This summer’s visible increase in brown-leaved or defoliated trees in West Marin has led to a resurgence of concern about sudden oak death.

“We’ve been getting a lot of calls from people who are worried,” said Steven Swain, the horticultural advisor for the UC Extension office in Marin. “This is a bad year to be an oak tree.”

There are two main problems plaguing local oaks. The primary, and most concerning, is the ongoing spread of sudden oak death. The second is an outbreak of the oak worm, a defoliator that appears cyclically every five to seven years. Oak trees are adapted to resist the oak worm, and though the trees appear to be in dire condition they should be healthy next year.

“Oak worm is more of a cosmetic issue,” Swain said. “Although it might push a few stressed oaks over the edge.” Worm damage can be distinguished from sudden oak death by the lack of intact, dead leaves.

The expansion of sudden oak death is a much more significant concern. Though Marin is one of the areas where the disease was first discovered in 1995, there are many areas that have remained uninfected until recently. “It skips over spots, and then in later years those stands may have high mortality,” said Mike Swezy, the natural resource manager for the Marin Municipal Water District. The distribution maps that were developed in 2006 are already out of date, Swain said.

The rate at which the disease spreads is based on weather conditions. The current spread was caused by a warm, rainy spring in 2005 that nurtured the spread of the disease, killing a larger number of trees for the last two years.

“What we’re seeing is an ecological disaster in slow motion,” said Swezy. “At this point, there’s not much good news out there.”

Biology
Sudden oak death was first noticed when tanoak, which are not true oaks, began dying off in the mid-1990s said Katie Palmieri, spokeswoman for the California oak mortality task force. In the late 1990s the disease was also found to be killing coast live oak, canyon oak, black oak and Shreve’s oak.

Phytophthora ramorum, one of a group of organisms called water molds that are not true fungi, was finally identified as the cause of the die off in 2000. It is not native to North America or Germany – the two places where it has been found. Scientists’ think it may be from south central or Southeast Asia, since that is where many of the organisms that it attacks best, such as tanoak and rhododendron, are from.

Phytophthora actually causes two separate diseases, Palmieri said. The first is sudden oak death, which is widely known and usually kills the plants that it infects. The second disease is ramorum blight, which generally is not fatal to host plants but causes leaves and twigs to die back while using them to reproduce and spread. Since the plants carrying ramorum blight are used to build up spore levels, they are a driving force in dispersal of the pathogen.

Spots emerge on the leaves of infected plants, producing spores that spread on wind driven rains, as well as and on people’s shoes or tires. California bay laurel is the most virile of those hosts, though redwoods, rhododendron and many other species catch it also.

Tanoak is the only plant known to be able to catch both ramorum blight and sudden oak death. “It has the least natural resistance,” Palmieri said. “ It gives itself the infection by raining spores down on its own trunk.”

Most oaks are expected to have reasonable resistance to sudden oak death, through there will be local die-offs. “Oaks are promiscuous, and they easily hybridize and cross between species,” Swain said. This promiscuity gives them extra genetic variability and hence resources to survive diseases.

Conversely, tanoak has no species that it can hybridize with, and Swain is concerned it could eventually go extinct. “We just have to wait and see. Chestnut blight in the Appalachians took about 40 years or longer to work its way through the entire population – it was the dominant canopy tree of the eastern seaboard, and its gone now.”

Tanoak is one of the few trees that lives among redwoods, and many species, such as squirrels and deer, depend on its large, fleshy nut for protein. The tree is also an important part of the cultural heritage of local Native Americans, Palmieri said.

Prognosis
Patchy tree loss from sudden oak death is inevitable, which leaves many landowners wondering what practical steps they can take to protect their property. One recommended step is the treatment of oaks and tanoaks with nutrient supplements such as azomite or agri-fos, though the second is the only one endorsed by officials such as the UC cooperative extension.

The supplements can be applied externally or injected, and are only effective on trees that are still healthy. Because the process is time consuming and expensive, it’s not a large-scale solution but may save individual trees to which people have aesthetic, emotional or cultural attachments.

A second small-scale solution is the removal of selected bay trees that are close to individual healthy oaks that landowners hope to save. This is a solution that experts suggest reluctantly. Bays are an important part of the local ecology, more so because they produce large nuts and are resistant to sudden oak death. There is also a risk of creating large unwanted clearings if the bays are removed and the oaks die anyway, which is a strong possibility.

The only other option for preventing spread of the disease is good hygiene. Everyone, hikers especially, should clean their boots and their tires or risk transporting the spores.
Ranchers, and others who are managing large properties, should consider planting sudden oak death resistant trees in their understory to diminish the possibility of weed invasion and erosion if the oak canopy is killed off. Management of dead trees can help prevent fire hazard. The best solution is to fall the trees and remove their branches so they can’t act as ladder fuel, carrying flames from the ground level into the forest canopy.

The picture, however, isn’t as bleak as it may seem. The organism isn’t expected to spread far inland since it appears to favor moderate temperatures and moist areas, especially the coastal fog belt. And whatever happens, the rate of change is slow.
“There are probably thousands of trees that have been hit by the oak moth in West Marin,” Swains said, “The number of trees that are dying from sudden oak death are comparatively few, probably on a scale of tens or twenties.”