tip necrosis with angular spotting in California bay laurel (Umbellularia californica) (Figure 2) and bleeding in oaks (Quercus spp.) (Figure 3). Large necrotic spots are symptoms in California coffeeberry (Rhamnus californica) and toyon (Heteromeles arbutifolia), and edge necrosis is a symptom in bigleaf maple (Acer macrophyllum) and California buckeye (Aesculus californica). However, other plant pathogens including other Phytophthora species cause identical symptoms on these host plants. Therefore, laboratory tests are necessary to determine if Pr is present.
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CitationBlomquist, Cheryl; Kubisiak, Thomas L. 2003. Laboratory diagnosis of Phytophthora ramorum from field samples. American Phytopathological Society online symposium on Sudden Oak Death. http://www.apsnet.org/online/SOD/Papers/blomquist_kubisiak/default.htm. [Date accessed:] June 26, 2003.
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