Regional Gardening Reports :: National Gardening Association

In the Garden:
Northern California Coastal & Inland Valleys
October, 2000
Regional Report

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In Northern California, the native oaks are under attack by sudden oak death.

Sudden Oak Death

Sudden oak death (what a terrible name!) is threatening to wipe out California's majestic oak woodlands. It first appeared in Marin County only 5 years ago and potentially threatens oaks in the counties along the coast and San Francisco Bay, including Mendocino, Sonoma, Napa, Solano, Contra Costa, Alameda, San Mateo, Santa Clara, Santa Cruz, and Monterey.

The disease has never been seen before in California and is killing not only coast live oaks (Quercus agrifolia) but also black oaks (Q. kelloggii) and tan oaks (Lithocarpus densiflorus) despite the best efforts of foresters and scientists to halt the rapid demise of these stately trees.

Sudden Oak Symptoms

Symptoms of sudden oak death occur first in the new foliage, which wilts or droops suddenly, then turns yellow or brown. Next, cankers appear on the trunk and limbs, which secrete an oozing sap that ranges in color from deep red to black.

The Disease

It's thought that the cause of the problem may be the fungal disease Phytophthora. Although the fungus is related to the same organism that caused the Irish potato famine in 1845, this particular strain does not match the 60 known species of Phytophthora. It appears to be related to the strain that has killed thousands of cedars along creeks, streams, and tributaries in the Pacific Northwest.

How It Spreads

It's not known for sure how the fungus spreads from county to county. It may have been trapped in the soil on the bottom of someone's hiking boots or in the tread of a bicycle tire. It may have been moved from one place to another on firewood. Perhaps insects carry the disease as they move from tree to tree. Fungal spores move best when carried by water, but cankers have been found high in the branches of infected trees, indicating that the disease may have been spread by wind.

The Potential Loss

Oak trees are perfectly adapted to the alkaline soil and dry summer season in California. Deeply rooted with a spreading canopy of leathery leaves, these trees are as much a part of the California landscape as the Golden Gate Bridge. An estimated 10 million acres of oak trees cover the hills and valleys of California. The evergreen coast live oak grows to 70 feet tall, features a smooth, dark gray bark, and is a common sight along California roads.

Imagine California without these glorious trees. Not only would it change the landscape dramatically, it would also change the ecosystem radically. Many creatures depend on the oaks for food and shelter. The acorns provide winter fodder for deer and squirrels. The leafy canopies provide secure nesting sites for many types of birds including jays and robins.

What to Do?

So far only 3 of California's 19 varieties of oaks have been infected with sudden oak death. However, the fungus is apparently very adaptable and may eventually change to infest other varieties of oak trees.

The best control is to prevent its spread. Spraying infected trees is not effective.

To prevent the spread of sudden oak death, hikers and bikers are encouraged to remove soil from their shoes and tires after walking or biking in oak woodlands. Also, don't carry oak firewood between counties or out of state. Although these measures may help in a limited way, more research is needed to prevent the oak trees of California from going the way of the elm trees of the East Coast.

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