Fungi, indeed, are important in the natural decline and death of mature oaks in undisturbed woodlands. But Rizzo has never witnessed one this virulent. Once in a tree, the fungus produces enzymes that dissolve the dead outer and living inner layers of bark, producing oozing sores. As the disease progresses into the wood, the tree becomes weak and vulnerable to bark beetles, which burrow and choke a tree's circulatory system. A year or more later, trees appear to die rapidly, turning from apparently healthy green to brown within a few weeks.
Scientists still have a lot to learn about how the disease spreads and damages oak trees.
But scientists don't know whether they live in soil or leaf litter and how they spread from tree to tree or over long distances. The fungus thrives in cooler, wet conditions found along the California coast. But scientists don't know whether individual trees or dense stands are vulnerable and if positions on north or south slopes matter. While insecticides can control beetles and prolong a tree's life, scientists need to test chemicals in the lab to find a fungicide that might control the fungus. A new field experiment was launched this month to learn whether oaks under stress from lack of water could be vulnerable to the disease.