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How to Manage PestsPests in Gardens and Landscapes
Sudden oak death is the name given to an epidemic, first detected in 1995, that affects three true oak species—coast live oak (Quercus agrifolia), California black oak (Q. kelloggii), and Shreve oak (Q. parvula var. shrevei)—and tanbark oak (Lithocarpus densiflorus). Additional species affected include rhododendron (Rhododendron spp.), madrone (Arbutus menziesii), California huckleberry (Vaccinium ovatum), California bay laurel (Umbellularia californica), California buckeye (Aesculus californica), big-leaf maple (Acer macrophyllum), toyon (Heteromeles arbutifolia), and manzanita (Arctostaphylos spp.). A previously undescribed pathogen, Phytophthora ramorum, has been identified as the infectious agent. The disease is currently known to exist in the coastal ranges in California, between Big Sur in Monterey County and southern Mendocino County. Sudden oak death has been confirmed in Alameda, Marin, Mendocino, Monterey, Napa, San Mateo, Santa Clara, Santa Cruz, Solano, and Sonoma counties. This disease has also been reported from several locations in southern Oregon. Within the affected counties, both the severity and occurrence of the disease vary considerably across the landscape. Infected trees are abundant in Marin, Mendocino, Monterey, Santa Cruz, and Sonoma counties. It is not known if the geographic distribution of infected trees is associated with climatic variables. Attempts to assess distribution patterns within California through surveys are currently under way. The numbers of infected and dead trees are suspected to be in the tens of thousands. Before the recent discovery of sudden oak death in California, P. ramorum had not been known in North America. The pathogen was first reported in Europe in 1993 where it was infecting rhododendron (Rhododendron spp.) in nurseries but was not recognized as a new species until 1999. Death of oaks and tanbark oaks in areas of California outside the ten reported counties does not appear to be caused by this new Phytophthora species. Based on field observations and laboratory cultures, mortality of oaks and tanbark oaks outside the infested areas appears to be a result of the normally expected causes, and oaks in these areas are not dying at a greater rate than previously observed. IDENTIFICATIONAt present, the only definitive ways to diagnose a sudden oak death Phytophthora infection in a tree are by culturing the pathogen or by amplifying the DNA using PCR (polymerase chain reaction). No single field symptom is sufficient for diagnosis. Molecular probes for routine detection of the DNA of the pathogen are under development, though these may not discriminate between active and inactive infections. When trees with characteristic symptoms of sudden oak death are found within forests or woodlands where the disease is already confirmed, these trees are likely to be infected with P. ramorum. Infected trees are typically found in the proximity of other infected and dead trees. To date, this disease has been found infecting oaks in forests, woodlands, and urban-wildland interfaces. Symptoms of sudden oak death differ among the known hosts (Table 1). One characteristic of this Phytophthora infection in oaks and tanbark oaks is the sudden simultaneous leaf death on a major stem or leaf death on an entire tree, an observation that gave rise to the term "sudden oak death." The occurrence of leaf death may occur a year or more after the initial infection by the pathogen and many months after the tree has been girdled.
OaksOn coast live, California black, and Shreve oaks, the primary symptom is dark, hardened sap exuding from the main trunk, most often near the base of the trunk and up to about 6 feet above the ground. These seeps ("bleeding") are always found above the soil level on the trunk and can also occur on exposed roots. Less frequently, this seeping is found much higher on the main stem. Seeps often appear to emerge through intact bark. They may take the form of discrete red, brown, and black droplets, viscous oozing, stalactite-like drips, and red to brown stains on the trunk. Mosses and lichens are killed where they are contacted by the exudate. Their death may often be the only reliable indication that a tree is seeping. The seeping is the external manifestation of an underlying, diseased area of the tree, referred to as a canker. Removal of the surface bark reveals discolored brown tissue, separated from healthy bark by a distinct black zone line. This zone line represents the active front of the Phytophthora infection. The foliage may appear healthy until shortly before it turns brown. Before turning brown, the leaves can be olive-green, pale green, and yellow-green for a period of weeks to months. Infected coast live oaks may also lose leaves before they die. Tanbark OaksThe most consistent initial symptoms in tanbark oaks are wilting branch tips and dead leaves interspersed among green leaves throughout the tree. Thus, a symptomatic tree may show a number of pale green, light brown, and darker brown leaves among the darker, evergreen leaves. Seeping is not a consistent characteristic of infected tanbark oaks, though this may be prominent on some trees. While understory and seedling tanbark oaks infected with sudden oak