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/