Understanding and Managing Sudden Oak Death in California

SIDEBAR 1-5—Diagnosing SOD

Within the known range of P. ramorum in California, a preliminary SOD diagnosis can be made based on canker symptoms and the distribution of diseased trees in the field. One hallmark of P. ramorum is that it causes high rates of mortality in trees which appeared to be healthy and vigorous prior to infection. Many other mortality agents primarily affect low vigor trees, or are slow-acting, so that trees decline gradually over decades.

Confirmation of SOD in oaks and tanoaks is based on detecting P. ramorum in stem cankers. Tissue samples are cut from the edge of the canker below the outer bark surface, typically at the border between healthy and symptomatic tissue. The tissue samples pieces are placed in petri plates containing a selective medium. If the pathogen is still viable in the sampled tissues, it will grow from the pieces into the medium. The resulting cultures can be identified based on characteristics seen under a microscope. Alternatively, molecular identification methods can be used to positively identify the DNA of the pathogen in the sample. These and related methods are also used to detect P. ramorum in other plant tissues, in soil, and in stream water.

Foliar infections caused by P. ramorum on California bay and tanoak may appear years before trunk cankers develop.. Confirmation of P. ramorum from California bay leaves or tanoak leaf/twig samples using selective media or PCR provides positive evidence that an area is infested with the pathogen even if stem canker symptoms have not yet appeared. Many of the initial detections of P. ramorum within infested areas in California were made by sampling California bay foliage.

For the purpose of SOD management, you may assume that a site with typical SOD symptoms on foliar and/or canker hosts is infested if P. ramorum presence has been confirmed at nearby sites (up to several kilometers away). If your location is more than a few kilometers from the confirmed range of SOD, you may want to confirm that P. ramorum is present before choosing management actions (Part 2). In areas well beyond the known range of P. ramorum, especially in counties not currently listed as infested, confirmation by California Department of Agriculture (CDFA) or University of California (UC) plant pathologists should be obtained. County Agriculture Department and UC Cooperative Extension offices can provide information about official sampling by CDFA and UC personnel.