Sudden oak death
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 Canker on an infected oak
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| Phytophthora ramorum Werres et al. 2001 |
Sudden Oak Death (Phytophthora ramorum) is a water mould infection which afflicts some oak species and some other trees and shrubs, causing them to die rapidly. It also causes a non-fatal leaf disease in many other plants such as rhododendrons and California Bay Laurel. There is no known cure but new studies show that periodic wildfires help protect against the mould.
It was first discovered in California in 1995 when large numbers of Tanoaks (Lithocarpus densiflorus) died mysteriously, and was described as a new species of Phytophthora in 2000. It has subsequently been found in many other areas including Britain, Germany, and some other U.S. states, either accidentally introduced on nursery stock, or already present undetected. The origin of the disease remains unknown, although it was reported infecting European nursery rhododendrons and viburnum in 1993. In tan oaks, the disease may be recognized by wilting new shoots, older leaves becoming pale green, and after a period of two to three weeks, foliage turns brown while clinging to the branches. Dark brown sap may stain the lower trunk's bark. Bark may split and exude gum, with visible discoloration. After the tree dies, suckers will sprout next year, but their tips soon bend and die. Ambrosia beetles (Monarthrum scutellare) will most likely infest a dying tree during midsummer, producing piles of fine white dust near tiny holes. Later, bark beetles (Pseudopityophthorus pubipennis) produce fine red boring dust. Small black domes, the fruiting bodies of the Hypoxylon fungus, may also be present on the bark. Leaf death may occur more than a year after the initial infection and months after the tree has been girdled by beetles.
In Coast Live Oaks and Black Oaks, the first symptom is a burgundy-red to tar-black thick sap bleeding from the bark surface.
Promoting tree health may help prevent disease and insect infestations. Make certain roots and soil remain undisturbed, prune dead and dying branches out, and landscape below oaks with mulch or drought-tolerant plants. When a tree dies, experts recommend cutting it down and grinding down the stump. Firewood should be covered with 6 mil plastic, sealed at the edges with dirt or rocks, for six months to prevent beetle emergence.
In addition to oaks, many other forest species may be host for the disease, including rhododendron, Madrone (Arbutus menziesii), Evergreen Huckleberry (Vaccinium ovatum), California Bay Laurel (Umbellularia californica), Buckeye (Aesculus californica), Big-leaf Maple (Acer macrophyllum), Toyon (Heteromeles arbutifolia), manzanita (Arctostaphylos spp.), Coast Redwood (Sequoia sempervirens), Douglas Fir (Pseudotsuga menziesii), Coffeeberry (Rhamnus californica), Honeysuckle (Lonicera hispidula) and Shreve's Oak (Quercus parvula v. shrevei). Disease progression on these species is not well documented but hikers have observed dead Douglas Firs with massive quantities of red frass surrounding their base. Redwoods exhibit needle discoloration and cankers on small branches, with purple lesions on sprouts that may lead to sprout mortality.
The disease is known to exist in California's coastal ranges between Big Sur and southern Mendocino County and is confirmed in Alameda, Marin, Mendocino, Monterey, Napa, San Mateo, Santa Clara, Santa Cruz, Solano and Sonoma counties. It is reported in Oregon as well as of 2002.
