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Sudden Oak Death—What Gardeners Need to Know About a Deadly Pathogen on the Move
Plants & Gardens News | Volume 19, Number 3 | Fall 2004/Winter 2005
by Niall Dunne
Well, for a while it looked as if it had finally arrived in New York. Sudden oak death (SOD), that is. On July 2, the USDA's Animal and Plant Health Inspection Service (APHIS) announced that it had detected the pathogen on a red oak tree (Quercus rubra) in the Tiffany Creek Preserve, a 196-acre park in Nassau County, Long Island. However, as I'm writing this, one month later, it seems as if the initial tests produced false-positive results. Nonetheless, the park has been quarantined as investigations continue, and anxious officials wait for the final word to come in.
First discovered in 1995 in Mill Valley, California—but not properly identified until about five years later—sudden oak death has become a major problem in some western U.S. forests. The fast-spreading disease has killed tens of thousands of coast live oaks (Quercus agrifolia), Shreve oaks (Q. parvula var. shrevei), and tanoaks (Lithocarpus densiflorus) in coastal regions of California and southwestern Oregon.
Last year the disease was found infecting ornamental plants in nurseries in Washington and Oregon, as well as in California's Central Valley, sparking a nationwide investigation to see if SOD had spread eastward from its point of origin via the plant trade. So far more than 150 infected plants have been found at garden centers—and at least three homes—in 21 states.
Most forestry experts believe that if SOD establishes itself in the East, oaks trees in this part of North America could suffer a massive decline. Laboratory tests have shown that at least ten oak species native to the eastern U.S., including the ubiquitous northern red oak and the northern pin oak (Quercus palustris), can be infected by sudden oak death. As if oaks didn't have enough to worry about: Weakened and stressed by acid rain, extreme deer browse, and gypsy moth infestation, these majestic, ecologically important trees have been regenerating poorly in our forests for decades.
Gardeners need to be aware of sudden oak death to protect oak trees and other susceptible plants on their own property. (The pathogen is known to infect over 60 woody plant species besides oaks, including ornamentals like rhododendrons.) They also need to make sure they don't inadvertently spread the disease to landscape and forest trees by growing contaminated plants in their yards.
Biology of a Killer
Infection of susceptible oaks and tanoaks is usually lethal—and ugly. The most obvious symptom is the formation of large reddish-brown to tar-black bleeding cankers on the lower trunk and stems. Cankered trees can actually live up to three years, but once crown dieback begins, leaves brown off and drop in a matter of weeks—hence the "sudden" in the disease's common name. Infection of other host species most often results in nonlethal damage to leaves, but these species facilitate the spread of the disease to the more vulnerable plants.
The culprit is a microscopic funguslike pathogen called Phytophthora ramorum. (Technically it's not a fungus but an oomycete, or water mold.) It belongs to a genus famed for its parasitic prowess; indeed, the word phytophthora translates to "plant killer." Phytophthora infestans, for instance, caused the Irish potato blight in the late 1840s. And P. cinnamomi has been ravaging forests and shrublands around the globe for centuries.
Unlike most phytophthoras, which attack plants at their roots and are spread solely in soil and water, SOD attacks the aboveground portion of plants and produces aerial spores. Wind-driven rain and rain splash enable dispersal of the spores from host to host. Phytophthora ramorum enters its oak host's vascular system through the bark. It then breaks down plant defenses and cuts off the flow of food from the leaves to the roots, killing the plant from the inside out.
Recent genetic sequencing of the pathogen has revealed that it is alien to North America. Scientists have speculated that it entered the country in the San Francisco Bay Area as a stowaway on a food or plant shipment, probably from Asia. One researcher has hypothesized that sudden oak death originated and coevolved on Rhododendron in the Himalayan region, and that international trade in nursery stock has been responsible for its movement around the world.
North America is not the only country where Phytophthora ramorum has found a new home. The pathogen was discovered on rhododendrons in Dutch and German nurseries in 1993. In 2003, the U.K. detected it at 300 nurseries around the country, mainly on potted shrubs such as viburnums, camellias, Pieris, and rhododendrons.
