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.

The 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.