I once heard a speaker, talking about the spread of a plant disease, refer to a disease-friendly environment as a “free ticket to Disneyland” for the disease.

With the current El Niño weather pattern, it looks like Phytophthora ramorum — the organism that causes Sudden Oak Death disease (SOD) — is getting that free ticket this year. Water, specifically rainwater, is a key factor in the spread of the disease — but it is not the only factor. To propagate and infect susceptible oaks and tanoaks, the disease needs the leaves of foliar host plants, especially California bay laurel trees.

Recently I had the opportunity, at two separate conferences, to hear two leaders in the field of SOD research in California discuss their current understanding of the disease. Both emphasized the importance of seasonal weather patterns in the spread of SOD, saying that spring rains lead to more infections. The oak tree die-offs occur about three years after the fact.

The experts emphasize, however, that  a combination of factors leads to the death of oak trees:  susceptible tree species, the presence of the disease organism and an environment favorable to infection. That is the classic “disease triangle.” Remove one corner and there is no disease.

Considering the current SOD map of the Napa area (oakmapper.org ) perhaps explains why all of the confirmed SOD sites are in forested areas and none are in the urban center. But as the years go by, it is clear that the disease is spreading in the wooded areas surrounding the city. In addition, there are some unconfirmed sites in town.

At the December 2009 American Society of Consulting Arborists Annual Conference in Monterey, Dr. Dave Rizzo of UC Davis, explained how in the past, SOD infections have spiked about three years after rainy spring weather. It is called Sudden Oak Death because the trees may “flame out” suddenly, but trees may be infected for several years before the damage actually kills them.

He also noted a key feature of the spread of the disease: There are more than 100 foliar host plants but, so far, when trees are infected, California bay laurel trees (Umbellularia californica) have always been present. So, the bay laurels are a key part of the environment that is conducive to the disease. In addition he noted that the risk models they have developed show a strong potential for increasing SOD infestation in east Napa County.

Ted Swiecki, another expert in the field who spoke at the California Tree Failure Report Program at Filoli in January, primarily covered how trees killed by SOD tend to fall apart. He, too, mentioned that late spring rains “bump up spore levels,” referring to ascospores, the microscopic structures, something like germs, that infect the trees and foliar hosts.

If you are concerned about this disease attacking your own trees, keep some basic facts in mind.  In and around the city of Napa, the primary at-risk tree species is coast live oak (Quercus agrifolia). The second is black oak (Q, kelloggii). We do not have many of the most susceptible tree species, tanoak, in our local woodlands.

Location is critical. If your property is in town, there is far less risk of your trees becoming infected. The risk increases with increasing proximity to woodlands, especially dense forest and areas along creeks.

California bay laurels are the primary carrier of the spores. The risk of infection increase with the proximity of oaks to bay trees. Rainwater dripping from infected bay leaves directly onto oak tree trunks is the most likely pathway for infection. Pruning, or removing the bay trees to a distance of at least 15 feet from the oaks greatly reduces the risk of infection.

The current chemical treatment, registered in California, is Agri-Fos, a systemic fungicide. It has proven to be effective as a prevention, but not as a cure. It can be injected directly into the tree trunks or mixed with Pentra-Bark, a soapy penetrant, and sprayed onto the trunk and limbs. The material induces a short-term  defense response in the tree. It must be applied twice the first year and once a year after that. It is available in retail garden centers, so homeowners can apply it to their own trees with a garden sprayer. 

If you decide to use it, read the label and be aware of its properties. It is considered safe for the person doing the application if they follow the label directions and precautions. The recommended time for application is the cooler months, so there is still time to apply it this winter and spring.

Bill Pramuk is a registered consulting arborist. Visit his Web site, www.bill

pramuk.com, e-mail questions to info@billpramuk.com or call him at 707-226-2884.