Foliar and Canker Hosts
We have learned a lot about sudden oak death (SOD) and its causative agent, the water mold Phytophthora ramorum (P. ram), since our last report (see Pacific Horticulture, October ’04). Many of us are now aware of the central role played by California bay laurel (Umbellularia californica), a foliar host that provides an ideal environment for P. ram to reproduce. Foliar hosts produce the spores that are responsible for spreading SOD; some, such as Rhododendron, Camellia and Pieris, may be killed by P. ram. Without suffering significant damage, bay laurel acts as a primary driver of the disease, for several reasons: it produces copious quantities of spores, it frequently overtops oaks and spores are literally “rained down” on oak canopies, and it is ubiquitous in many woodlands, where it grows in close association with susceptible oaks.
Canker hosts include several tree species in the red oak group (Quercus spp.), and tan oak (Lithocarpus densiflorus), which is unique in acting as both a foliar and canker host. Canker host species generally do not produce spores that spread the disease, but suffer varying levels of mortality. While several species of red oaks have been found to exhibit some degree of resistance to SOD, tan oak is devastated wherever it is infected by P. ram. In susceptible trees, opportunistic organisms such as ambrosia beetles, bark beetles and Hypoxylon fungus (which frequently forms hemispherical black knobs on diseased trees) may contribute to tree failure. SOD symptoms can mimic those caused by several common plant diseases, including other species of Phytophthora, and a final diagnosis can only be ascertained with a lab test.
In the past few years it has been confirmed that P. ram is an exotic organism, although we still have not pinpointed the region of its origin. Just as Native Americans had no natural resistance to smallpox and measles, native oaks have not yet had time for selection of genes resistant to the pathogen; weak, ailing trees and strong, healthy trees alike may succumb to the invader. Changes in the forest are also undoubtedly a contributing factor in the spread of the disease. Because of the absence of fire in many areas, woodlands have filled in, creating the damp, shady conditions favored by P. ram. Scientists are working to explain the complex relationship found between SOD prevalence, land-use history, and fire suppression. Meanwhile, community concern over the growing fire hazard is bringing long-overdue attention to the spread of P. ram and associated increased tree mortality in California woodlands. In Big Sur, the fires of summer 2008 offer the first opportunity for scientists to study the effects of major wildfires in an area with SOD.
Gardeners Can Help
Fourteen counties in California, and one in Oregon, are under quarantine, but diseased material can be legally moved within county boundaries. It is up to citizens to avoid spreading the disease to new areas in these counties, and to be aware when shopping for plants. Especially for those living at the interface of urban and wild lands, be cautious when purchasing new landscape plants that may be foliar hosts to P. ram; this includes dozens of native plants. Symptoms of SOD may be masked in plants at the nursery; either because leaves have been heavily sprayed, or because the disease has not yet manifested symptoms. A recommended precaution is to quarantine new plants, in an area removed from vulnerable landscapes, for three eight to weeks before planting. Near wild lands, it is best to avoid planting primary hosts such as bay laurel, tan oak, Pieris, Rhododendron, and Camellia.
Because the structural integrity of trees killed by SOD may be compromised, even when leaves are still green, a professional arborist should be called to assess damage. If trees must be cut down, leave the material on-site, if possible, and use for firewood. Firewood should be stored away from unaffected trees, and kept dry. All tools should be cleaned before they are moved off-site.
Still No Sure Cure
Unfortunately, despite continued tests on alternative treatments, there is still no effective treatment for SOD. The phosphonate product, Agri-Fos, can be used to protect high-risk trees from infection; once a tree is infected, phosphonates can only slow the progression of the disease. Such treatment is only justified for high value trees, such as trees near homes, or heritage oaks, or infected trees that pose a hazard to people and property. One strategy now gaining favor is the selective pruning and removal of bay, especially where it occurs within fifteen feet of a desirable oak. Although bay laurel is also a natural member of the woodland community, it may be more abundant now than it was historically, and removing a few bay laurel trees (especially saplings) may be an effective preventative measure. The newest treatment to show promise is a March application of fine commercial-grade compost, mixed into the top few inches of soil around the trunk of the tree, from the root collar to two to three yards away. It is thought that compost reduces rain splash of spores, and introduces microbes that compete with P. ram. Property owners may want to protect high value trees by implementing all three control measures: phosphonates, selective pruning and removal of bays, and compost amendment.
For more information, see the California Oak Mortality Task Force website, www.suddenoakdeath.org, Information on protocols for treatment can be found at www.matteolab.org. Another good site is www.phytosphere.com. For information on lab testing, call the local county agricultural commissioner.
The author wishes to acknowledge the assistance of SOD researchers, Nathan Rank, David Rizzo, and Matteo Garbelotto, in the preparation of this Lab Report.