Many people have asked about their oak tree—does it have sudden oak death? I answer something like this: “Probably but perhaps not”.

Phytophthora ramorum, Sudden Oak Death (SOD), has become a staple of California coastal hardwood forests since first discovered here in 1995. Today SOD extends from Big Sur to southern Oregon and 80 kilometers inland. Hillsides of dead tanbark oaks can be seen in Monterey, Santa Cruz and Humboldt Counties. In Marin it has killed coast live oaks, tanbark oaks and black oaks at China Camp. Thousands of dead tanbark oaks can be seen around Kent Lake in the upper Tomales Bay watershed. We again have clusters of mortality in Inverness this year, and probably more next year. The fungus, one of over sixty species of the genus Phytophthora (one of which is the Irish potato blight), has affected dozens of other common native woody species including California bay, madrone, buckeye, rhododendron and even redwood, Douglas-fir and many others. Though bay seems to be a primary host, the fungus is most virulent in tanbark oaks. The SOD fungus slowly penetrates, infects and then kills the inner bark tissue and spreads to the point that it eventually girdles and kills the diseased tree. The pathogen is spread via air, soil and water. It incubates in our cool moist climate and increased mortality is noticed two years after each wet winter. Some more pessimistic projections are that tanbark oaks will completely disappear from the north coast within a few centuries or sooner. Genetic resistance in tanbark oak is as yet unknown.

We noticed abundant bay and tanbark oak trees with dead foliage in Inverness this spring. I obtained some samples and sent them to the California Department of Food and Agriculture for tissue culture and analysis. To my surprise SOD was not identified as the culprit. The diagnosis was Phytophthora nemorosa. This species is so new that it has not even been completely described, though it is thought to be a native and appears to be less toxic to trees than the SOD fungus. The UC Oak Mapper indicates confirmed SOD occurrences in Tomales Bay State Park and also near Bear Valley. Monitoring and research locations are scattered around West Marin. According to Yana Valachovic of UC Extension SOD seems to be everywhere.

After two wet winters here many madrones here have just a few succulent healthy green leaves scattered among lots of dead twigs and foliage. If you look closer you will notice that many twigs, and even some larger branches, have been girdled by a stem canker. This is very likely to be SOD at work. Other possibilities are two madrone canker diseases that have been around for some time.

There is no landscape level treatment. Foresters are expecting a successional transition to more tolerant species—probably bay. For important healthy landscape trees in affected areas annual phosphite treatments of “Agri-fos” are thought to be effective. Agri-fos can be applied as a spray or injected by a qualified arborist. For more information please visit www.suddenoakdeath.org or www.forestdata.com .

Tom Gaman is a registered forester living in Inverness. tgaman@forestdata.com

August 2006.

Home