The Trail Companion

Fall 2000

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).

Tanoak (Lithocarpus densiflora).

Photo by Brother Alfred Brousseau, St. Mary's College.

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.
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 problem.
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.).

Coast live oak

Photo by Beatrice F. Howitt, California Academy of Sciences.

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 mortality previously.
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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The Trail Companion

Fall 2000

Sudden Oak Death

Fungi and Beetles

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