My West Marin garden is surrounded on three sides by the native forest of the Inverness Ridge, which includes magnificent Douglas firs, towering bays, elderly California buckeyes, madrones, coffeeberry and manzanita, as well as coast live oaks and tanbark oaks.

I didn’t know when I planted the garden eight years ago that existing and new oaks were to become susceptible to sudden oak death, and that most of their forest companions were foliar carriers of the disease. Over the past four years or so, we’ve learned to adapt our gardening and landscaping to accommodate the reality of this pathogen and its spread. Living at the urban/wildland interface has its rich rewards and, as we now know, its drawbacks of nature being, well, nature. A few lessons from our experience may be useful to others living close to or adjacent to native woods.

The disease is caused by a pathogen, Phytophthora ramorum, a non-native organism now common in many urban/wildland areas of California’s coastal counties. It thrives in warm, damp weather. Not much is yet known about the spread of the pathogen, though oaks and tanbark oaks near or under affected non-oak host plants are most susceptible.

Known carriers of the pathogen include the common woodland trees and shrubs listed above, but particularly California bay laurel (Umbellularia californica). These carriers seldom die of the disease but evidently help it to spread. Tanbark oaks have the highest mortality and are unlikely to survive if affected. California live oaks are more likely to survive and may live for as long as two years after contracting the disease.

The woods surrounding our home are filled with dead and dying tanoaks, and because we live near the boundary of the Point Reyes National Seashore, I talked with Allison Forrestal, a Bay Area fire ecologist with responsibility for sudden oak death at the park headquarters. She and other researchers are gathering data on the disease, its spread and likely effect on the ecology of woodland areas. For example, the acorns of tanoaks are the preferred food of the dusky-footed wood rat, itself the preferred food of the northern spotted owl. Certainly, the presence of so much dead wood will present a fire hazard for a year or two.

The only available treatment for sudden oak death is a preventative, not a cure. It helps boost the immune system of oaks and tanoaks growing near bays and other carriers. A phosphonate compound, AgriFos, should be applied by a trained arborist and probably limited to only a few specimen trees in the garden. It will turn the green moss on tree trunks black temporarily, and cannot be guaranteed to save the tree. We chose to have three, large coastal live oaks (Quercus agrifolia) and one large, specimen tanoak (Lithocarpus densiflorus) sprayed last fall, hoping to help them survive.

Unfortunately, even healthy trees can succumb to an attack by this pathogen. However, any steps to keep your trees as healthy as possible will limit damage caused by this and other pests. For oaks, this means preserving the original ground level under the tree out to the dripline. Excavations, paving or fill near the tree’s trunk can harm the tree’s root system. Avoid watering in this same area in summer, and limit plantings to those that do not need summer irrigation and that do not carry sudden oak death. Shrubs such as rhododendron, coffeeberry and manzanita are known carriers.

If oaks or tanoaks in your garden die, the general principle is to keep the wood on your property if possible. Consider leaving them standing if they are not hazardous. Although not pretty, they will provide habitat for wildlife. Chip the smaller branches and debris, and stack the larger limbs for firewood. Store the affected firewood away from existing trees to reduce the risk of spreading the infection. Phytophthora ramorum is so widespread in woodland areas of Marin County, it is unlikely that what is in your, or your neighbor’s, garden, will cause more harm.

I highly recommend a Web site, www.suddenoakdeath.org. It was my most useful source of information and is regularly updated. It was reassuring to know that the California Department of Agriculture, the UC Agricultural Extension Office and the Point Reyes National Seashore are monitoring sudden oak death and conducting research to better understand how it spreads and why some trees appear to be resistant to this pathogen. ÊÊ Ê

The UC Marin Master Gardeners are sponsored by UC Cooperative Extension. The group’s two books, “Bay Area Gardening” and “Gardening Among Friends,” are available at local bookstores. For more information about integrated pest management, contact the office at 1682 Novato Blvd., Suite 150B, Novato 94947, or call the Master Gardeners desk at 499-4204.

source : www.marinij.com