Oak disease: What’s a gardener to do?
There’s a new pest in town, and home gardeners are wondering what, if anything, they should do to protect their plants and gardens. The disease, sudden oak death, is caused by a newly discovered fungus, Phytophthora ramorum. To date, more than 60 hosts -- including some oaks, camellias, viburnum and lilac -- have been identified in North America and Europe, and the list is growing.
Sudden oak death has killed tens of thousands of coast live oaks and tanoaks in Northern California in the last decade and recently was found on plants in two Southern California wholesale nurseries, yet experts say Southland gardeners need not panic.
The fungus has not been detected in natural areas in Southern California, says Dave Rizzo, research advisor and associate professor of plant pathology at UC Davis, and it would be difficult for the disease to establish itself in this region. Two factors are key to its spread, and neither exists here: a cool, moist environment and nearby associated hosts, primarily California bay laurel and native rhododendrons, both common in northern wildlands.
Little is known about the origin and life cycle of the disease-causing organism, but researchers say they are learning more each day. Symptoms of sudden oak death, including trunk cankers, tip dieback and leaf spots, vary from host to host and resemble symptoms of other diseases. Laboratory culturing is the only way to confirm the fungus. The disease is deadly to some hosts. Others, such as California bay laurel, only harbor the fungus and nurse the spores, which are carried to vulnerable plants by water and wind. There is no chemical control for sudden oak death.
Twelve California counties from Santa Cruz to Humboldt are quarantined (Los Angeles County is not), making it illegal to move or transport host plants and associated materials, including cut and chipped wood. If you travel through infested areas, especially forests, take time to remove soil from auto and bicycle tires, your shoes and your pets. At present, all California nurseries are under strict state and federal guidelines that restrict movement of any host plants and require careful monitoring and changes designed to help prevent future outbreaks.
Though local conditions are not conducive to sudden oak death, you can fortify your plants in case the fungus makes its way into your garden. “Be cautious, not alarmist,” Rizzo says. “Avoid overhead irrigation. Don’t over water native oaks, and don’t plant thirsty ornamentals, especially other hosts, around and under them.”
For more information, the Los Angeles County Agricultural Commissioner’s Office (acwm.co.la.ca.us) has a pamphlet, “Sudden Oak Death and Care of Native Oaks,” that can be downloaded. The California Oak Mortality Task Force website (www.suddenoakdeath.org) provides details and updates. In early May, the task force will offer two free all-day training sessions for nursery workers and others in Southern California.