Overall, it appears that sudden oak death exhibits a preference for cool temperatures with relatively high moisture. This and other factors (such as the presence of known host species) put Pacific coastal regions and much of the central and southern Appalachian region of the U.S. at especially high risk.
No cure has yet been developed for SOD; the only way to kill the pathogen right now is to cut and burn infected plants.
Nursery Nightmare
Sudden oak death is presenting a serious challenge to plant-health officials and nursery owners. In early 2002, the USDA quarantined 12 counties in California and one in Oregon in an effort to prevent the artificial interstate movement of SOD. With the discovery of the pathogen in nurseries outside the quarantine zone in 2003, however, it became clear that wider-ranging action was necessary. APHIS has now restricted movement of host nursery stock from all California nurseries and developed a federal nursery protocol and action plan, and the U.S. Forest Service has begun a nationwide survey of nursery perimeters and forests.
But it's hard to regulate the movement of a disease when species that can act as potential vectors are still being identified. In March of this year, during a routine inspection, SOD was discovered on Camellia cultivars at the Azusa, California, nursery of Monrovia, one of the nation's largest plant wholesalers. The names of these cultivars have yet to be released, but it's known that many of them were shipped around the country—Monrovia supplies around 5,000 garden centers nationwide.
The USDA has been relentless in tracking down these shipments. Monrovia has been forthcoming about the incident and has set up an informational section on its website (www.monrovia.com) to address customer concerns. The company advises anyone who has purchased a camellia from it or other California growers in the last 12 months to contact his or her local or state agricultural department for instructions.
Of course, since SOD has been in California for at least a decade, this may be just the tip of the iceberg. Rigorous sampling and testing of known host plants might not be enough to stop the spread of SOD from regulated areas to nurseries and home gardens elsewhere, and perhaps from these into nearby landscape or forest.
Advice for Homeowners
So what can individual gardeners do? First of all, be aware of the general symptoms (bleeding cankers on bark and leaf dieback) of the disease on host oaks and tanoaks. Unfortunately, these symptoms are not specific to SOD alone and so can be confused with those of other, less serious oak disorders such as oak wilt and oak decline. (Photos of sudden oak death and some of its look-alike disorders can be compared on the USDA Forest Service website, www.na.fs.fed.us/SOD/.) Only a lab test can confirm the presence of the Phytophthora ramorum pathogen.
Become familiar with the known foliar hosts of the disease. So far more than 60 of these have been identified, including natives like huckleberry (Vaccinium ovatum), bigleaf maple (Acer macrophyllum), mountain laurel (Kalmia latifolia), and witch-hazel (Hamamelis virginiana). In addition to rhodies and camellias, popular ornamentals such as some hybrid viburnums, European yew (Taxus baccata), and common lilac (Syringa vulgaris) can also act as hosts to the pathogen. (For a full list of associated plants, visit the USDA website listed in "SOD Resources.")
Infection of foliar hosts usually results in nonlethal leaf and twig damage, such as leaf spots, needle and tip blight, and shoot-tip dieback. Again, these symptoms are fairly nonspecific; your best bet is to avoid purchasing SOD-susceptible nursery plants that exhibit them. When you plant known foliar hosts of the disease, situate them as far away as possible from susceptible oak species.
If you live on the West Coast, especially in already-infested areas, take precautions immediately. The nonprofit Oak Mortality Task Force has published a homeowner's guide to SOD on its website (www.suddenoakdeath.org) geared especially for western gardeners. It includes detailed advice on how to identify and dispose of infected trees. Though the chances of encountering the disease are still small in the rest of the country, gardeners should be vigilant and report possible sightings to their local cooperative extension service or to the USDA hotline.
As mentioned, there is no cure for SOD. Congress has recently introduced legislation to fund more extensive research, regulation, and monitoring of the disease. But there is some good news, especially for gardeners in infested areas who want to protect their prized oak specimens: A new chemical treatment has been developed that can prevent oaks and tanoaks from becoming infected. The phosphonate-compound treatment is either sprayed on or injected into high-risk trees, depending on the species. If applied within two months of infection, it can also prolong the life of infected trees.
Niall Dunne is the associate editor of Plants & Gardens News.
Illustration by Paul Harwood