Previous attempts to identify this pathogen were unsuccessful since it turns out that positive identification requires closely controlled laboratory conditions and samples must be cultured soon after collection. To date, this new disease organism has been isolated from trees exhibiting SOD symptoms in Monterey, Santa Cruz, San Mateo, Marin, Sonoma and Napa Counties. However, the Phytophthora has not yet been officially described and given a scientific name. This new Phytophthora appears to be genetically distant to most of the other 60 species within the genus Phytophthora. In California, there is a substantial overlap between areas where rhododendrons grow and areas where the oak species known to be susceptible to the new disease are found.
Rhododendrons are also extremely common ornamental species throughout the state.
The UC team is now intensifying efforts to examine natural populations of rhododendron and other non-oak hosts. Further host-range studies are ongoing in the laboratory and greenhouse. Eventually, the leaves on the trees die and the crowns turn brown. While the name Sudden Oak Death implies that trees are killed rapidly, scientists believe that it can take months or even a year or two from the time of the initial infection for death to occur. The boring insects and the Hypoxylon fungus are believed to play a secondary role, attacking trees that have already been weakened by the new Phytophthora, thus hastening their demise.