With another rainy season approaching, it's time to pay attention to sudden oak death (SOD). The pathogen that causes the disease produces spores that spread during wet weather, so if you have oaks on your property or you frequent trails in oak woodlands, you can take steps to protect the mighty trees.
Only four species of oaks are killed by SOD: coast live oak, California black oak, Shreve oak and canyon live oak, as well as a related species, tanoak. The pathogen that causes the disease is spread primarily by other plant species, in which it causes leaf and twig diseases, including California bay, Douglas fir, coast redwood and a long list of nursery host plants.
On public lands, the best way to avoid spreading SOD is to stay away from areas with these oaks during wet weather. If you do hike or bike on muddy trails, be sure to wash off and disinfect your shoes and bike tires thoroughly before leaving the trail so that you do not transport spores to unaffected areas.
In Santa Clara County, SOD is most common along ridge tops.
Oaks planted in suburban landscapes or those that grow far from any of the host plants are unlikely to contract SOD. Nursery plants have been inspected for SOD in the past few years, but any host plants added near one of the four susceptible oaks before that may harbor the pathogen. If in doubt, trim any host-plant leaves so they do not come in contact with or drip rainwater onto the oak tree's trunk or main branches.
Although SOD can be confirmed only by a laboratory test, its main symptom in oaks is sticky and spotty bleeding from the bark, with no foul odor, no wounds or cracks, and not in insect holes. In California bay trees, the most common species likely to infect wildland oaks, especially if its leaves drip rainwater onto oak trunks, SOD presents as brown leaf tips and patches.
If you suspect SOD in your oak or host plant, call the County Agricultural Department for a free lab test. Before you do that, however, be sure to go through the checklist at www.suddenoakdeath.org to make sure it's not another disease.
If you have a valley oak or a blue oak, it's not SOD.
Many other pests, pathogens and injuries can affect oaks. A common cause of oak decline is any construction or grading under the canopy (or within 20 feet of the trunk), which can destroy roots, most of which are located close to the surface.
If you have confirmed the presence of SOD on your property and want to protect other oaks nearby that are unaffected, the state has approved Agri-Fos fungicide treatments to prevent or slow the progression of the disease. These treatments are most effective if applied in November or December.
The California Oak Mortality Task Force has scheduled a workshop in Woodside in October for homeowners who want to apply this treatment themselves. Depending on the size of the tree, the chemicals could cost $100-$200 or more per tree.
For the latest information on SOD, details on the October workshop, a full list of host plants and management guidelines for gardeners and landscapers in English and Spanish, visit www.suddenoakdeath.org.