By Barrie D. Coate, arborist

As all of us in the Santa Cruz Mountains know, the sudden death of tan bark oaks (Lithocarpus densiflora) has left large tan oaks which appeared healthy last year declining and dead this summer.

As though this weren’t devastating enough to the property of those who have lost large tan oaks, the two beetle species which "finish off" the weakened tan oaks leave them to attack coast live oak (Quercus agrifolia).

Coast live oak is the oak species which is most commonly seen within 100 miles of the coast, from Mendocino County down into Mexico.

Indeed, when you look at the mountains surrounding the Santa Clara Valley, 80% or more of the green "domes" you see are coast live oak. The fact that deadly insects are attacking this dominant species is far more important than its attack on tan oaks because of the relatively large proportion of our natural tree cover which is provided by coast live oak.

When oak bark beetles (Pseudo-pithiphorus agrifoliae) and then oak ambrosia beetles (Monarthrum scutellare) build a heavy population in a tan oak, the mature adults leave the dying tree and attack nearby coast live oak. This is the oak species most common in this area. It has dark green, 2-inch long cupped leaves with soft spines at the ends of each lateral view.

Thanks to Dr. Pavel Svihra and others of the University of California Cooperative Extension Service, work is underway to explain where and why this sudden phenomenon has originated.

The latest information is that the female brings ambrosia fungus (Monilia brumea) from an infected tree to a previously healthy tree (as the elm bark beetle brings Dutch elm disease to elms, which are then killed by the disease, not the beetle) and the oak is killed by the disease in a single season.

What to do?

If you have tree with 15 to 20 dark, oozing patches on the bark at 1- to 4-feet above the ground, the ambrosia beetles have functionally done enough damage that the tree cannot recover. Those trees must be cut down and either hauled away or cut up and stacked under 6 mil clear plastic covers tightly sealed to prevent adult beetles from leaving.

If 1-inch round dark patches with a white spot in the middle are seen in branches or trunks from the ground to the top of the tree, and white very fine textured frass is found on the trunk or ground, oak bark beetle is the culprit.

If more than a few of these are seen, it is probably too late to treat the tree and it should be removed to a dump, or the wood covered. If 1- to 2-inch diameter solid clusters of charcoal-like lumps are seen on a trunk, they are probably a Hyposylon fungus which feeds primarily on deadwood. Collections of these indicate the portion of the trunk to which they are attached has been dead for more than a year.

In either case, Dr. Svihra suggests spraying all woody parts of uninfested coast live oaks with the insecticides Astro or Dragnet between March and April and again between mid-August and mid-September. These chemicals are not available to the homeowner and they must be applied by a certified pest control operator. They are non-phytotoxic when used as directed.


An earlier story...

In mixed hardwood forests of Marin County and the Santa Cruz Mountains, tanbark oaks (Lithocarpus densiflora) are dying. For example, several large tanbark oaks may be seen dying across from Caltrans slide repair on Highway 17.

Dr. Pavel Svihra, a plant pathologist with the University of California Cooperative Extension Service in Marin County, has been conducting research into the causes of this phenomenon.

The first incidence of sudden tanbark oak death was reported in 1995 in Mill Valley.

Tanbark oaks are primarily an understory tree, which eventually grow up into the canopy of the surrounding companion species, madrone, coast live oak, bay trees and occasionally Douglas fir and redwoods. From a forester’s viewpoint, they are "junk" trees because their wood is not very useful and their vascular tissue is easily damaged by even low temperature forest fires. From an arborists’ point of view, once adjacent sturdier trees are removed, the tall, small diameter trunk specimens appearing in the understory are hazardous, because they have a high probability of "wind fall" (falling over).

For the homeowner in the native forest, however, these trees may provide critical screening between neighbors or shade for a home. For those of us who have moved to the Los Gatos mountains for the green canopy and forest density, the death of twenty to thirty tanbark oaks on or adjacent to our property can be catastrophic to the enjoyment of our outdoor living space.

Several disease symptoms and insect populations have been found on trees during the first appearance of symptoms as well as in dead trees. It must be emphasized that these are secondary pests that attack weakened trees, and are not the primary causes of tree death. Among the assorted pests identified have been inconsistent infections by oak root fungus (Armillaria mellea) and water mold disease (Phytophthora species). Insects include oak bark beetle (Pseudopityophthorus pubipennis and ambrosia beetles (Monarthum scutellare species).

After the tree dies, charcoal fungus (Daldinia species) will often appear on the trunk. This disease only feeds on dead wood, and did not kill the wood.

In each of the several pathology lab tests conducted, three diseases were found in the lower trunk and root systems, but none where disease species initiate decline.

The only recommendations:

1. Promptly remove dead trees.

2. If the wood is kept, cover it with 6 mil plastic, sealed at the edges with dirt.

3. Express your support of research carried on by the University of California Cooperative Extension Service by addressing letters to The Chancellor of the U.C. Extension, 1682 Novato Boulevard, Suite 150 B, Novato, CA 94947.


(c) 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 mountain network news All rights reserved.