WEARVERVILLE (AP) — Scientists say a disease that kills oak trees along the California coast has spread into yet another county.

California Department of Food and Agriculture spokesman Steve Lyle said Monday that an outbreak of the Sudden Oak Death has now been identified in Trinity County. This makes 15 counties with trees infected by the disease that can be fatal if it spreads to the trunk.

Lyell says the area has been quarantined.

UC Berkeley’s California Oak Mortality Task Force says that infected oak trees have been found in a five-acre area of Trinity County about half a mile from the nearest outbreak in nearby Humboldt County.

Since its discovery in 2000, three million trees have died of the disease, making it the leading killer of trees in the California coastal forests.