The popular press has focused on the more sensational aspects of the potentially devastating disease called sudden oak death. Here, our laboratory reporter presents the facts about the disease as they are now known.
Sudden oak death and oak death syndrome are names given to a recently recognized disease in oaks in central coastal California. It is so named because infected trees suddenly change color from green to yellow or tan and then to brown. Though trees may have had the disease from six months to more than two years, the rapid color change associated with the death of the trees is a distinguishing feature. After changing color, leaves may begin to drop or may remain attached for up to a year or more.
The disease first was observed in Marin County in 1995, where tanoaks, or tanbark oaks (Lithocarpus densiflorus), were dying. Although there was some concern, this species is not of direct commercial value (sometimes even considered a weed tree), so not as much attention was given as should have been. In 1997, a diseased coast live oak (Quercus agrifolia) was found in a wooded area of Marin County. By 1999, the disease was found on trees in Monterey and Santa Cruz counties and, shortly after, in Sonoma County. Since then, it has been found in San Mateo, Santa Clara, Napa, Solano, Mendocino, and Alameda counties in California and in a remote isolated area of Curry County in southwestern Oregon. In addition to these two species, California black oaks (Q. kelloggii) and Shreve oaks (Q. parvula var. shrevei) have been found to be susceptible to the disease. The disease has been confined to forested areas, with the most severe outbreaks in Marin, Santa Cruz, and Sonoma counties plus a large area in the Big Sur region of Monterey County. The areas affected are separate and not contiguous; only one diseased oak has been found in Alameda County and only a few trees in Solano County. There is a report of a single infected tree in Prunedale, a considerable distance from the affected area in Monterey County.
Symptoms of the disease are much the same in true oaks (Quercus) as in tanoaks (Lithocarpus), but there are a few differences. In tanoaks, the first symptoms may appear as a drooping or wilting of the new growth; this is not found in true oaks. In both, black or brown areas (cankers) develop on the lower main trunks (mostly on the lower five to six feet, though as high as twenty feet on oaks and thirty feet on tanoaks), but these may be difficult to discern. Red to black sap exudes from these cankerous areas, a symptom of infection. This seepage is toxic to mosses and lichens found on the trunks, and small patches of these dead plants serve as another symptom. The seepage eventually dries to a shiny crust.
Yet another indication of the presence of the disease is the appearance of fruiting bodies of the fungus Hypoxylon thouarsianum, a secondary fungus that rapidly invades diseased, weakened, or dead woody tissue. These fruiting bodies are hemispherical, from one-fifth to one and one-fifth inches in diameter, and are khaki green and powdery when they first appear but soon turn brown or black. This fungus is not always found on tanoak, and particularly not on young trees.
Two species of ambrosia beetles (Monarthrum scutellare and M. dentiger) tunnel into the sapwood of the cankerous zones on diseased trees. The western oak bark beetle (Pseudopityophthoras pubipennis) burrows into the phloem and outer bark of cankerous areas. These beetles, attracted only to weakened trees, produce large quantities of boring dust, light or dark colored depending on the beetle. This dust collects on the bark, particularly in crevices, and on the ground and is further evidence of the disease. In many trees, the beetles appear to hasten the spread of the diseased areas, and eventually the tree is girdled and dies. Tanoaks may die without the beetle intervention; live oaks have died without the presence of beetles following inoculation of the trees by researchers.
In the summer of 2000, a plant pathologist at Davis isolated a fungus from infected trees that appeared to be a new species of Phytophthora. A visiting European researcher thought it resembled a fungus isolated from rhododendrons in Germany in 1993. In 2001, DNA studies showed the fungus from oaks to be the same as the fungus found in Germany, where studies had shown it to infect a number of rhododendrons as well as Viburnum x bodnantense. They named the fungus Phytophthora ramorum because it infects leaves and branch tips in rhododendrons; the name is derived from ramus, which means branch.
Research has now shown Phytophthora ramorum to be the causal agent of sudden oak death. In 2001, the fungus was found in rhododendrons in a nursery in Santa Cruz County, near an outbreak of the disease in oaks. To date, only two cultivars of rhododendrons have been found infected, neither of them in the group known as azaleas. The fungus has also been found in a number of native West Coast plants, including evergreen huckleberry (Vacccinium ovatum), California bay (Umbellularia californica), buckeye (Aesculus californica), manzanita (Arctostaphylos manzanita), toyon (Heteromeles arbutifolia), honeysuckle (Lonicera hispidula), big leaf maple (Acer macrophyllum), and coffee berry (Rhamnus californica). It tends to be found on these plants wherever the fungus occurs in nearby oaks. It infects stems of oaks (usually resulting in their death), Vaccinium in California, and Viburnum in Europe. In Oregon, it infects stems of Viburnum, Lithocarpus, and Rhododendron macrophyllum, killing the latter two. In all other susceptible plants, it produces either leaf spots or leaf spots combined with twig blights. Though reported recently on coast redwood (Sequoia sempervirens), only fungus spores were found on small sprouts of a few large trees. Subsequent inoculations have shown that redwoods are not susceptible.
The causal organism is in a group of fungi that are significant plant pathogens. Most Phytophthora species are soil-borne organisms attacking roots and crowns of various plants. Some species have wide range of host plants: P. cinnamomi has been reported on over 600 species of plants. In the Pacific Northwest, P. lateralis is a root pathogen that attacks only two species of plants. The late blight fungus (P. infestans) attacks tomatoes and several species of Solanum, including potato, and is the causal agent responsible for the terrible Irish potato famine in the 1840s. Like P. ramorum, it is an aerial pathogen.
The subject of control is of great concern. Trees not infected in areas where most trees are infected need to be tested for resistance. Unfortunately, this is difficult with oaks as they are challenging to propagate vegetatively. A number of rhododendron cultivars do not show symptoms in the nursery where the fungus was found on two others; these are to be inoculated to determine their susceptibility. Currently, in some areas where the fungus occurs, nearby plants are being sprayed experimentally with copper sulfate pentahydrate, mixed with tanate, picrate, ammoniate, and formate surfactants to reduce copper toxicity.
The information presented here is from published articles, firsthand observation, and conversations with researches currently working sudden oak death, including M Maggi Kelley, Brice a McPherson, David M Rizzo, Richard B Standlford, Andrew Storer, Pavel Svirha, and David L Wood. At presstime, Canada had imposed a quarantine on imports of plants and soils from California and Oregon, a situation which could have serious implications for the nursery industry in those states. More information will be presented as it becomes available.