The Trail Companion
Sudden Oak Death
By Bob Kelly.
You may have heard about the sudden death of Bay Area
tanoaks (Lithocarpus densiflora) and coast live
oaks (Quercus agrifolia).
This problem has become so serious that a state of
emergency may be declared in Marin County. Deaths of
tanoaks and coast live oaks have reached epidemic
proportions. Researchers have dubbed the syndrome Sudden
Oak Death, or S.O.D.
Photo by Brother Alfred Brousseau, St. Mary's
In 1995 the first
mysterious death of tanoaks was reported in Mill Valley,
but are now occurring from Big Sur to Santa Rosa. If you
hike in the Coast Ranges, you are likely to see infected
tanoaks with all or most leaves brown and dead on the
branches. Tanoaks and coast live oaks grow together in
native settings and now coast live oaks are dying with
similar symptoms. Black oak (Quercus kelloggii)
can be affected as well.
Fungi and Beetles
Recent research has implicated a newly identified species
of Phytophthora fungi in causing this dieback - a
genus that has caused a great deal of trouble, including
the Irish Potato Famine and die-offs among other tree
species worldwide. Exactly how the fungus infects trees
isn't fully understood and a variety of environmental
factors may predispose the trees to the onset of this
The symptoms first appear
as wilting of leaves, especially in new shoots.
Approximately 2 - 3 weeks later the foliage turns brown but
remains on the branches. At this point Hypoxylon
spp. fruiting bodies, commonly called charcoal mushrooms,
appear in patches on the trunk and or lower branches. These
look like dark brown to black granules and stains on the
bark. Sometimes before all symptoms are visible, the trees
also become vulnerable to several wood boring insects,
principally the western oak bark beetle
(Pseudopityophthorus pubipennis), then the
ambrosia beetle (Monarthrum spp.).
These beetles attack the lower trunks of susceptible trees
but may advance 20 feet or more up the trunk and into the
lower branches. These are native insects, long associated
with these trees, and are not known to have caused
|Coast live oak
Photo by Beatrice F. Howitt, California Academy of
So which comes first,
disease or insect? It is not clear if the insects are
vectors or the trees become sick and are predisposed to
attack. It could be a combination of the two.
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