death Phytophthora may die without any external bark discoloration or bleeding, proof of infection has been determined by isolation of the pathogen from discolored areas in the cambium (tissue under the bark) of these trees. The pathogen has also been found on the leaves of these trees. Infected trees with brown foliage are effectively dead, although there may be some sprouting from the tree bases. Many of these new shoots will wilt and die within a growing season. Additional Indications of Sudden Oak Death. Oak and tanbark oak trees infected with sudden oak death can also be infested with bark beetles and have Hypoxylon fungus growing on the trunk. These secondary pests are not unique to trees with sudden oak death but can serve as a good indication that the tree is ailing. Beetles in the family Scolytidae, such as the western oak bark beetle (Pseudopityophthorus pubipennis) and ambrosia beetles (Monarthrum dentiger and M. scutellare), may be associated with infected oaks and tanbark oaks in unusually large numbers. These insects are normally found in severely weakened and recently killed or fallen trees. The entrances to beetle tunnels closely track the seeping on the bark of trees bearing green foliage and are marked with red or white frass (wood residue from the beetle’s tunneling activities) around and beneath the tunnel entrances. Extensive observations indicate that nearly all coast live oaks that die following infection with P. ramorum have been colonized by these beetles before dying. Fruiting bodies of the fungus Hypoxylon thouarsianum are frequently found associated with both active and older seeping areas on the lower portions of the trunk of living oaks and tanbark oaks infected with sudden oak death Phytophthora. Fruiting bodies are somewhat flattened, dome-shaped structures that start out khaki green in color, but turn brown and then black as they age. These fruiting bodies indicate the presence of dead wood and are normally present on dead branches of living trees and on the trunks or branches of dead trees. In areas affected by sudden oak death, these fruiting bodies are rarely observed on the trunks of living trees in the absence of P. ramorum infection.
Other HostsSymptoms vary considerably among the other hosts and are primarily expressed in the leaves. Rhododendron species exhibit foliage symptoms, including brown spots and patches, particularly at the leaf tips. Twig and stem dieback are also common and may result in the death of plants. On huckleberry, the disease primarily causes twig and stem dieback, and ultimately, plant death. Leaves may exhibit necrotic patches leading quickly to abscission. In madrone, the symptoms are spots and necrotic areas on leaves, twig cankers, and stem dieback. Large branches and even entire trees may be killed. However, disease caused by P. ramorum is difficult to distinguish from cankers and twig dieback caused by Nattrassia mangiferae and Fusicoccum aesculi. Symptoms on manzanita appear to be similar to those on madrone. Bay laurel, buckeye, and big-leaf maple leaves show necrotic lesions; lesions on petioles and small twigs have been noted on California buckeye. For these species, the foliage symptoms may be similar to other endemic diseases. Seeping has not been noted on these non-oak hosts. DISEASE CYCLEThe biology, expression, and results of infection by P. ramorum can vary considerably among host species. The expression of symptoms is best understood for oaks and tanbark oaks but less so for shrubs such as rhododendron, huckleberry, and other known host tree species. In coast live oak, artificial inoculations of mature trees produced cankers up to 22 inches long (mean length 11 inches) within 16 weeks. Cankers in similarly inoculated tanbark oaks were up to 28 inches (mean length 18 inches). A number of inoculated trees were completely girdled (i.e., the infection spread around the entire circumference of the tree) by the pathogen during this 16-week period. Browning of the foliage occurred on several inoculated trees within one year. It is not clear whether viable spores are released from oaks and tanbark oaks. Where P. ramorum infects the leaves of host plants (particularly rhododendron, bay laurel, and tanbark oak), it has been found to readily produce spores on the leaf surface. The vertical distribution of cankers on oaks and tanbark oaks and the presence of foliar and twig infections on other hosts is consistent with both aerial and rain splash modes of spore dispersal that are typical for many species of Phytophthora. There is no evidence for transmission by insects or other vectors. Phytophthora ramorum appears to establish discrete infections in its hosts. In oak hosts, and especially in tanbark oaks, multiple independent cankers are common. These bark cankers may then expand and colonize more tissue, leading to girdling of the tree. Infection in the other hosts may be initiated in the leaves or stems. DAMAGEThe ecological consequences of sudden oak death Phytophthora are unknown but certainly will be significant. Oak trees provide shelter for many animals, and their acorns are an important food source for wildlife. Sudden oak death is particularly prevalent in state parks and protected watersheds. Although other trees and woody species will likely grow in forests that have lost large numbers of oaks and