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Sudden oak death |
As of November 11, 2006, there were about 47 regulated host species (hosts proven by Koch's postulates); that is up from 29 two years earlier. An additional 58 plant species are considered associated hosts (hosts strongly indicated by isolation or molecular test; up from 31 two years earlier). A complete host list can be downloaded from USDA Animal and Plant Health Inspection Service here. Hosts include the following:
The pathogen is a newly described species, Phytophthora ramorum, a member of the Oomycota. It produces at least two kinds of spores:
Oospores have not been found in nature, though they may occur.
Although airborne Phytophthora species that infect above-ground plant parts are known from agriculture, forest pathologists have generally thought of these fungi as soil-borne, root-infecting pathogens. However, looking for Phytophthora ramorum in leaves, stems and branches has opened a window into additional Phytophthora species that had not been studied before. Everett Hansen at Oregon State University has a new species, P. nemorosa, that he will be formally describing shortly, and there are additional new species being found. Some of these cause diseases similar to those caused by P. ramorum, but so far they appear to be less aggressive and are presumably native to western North America.
Little is known of environmental factors in this disease. The pathogen has a temperature optimum of 15-20 C. The restriction of the disease to coastal areas may indicate restrictive moisture requirements or may simply mean the fungus has not yet had an opportunity to spread to other areas.
The pathogen presumably survives periods unsuitable for growth as mycelium, chlamydospores or as oospores.
Strangely, sporulation has not been detected on stem and branch cankers and must be rare if it occurs at all. Thus, the lethal disease on woody hosts apparently does not contribute to inoculum. Hosts that get lethal cankers probably get infected from inoculum produced on foliar hosts. Abundant sporulation has been found on foliar lesions in the field and can readily be induced on lesions in the lab. Lesions on leaves dried for several months can still sporulate.
The pathogen may enter new areas when soil is carried on vehicles or animals or perhaps when infected host material is carried into new areas. The soilborne phase may be mycelium, chlamydospores and perhaps oospores in infested host material, or spores in soil. Inoculum may also be carried in streams. It is most commonly detected by placing a leaf of a highly susceptible host, such as Rhododendron, in the stream as bait.
The pathogen has been found in soil and may be soilborne but infections are exclusively on above-ground plant parts; the fungus does not infect roots. Experimental inoculations indicate that infection can occur without wounding, both on leaves and stems.
As stems die, succulent shoots may wilt and leaves turn brown. Mortality may precipitate or be accompanied by attack by ambrosia beetles, oak bark beetle, the saprot ascomycete Hypoxylon thoursianum, and Armillaria species coming up from the roots.
There is no root infection and cankers do not extend below the soil line.
The disease was first reported in 1995 in Mill Valley of Marin County, just north of San Francisco. It is now confirmed in 12 counties of California, from Monterey County in the south to Humboldt county, well into northern California. Humboldt and Contra Costa Counties are recent expansions of the distribution within California. Between 5 and 12% of California's redwood/tanoak forests are infested.
In addition, the disease was found in southwestern Oregon in July 2001. Near Brookings, in Curry County, Oregon, a number of infection foci were found over an area of nine square miles. An eradication campaign was quickly organized, involving cutting and burning all vegetation in and near the foci. During this process additional foci were discovered and treated. Discoveries continued, however, and as of December 2005, the disease had been found and eradication treatments conducted on 51 sites totaling 88 acres. Monitoring has shown that the pathogen can survive cutting and burning of infested areas. Now treatments include herbicide to prevent sprouting, which can carry the disease, or repeated cutting and burning until no inoculum can be detected. In 2005 and 2006, the number of trees infected and acres infested increased. In 2006, 35 new infested sites were discovered, and some previous sites expanded. It remains to be seen whether this attempt at eradication actually turns out to be more of a slow-the-spread program.
How the fungus made such a large jump (if indeed the forests between Oregon and the California infestation are not more or less continuously infested) is not clear. This disease seems to be spreading as fast as any disease introduced to North America thus far. If there is any reason for optimism at all, it is that, thus far at least, the disease seems to be restricted to coastal areas.
However, spread of the disease to other parts of the continent and new introductions from abroad are considered likely. The susceptibility of northern red oak and many other important species suggests great devastation may occur if the environment in the eastern U.S. is conducive.
In May 2003 the pathogen was found on some shrubs (arrowwood and andromeda) in a nursery just east of Portland in northwestern Oregon, along interstate highway 5. Apparently the suspicion is that the fungus was introduced there with plants from Europe rather than from the established population in the U.S. To eradicate the pathogen, many plants were destroyed in the nursery, plants recently shipped are being traced and inspected, and the neighborhood within 1/4 mile is being intensively inspected. The nursery has been quarantined to prevent further spread.
In early 2004, the disease was found in Monrovia Nursery near Los Angeles, California. This large production nursery ships extensively throughout the United States. Traces of shipments, and surveys in the target states, soon revealed that the disease was present in 13 other states. The numbers of positive nurseries, by state, receiving shipments from the source in southern California were: California (38), Alabama (1), Florida (5), Washington (6), Oregon (9), Texas (5), Colorado (1), Georgia (13), Louisiana (5), Maryland (1), North Carolina (9), New Mexico (1), Tennessee (2), and Virginia (1).
In January 2006, the pathogen was found in a stream draining a nursery in Pierce County, Washington. The nursery had plants that tested positive in 2004 and 2005 and eradication procedures required by USDA were followed. Presumably the infestation arose from the infested nursery stock shipped from southern California.
The pathogen itself was discovered and named in The Netherlands at about the same time that the etiology of sudden oak death was being elucidated. In Europe, however, trees are less often affected and the problem is mostly restricted to nurseries and gardens on Rhododendron and Viburnum. Recent surveys have discovered the pathogen in the Netherlands, Germany, France, Poland, Italy, Spain, Belgium, Sweden, and the UK. In the UK, where Pieris and Camellia have also been found infected, surveys in 2001 and 2002 showed an apparent rapid infestation of 140 nurseries during that time. Nursery stock is suspected as being the pathway of movement of the pathogen, although genetic evidence suggests that the population in California and that in Europe probably had separate origins. The native range of the pathogen is still unknown.
Realistically, little or nothing can be done to treat infected plants or protect plants from inoculum. An experimental effort to stop local spread in generally infested areas is being conducted in northern California; this consists of removing bay laurel and tanoak, followed by pile and broadcast burning. However, most effort is directed at limiting the spread of the disease into new areas, particularly because the evidence available suggests that eastern oak species are just as susceptible as California oak species in inoculations.
Sudden oak death (SOD) is a new disease that became established around the San Francisco Bay of California and has since been found in southern Oregon and more recently disseminated by man to other parts of North America.
The fungus is almost certainly introduced to California, and the same fungus has since been found in Europe, but the origin and nature of the introduction are unknown. Virtually all the facts point to a recent introduction:
Sudden oak death may develop into one of the most devastating diseases to hit North American forests, but this is uncertain. Some say its potential was overblown, it is killing relatively unimportant tree species, and doesn't seem to be establishing itself away from the Pacific coast. Even in the 12 California counties that were invaded by 2004, however, the hosts that are readily killed are dominant on 1.5 million acres, and the impact to wildlife and other species may be great (®).
The discovery of the disease on coast redwood and Douglas-fir is another in a line of alarming findings. These are among the most treasured and valued tree species of the Pacific coast. However, occurrence of the disease on these hosts is very limited in distribution and severity, and no symptoms have been seen on overstory trees. Disease on these hosts may be restricted to areas with extremely high inoculum loads (because of more susceptible species, especially bay laurel) and especially conducive environment.
