Oak Sudden Death
January 6, 2001
Our oak trees are dying. Broad concern in Northern California about Sudden Oak Death is putting yet another forest health issue before the public. Millions of dollars are being spent to combat and research the outbreak, and nobody knows how, if and when it will end.
Reaching under the bark, this fungus infects the bark, the cambium layer and enters into sapwood, allowing invasion of the tree by secondary insects—oak bark beetles and 2 species of ambrosia beetles, which kill the tree. Then Hypoxylon fungus rots the wood and produces dark green and black hemispherical fruiting bodies. The deadly Phytophthora spreads, probably by air, to nearby healthy trees creating pockets of dead oak trees.
Since initially being recognized in tanoak and named by Marin Cooperative Extension’s Pavel Švihra in 1995, the infestation has spread to coast live oaks and black oaks. Dr. David Wood and Dr. Brice McPherson, entomologists at UC Berkeley, have been monitoring plots in Marin County since April. UC Davis professor Dr. Dave Rizzo last summer identified the culprit as the 61st known species of the genus Phytophthora---similar to that killing Port-Orford-cedars in California and Oregon.
Some 160 members of the newly organized California Oak Mortality Task Force met in mid-October and toured affected Marin County forests. We gloomily visited coastal oak woodlands, all populated with scattered dead trees. Thousands more are infected. This threat looms over 1.2 million acres of coast live oak woodlands, and millions of acres of mixed evergreen forest. The task force organized into committee and proposed to meet again in Marin January 31, 2000
Although the mortality is by far the most severe in Marin and Santa Cruz Counties, surveyors believe the syndrome is a cause of coastal oak mortality from Monterey to Humboldt Counties. Innumerable pockets of mortality exist, but by no means all are attributable to the new Phytophthora.
Trees go from apparently healthy, with green foliage, to dead in a single season. To determine if a green tree may be infected, look for dark seeping stains in the lower 4-7 feet of the trunk. These indicate fungal activity in the bark. Also look carefully for “oak blood”—droplets of dark liquid oozing from crevices in the bark. Under the bark surface there is a characteristic black perimeter “zone line” which delineates tissue killed by Phytophthora from healthy tissue.
The fungus’ spread may be climatically limited to fog belt forests as it readily survives prolonged drought and thrives in cooler moist climates. It is killed in the laboratory by high temperatures and that may limit its spread inland. Few management guidelines exist, yet there is no question that this threat to our hardwoods is a complex, poorly understood, and expanding problem.
If you see Sudden Oak Death please report it online. For much more information here are the websites to explore:
Sudden Oak Death http://cemarin.ucdavis.edu/index2.html
Berkeley Oak Research http://himalaya.cnr.berkeley.edu/oaks/