Beetles were the focus after scientists associated them with large numbers of dying oaks . But evidence suggests pesticides sprayed on trunks to control beetles prolong tree life for only a year or more. Then Rizzo isolated a fungus this spring that is the only organism found in each of three affected oak species. The fungus is a member of the genus Phytophthora but has yet to be named. It does not match any of the 60 known Phytophthora species in the world, based on DNA testing.
Fungi can be both beneficial and destructive to oaks .
Nearly 200 species of fungi are known to cause disease in California oaks . 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.