Sudden Oak Death on the West Coast

July-August 2005

PHYTOPHTHORA RAMORUM, the disease that causes Sudden Oak Death (SOD) in coastal areas of the Pacific North-west is still active and spreading. Currently, the disease can be found in the forests of 14 counties in California and 1 county in Oregon. The disease has been associated with the death of several thousand coastal oak species in the 15 regulated counties.

Unlike it’s name, it can take one to two years before the trees are killed. SOD causes lethal stem infections on oaks (Quercus spp.) and tanoak (Lithocarpus densiflorus) as well as nonlethal foliar infections on a broad range of trees and shrubs. It’s important to note that this disease does not kill most of the susceptible host plants, but it does cause various foliar infections.

Recently I took a trip to the west coast to examine this disease up close and meet with California and Oregon researchers about P. ramorum. We visited a site near Santa Cruz where P. ramorum was confirmed. It was truly amazing – every time we found an infected plant, we would look up to find a California bay laurel (Umbellularia californica) just loaded with leaf lesions. The bay laurel is the main foliar host for P. ramorum on the west coast. It is the host that supports the fruiting/sporulating structures that cause new infections. The interesting part is that the bay laurel isn’t killed, it just expresses leaf lesions and leaf senescence. The bay laurel is the host that makes the fungus so successful in coastal forests of California. Whereas in Oregon, rhododendron seems to be the “bay laurel.” If P. ramorum makes its way to eastern forests, what will be our “bay laurel”?

The implications of P. ramorum on eastern tree species are quite significant. There have been a couple of studies where researchers have artificially infected native eastern species (e.g. Acer ruburm, Quercus alba) in a laboratory setting. The interesting part is that the lesions which develop on eastern species are larger than those on the coastal oak species that are dying in California and Oregon. But – can P. ramorum survive our freezing temperatures? That’s the golden question. And the answer? Well – maybe.

Phytophthora can develop some very tough resting spores, called chlamydospores. For some Phytophthora species, these chlamydospores have been found to persist in soil for many months. Some very preliminary experiments that exposed chlamydospores to freezing temperatures have indicated that these resting spores are, in fact, quite tolerant of some freezing temperatures. But this research has not yet been completed.

The host list for this disease is growing all the time as new species are successfully infected in research labs or found infected in nurseries and wildlands. Unfortunately, ornamental nursery stock has been found to be positive for P. ramorum at various retail garden centres, nurseries and landscapes in the U.S. Of the infected nursery stock found to be positive for SOD, Camellia, Rhododendron, Pieris and Viburnum spp. are the most common and thus represent the high risk nursery hosts for this disease in the coastal U.S. It’s not that the disease is necessarily killing these plants, but it is causing aerial infections.

Since many of the SOD host plants are grown as nursery stock and nursery stock is traded so extensively in North America, the U.S. nursery industry has fallen under restrictions to reduce the spread of this disease.

At this time, all nurseries in California, Oregon and Washington fall under regulatory programs whereby nursery stock is inspected and tested for P. ramorum. Unfortunately, there were five positives discovered in British Columbia last year (at retail garden centres and nurseries). And although B.C. is not regulated for this disease, their nursery growers decided to take a proactive approach to prevent the spread of P. ramorum. The B.C. nursery industry contracted out survey and testing procedures as well as a detailed certification program that outlines sanitation and best management practices that help prevent the spread of this disease in nurseries. Over 300 facilities in B.C. are on this certification program.

Recently I visited with some leading B.C. nursery growers and found that they are quite focused on implementing sanitation measures to mitigate the spread of SOD through soil, water and plants. Many facilities are re-designing nursery traffic routes to limit off-site vehicle access (e.g. customers leave their vehicle parked in a designated area and do not track foreign soil through the production facility). Truck wash stations, sanitary foot baths, fencing and ample signage give the visitor the impression that they need to act responsibly in the facility or they will not be allowed on site. Nurseries that are recycling their water are using disinfecting strategies to reduce waterborne pathogens (such as Pythium and Phytophthora).
SOD host material is inspected upon receipt and held in an isolated area if any testing is required. Many B.C. nursery growers avoid or have drastically cut their shipments of SOD host material from infested US coastal states. B.C. nursery growers are worried about the implications of this disease in their facilities, the landscape and forest.

Ontario nursery growers share the same concerns and some of our leading wholesale nurseries have started implementing sanitation measures and practices that can help prevent the spread of P. ramorum into our province. In Ontario this spring, the largest nurseries that do a lot of trade across Canada have been surveyed and tested for P. ramorum by the Canadian Food Inspection Agency (CFIA). This is a nerve racking process for nursery growers since a positive would mean destruction of several hundred to thousands of plants.

In addition, all SOD host plants (over 45 genera including Acer, Fraxinus, Quercus sp.) at the affected nursery would be placed under quarantine. For some nurseries, this could mean most of their inventory. A quarantine could, in fact, put them out of business. The nursery industry has been working cooperatively with the CFIA to identify possible infections and eradicate them quickly and effectively. Ontario nursery growers want to do their part to keep SOD out of the province.

There are quite a few good websites for information on SOD, but the California Oak Mortality Task Force does an exceptional job. Check it out at:

Click here to view accompanying pdf tables for this article.

— Jen Llewellyn, Ontario Ministry of Agriculture and Food (OMAF) Nursery Crops Specialist

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