Treesearch

Pathologists investigating the widespread death of oak trees in the forest ecosystems of northern California concluded, in 2000, that the problem was due to a new plant disease, dubbed sudden oak death (SOD), which is caused by the fungal pathogen Phytophthora ramorum. Since then this one organism has been the focal point of notable efforts to understand, monitor, and control SOD. While not disputing that P. ramorum is involved in the final demise of many oaks, there is a growing number of scientists and arborists who do not agree that this pathogen is the fundamental cause of the overall...

In preliminary studies on the susceptibility of plant members of the holm oak (Quercus ilex) forest, detached leaves of several woody species were highly susceptible when inoculated with zoospore suspensions of local isolates of Phytophthora ramorum (Moralejo and Hernández 2002). Since then, there have been reports of natural infections by P. ramorum on leaves and shoots of Q. ilex in the UK, and new findings on Viburnum tinus in nurseries across Europe. Taken together, this indicates that at least some areas of Mediterranean vegetation with a prevalence of the Arbutus unedo-Viburnum...

In California and Oregon, Phytophthora ramorum has an overlapping host and geographic range with two newly described homothallic Phytophthora species, P. nemorosa and P. pseudosyringae. P. nemorosa alone causes symptoms similar to those of P. ramorum, including lethal tanoak cankers, and P. pseudosyringae is associated with oak decline in Europe. However, epidemiological observations, namely broader geographic distribution and reduced virulence, suggest P. nemorosa and P. pseudosyringae are endemic in this region, while P. ramorum is hypothesized to have been introduced. Though molecular...

At the first Sudden Oak Death Science Symposium, we reported on the use of a single strand conformation polymorphism (SSCP) analysis for rapid identification of Phytophthora ramorum in culture. We have since assessed and improved the fingerprinting technique for detecting this pathogen directly from plant tissues. The improved SSCP protocol uses a single run PCR reaction with the same primer pair (ITS6/7) and consistently detects P. ramorum at 10 fg per reaction or above. It provides reliable diagnoses of P. ramorum, whether it is a single infection or dual infection (a second Phytophthora...

Phytophthora species are abundant in streams, widespread in soils and occasionally found in diseased plants in the tanoak forests of southwestern Oregon. It is time-consuming and expensive to identify hundreds of isolates to species using morphology or internal transribed spacer (ITS) sequencing. We modified a published Phytophthora single strand conformation polymorphism (SSCP) protocol (Kong and others 2003) to use fluorescent-labeling chemistry and an additional marker locus to allow quantitative matching of unknown isolates.

Currently, diagnosis of Phytophthora ramorum involves sending samples to a laboratory for traditional isolation and morphological characterisation, and/or PCR analysis. This can take as long as 2 weeks from sampling to final diagnosis. However, the Plant Health Group, Central Science Laboratory, has produced on-site DNA extraction and real-time PCR (TaqMan®) methods, which use field-stable reagents for diagnosis of symptomatic infections of P. ramorum, in under 2 hours (Tomlinson and others, 2005). In March 2004 this method was evaluated, with the help of the University of California,...

The northernmost range of coast live oak (Quercus agrifolia) is reported from the Ukiah Valley (Mendocino County, California). Here, field observations suggest that hybridization with interior live oak (Q. wislizeni) is important. Elsewhere in northern California, morphology of coast live oak can be highly variable (particularly foliar form) and this has commonly been attributed to "noise" from a polymorphic species belonging to a genus renowned for its variability. This variability can be worrisome to scientists working on sudden oak death, since it may result in confounding species in...

We developed a branch cutting inoculation method to provide a controlled system for studying variation in response to inoculation of coast live oak (Quercus agrifolia) with Phytophthora ramorum. This method has advantages over inoculations of trees in the field, in containing the inoculum and in allowing high levels of replication and the possibility of time series of responses. We previously reported significant tree-to-tree variation, with little population variation in lesion size using this method (Dodd and others 2005). Here we report on a time series in which branch cuttings were...

In 2003, isolates belonging to the Al mating type were reported from commercial nurseries in Oregon. Soon thereafter, we reported the presence of both mating types of P. ramorum in nurseries in Washington. AFLP, microsatellite, and RFLP of the Cox I region indicated the Al isolates belonged to the European (E.U.) lineage of P. ramorum. In 2003, 33 percent of the isolates analyzed belonged to European lineage, while the rest belonged to the North American (N.A.) lineage. Isolates belonging to the two lineages were found in the same block of plants, and even on adjacent plants. By using...

To manage and control Phytophthora ramorum successfully, it is important to know the amount of phenotypic variation within a given pathogen population. Because the pathogen has only recently been described, there are few studies on morphological and pathological variation of isolates from the United States. One study has compared growth rate on agar, aggressiveness to Quercus rubra (Eastern American red oak), and developmental stability of European and U.S. isolates. In this study, the U.S. isolates were significantly slower growing, less aggressive, morphologically more unstable, and had...