tanbark oaks, they cannot replace these hardwood trees from an ecological point of view. There is also a serious potential for wildfires resulting from the buildup of fuel from large numbers of dead trees. The economic costs associated with losses of these trees from landscapes and forests will be considerable. The cost of removing a large dead tree can exceed $1,000, presenting financial hardships for some property owners. The economic value of the land may be reduced by 30 to 40 percent with the loss of the aesthetic value of the oaks. MANAGEMENTThere is currently no known control for sudden oak death. The resting spores of other Phytophthora species are known to survive in soils for years and can be moved inadvertently in contaminated soil. Phytophthora ramorum has been isolated from plant debris in infested forests. It is likely that the spread of this pathogen in California has been facilitated by the activities of hikers, bikers, and vehicles, as well as by horses and deer. Preventing the movement of plant material (foliage and wood) may slow the spread of this pathogen to areas that are not infested. The lack of knowledge about the reproductive biology of this newly isolated species argues for restrictions on movement of such materials. Plants obtained from commercial nurseries may also serve as a means of spreading the pathogen. There are no prospects for saving trees infected with P. ramorum in forested habitats. For infected plants in landscaped settings, preliminary research suggests that control of this disease may be possible in the future. Because fungicides can help control other Phytophthora species in trees, experiments are underway to test such materials against this new Phytophthora species. In rhododendrons, P. ramorum may be controllable using treatments registered for use on other Phytophthora species. However, insufficient data are available to make recommendations at this time. The seeping symptoms of trees infected by other Phytophthora species can be very similar to those of sudden oak death. Reducing the excess water supply to a tree often can control these better-known Phytophthora species. The new Phytophthora has been isolated from hillsides in campgrounds, state parks, and recreation areas and does not appear to require overirrigation or low-lying wet areas to infect trees. The influence of bark beetles on the progression of sudden oak death disease in trees infected with Phytophthora ramorum is unknown. Although insecticides are registered for the management of bark beetles on oaks, their use is not recommended for the management of bark beetles associated with sudden oak death at this time. Dead trees are susceptible to structural failure because of infection by native decay fungi and wood-boring beetles, resulting in limb and trunk breakage. Therefore, trees killed by P. ramorum in residential areas should be cut down. The wood should be stored on site where possible and treated in a manner that dries it as rapidly as possible. Solarizing the wood by covering it with a clear plastic tarp in a location where it is exposed to sun will help kill the pathogen. This wood should not be moved from its source into regions where sudden oak death has not been reported. Movement within areas of infection should be minimized. WARNING ON THE USE OF CHEMICALS FURTHER READINGGarbelotto, M., P. Svihra, and D. M. Rizzo. 2001. Sudden oak death syndrome fells three oak species. Calif. Agric. 55 (1):9-19. Kelly, N. M., and B. A. McPherson. 2001. Multi-scale approaches taken to SOD monitoring. Calif. Agric. 55 (1):15-16. McPherson, B. A., D. L. Wood, A. J. Storer, P. Svihra, D. M. Rizzo, N. M. Kelly, and R. B. Standiford. 2000. Oak mortality syndrome: Sudden death of oaks and tanoaks. Tree Note 26, August 2000. California Department of Forestry and Fire Protection. Svihra, P., N. K. Palkovsky, and A.J. Storer. 2000. Sudden Oak Death: The Facts as We Know Them. Pest Alert #4, December, 2000. Univ. of Calif. Coop. Exten., Marin Co. ONLINE RESOURCESMonitoring sudden oak death in California California Oak Mortality Task Force Web page University of California Integrated Hardwood Range Management Program Web page Sudden oak death information from University of California at Berkeley State restrictions regarding wood infected with sudden oak death and regulatory sampling procedures PUBLICATION INFORMATION![[UC Peer Reviewed]](https://webarchive.library.unt.edu/eot2008/20090116034350im_/http://www.ipm.ucdavis.edu/IMAGES/logo-ucpeerreviewed.jpg){kind=logo}
Pest Notes: Sudden Oak Death in California Authors: B. A. McPherson, Insect Biology, UC Berkeley; D. M. Rizzo,
Plant Pathology, UC Davis; M. Garbelotto, Ecosystem Sciences, UC Berkeley;
P. Svihra, UC Cooperative Extension, Marin Co.; D. L. Wood, Forest Science,
UC Berkeley; A. J. Storer, Insect Biology, UC Berkeley; N. M. Kelly, Ecosystem
Sciences, UC Berkeley; N. Palkovsky, Center for Forestry, UC Berkeley;
S. A. Tjosvold, UC Cooperative Extension Santa Cruz Co.; R. B. Standiford,
Ecosystem Sciences, UC Berkeley; and S. T. Koike, UC Cooperative Extension,
Monterey Co. PDF: You need a PDF reader, such as Acrobat Reader version 8 or later, to view or print this PDF. If no reader is installed on your computer, you can download a free copy of Adobe Acrobat Reader. |
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