Sudden Death Looms for Oaks - sudden oak death tree disease hits Marin County, CA

American Forests, Summer, 2001 by Mary M. Woodsen

A swift-moving disease threatens California's signature tree, among others--and raises the specter of Dutch elm disease and chestnut blight.

It's not an obsession; after all, Kent Julin gets paid for looking at trees. Still, each day when he comes home he can't help but examine the two young coast live oak trees in his yard. Sometimes he imagines the bark of the Quercus agrifolia suddenly flecked with oozing, dark-hued droplets, though a closer look finds nothing amiss. In a passing breeze he even imagines spores lighting on the trees, although spores, the reproductive essence of microorganisms, are way too small to see.

But as forester for Marin County, California, Julin is all too aware that his trees, healthy though they appear, are at almost certain risk of contracting a new and deadly disease: sudden oak death (SOD).

Marin County is ground zero for sudden oak death. The disease was first reported in 1995 on a clump of tanoaks (Lithocarpus densiflorus), relatives of both oaks and chestnuts. The effected trees were in a backyard in Mill Valley--the Bay Area burg was Julin's boyhood home. By '97 the disease had appeared on coast live oaks and California black oaks (Quercus kelloggii), but in such low numbers that few people paid attention.

That changed when the disease hit big time in the summer of '99. Now tens of thousands of dead and dying trees mark the backyards and byways that wind among Mann's lovely hills. Santa Cruz County is just as bad off, and the disease is moving with lethal speed through Monterey, Napa, Sonoma, San Mateo, and Santa Clara counties.

This poses both ecological and economic angst. Wherever trees die en masse, erosion, stormwater runoff, and the threat of wildfires follow; watersheds can become more drought-prone, landslides more likely, and wildlife habitat deteriorated. Homeowners' property values can drop, for just one coast live oak--California's signature tree--can add upward of $10,000 to the worth of a home. Plus those dying trees have to be removed at close to $1,000 a pop.

And there's no cure in sight.

Adding to Californians' fears (along with those of horticulturists, plant pathologists, and foresters around the world) was an announcement in January that a disease on rhododendron nursery stock in Santa Cruz County--and a disease on rhododendrons in Germany and The Netherlands--was none other than sudden oak death. Oaks may not be big-ticket items in the national and international nursery trade, but rhododendrons are. And global trade drives the rapidly increasing spread of every class of invasive organisms.

Could SOD spread to other species of oak in North America? Perhaps. A worst case scenario would be one or more of our beautiful oaks proving so susceptible that a plague comparable to Dutch elm disease or chestnut blight would sweep major ecosystems, even continent-wide.

And what about native rhododendrous; are they also in danger? It's simply too early to tell, says Susan Frankel, a Forest Service plant pathologist and head of the California Oak Mortality Task Force, "The California Native Plant Society has an amazing database; they know where many of California's two native species of rhododendrons are located. We'll be hiking out to look for signs of disease as the season progresses. Fortunately, while rhodies are highly susceptible to several different phytophthoras, it's rare they die from them."

Then in February researchers found SOD on native evergreen huckleherry, a rhododendron relative, in California's Muir Woods. The huckleberries died to the ground, which raises the specter of blueberries and cranberries succumbing as well. The huckleberry is native throughout the Pacific Northwest, providing yet another avenue for spread of the disease.

Researchers likewise found SOD on Shreve's oak (Quercus parvula var. shrevei) a locally common denizen of coastal areas. By April, SOD had turned up on rhododendron relatives Pacific madrone (Arbutus menziesii) and hay laurel (Umbellularia californca), as well as viburnums in Germany, which raises concern not only for our native viburnums but for their relatives, honeysuckles and elderberries.

What causes sudden oak death? Though most reports call it a new and yet-unnamed fungal species in the genus Phytophthora (fy-tof-thor-a), it isn't--quite. Formerly classified as fungi, phytophthoras are adaptable and aggressive algae-like pathogens. (It was a phytophthora that precipitated the Irish potato famine in the mid-19th century.)

Nor are these pathogens new to California or its oaks. At least two related species attack oaks inadvertently overwatered by homeowners caring for their lawns, and the symptoms mimic SOD. Still, this phytoplithora could spread faster and further than most. Its spores are carried by soil and rain-splash and may be airborne as well.

Regardless how the tree becomes infected, the symptoms of sudden oak death are similar. Often the first sign is black or burgundy sap oozing from tiny holes in the bark; soon after, the tree wilts, turning starkly brown. Yet "sudden oak death" is a misnomer, says Frankel. "Trees are affected long before the symptoms show. By the time you see that oozing sap, your tree is almost dead. The pathogen has already attacked the bark, killing the tissue that carries sugars from the leaves to the roots and essentially girdling the tree."