On a hot summer day in 2008, a pair of plant disease researchers made an extraordinary discovery as they toured a hillside forest in San Mateo County: a stand of trees that had not been infected by the killer disease known as sudden oak death.

A healthy swath of forest, located on watershed lands owned by the San Francisco Public Utilities Commission, is now being used as a laboratory for the largest experiment ever conducted in the wild on a promising preventive treatment for this fast-spreading scourge.
If the experimental treatment works, it could blunt the epidemic’s devastating impact on oaks and on tan oaks, which are not technically oak trees but share many of their characteristics. That, in turn, will preserve shade for trail hikers and protect wildlife that rely on these native trees for acorns and habitat.
Sudden oak death, caused by the pathogen Phytophthora ramorum, has killed more than a million mature oaks and tan oaks in California over the past two decades, turning shady oak forests in Big Sur and elsewhere into weedy wastelands in as little as six years. Many parts of the utility commission’s 7,300 acres of forest on the Peninsula have already been hit hard by the disease. At least one out of 10 oak trees there died between 2006 and 2010, and many healthy trees were cut down to slow the disease’s spread.
The pathogen spreads via water, including windblown rain, and transplanted soil. Its advance has been considered inexorable, and the discovery of the uninfected patch of forest was considered sheer good luck.
“When we first came here, we couldn’t believe that there was no disease,” said Matteo Garbelotto, a forest pathology professor at the University of California, Berkeley, who is helping with this and other sudden oak death experiments. “It seems to go in the upper jet stream of the air. It moves along ridges, it affects the higher parts of the tree. and then it starts raining down and moving down.”
The uninfected patch of the forest was a tan-oak and redwood woodland, and the experimental treatment involves coating or injecting nearly 300 healthy tan oaks with Agri-Fos, a commercial fungicide. A similar number of nearby tan oaks, one of the state’s most common hardwood trees, were left untreated, for comparison. The experiments, begun in 2008, could cost more than $1 million.
This year, which began with a wet spring that was expected to help spread the disease, traces of the pathogen were found lurking in the hitherto uninfected forest.
“That suggests that within the next year or two we’ll find out whether the treatment works,” said Ellen Natesan, an ecologist with the utilities commission.
The fungicide has proved an effective tool for protecting residential oak trees, at a cost of $30 to $100 per tree, but efforts to protect oak forests in Oregon by dumping Agri-Fos from aircraft were largely ineffective. Scientists remain uncertain how well it can work in the wild.
“It boosts the immune response that trees have,” said Ted Swiecki, a plant pathologist working with the agency on the research. The “trick is getting this material into the plant so it can cause it to have this defense reaction. Short of doing a study like this, you really can’t tell how well it’s being absorbed into these larger trees.”
Regardless of how well the experimental treatment works, Ms. Natesan said it was unlikely that applying the substance to every oak in a forest would be financially feasible.
But she said, “You could create zones that are sudden-oak- death-free or have very little sudden oak death; you could use it to buffer the movement of the disease; you could create strips across the landscape to stop sudden oak death from moving quickly.”
(Source: The New York Times)