This new Phytophthora appears to be genetically distant to most of the other 60 species within the genus Phytophthora. The closest relative in the genus is Phytophthora lateralis , a virulent pathogen of Port Orford Cedar known to be present in natural stands of the Pacific North West and occasionally on Port Orford Cedar stock in nurseries in Europe. The presence of the same pathogen in two discrete and extremely distant areas raises important issues on the potential mode of transfer of an aggressive pathogen between continents but provides no information on where the pathogen may have originated. The pathogen may have originated in either known areas of distribution or in a third region of the world. In California, there is a substantial overlap between areas in which rhododendrons grow and areas in which the oak species known to be susceptible to the new disease are found.
Rhododendrons are also extremely common ornamental species throughout the state.
Rhododendrons were sampled at a nursery in Santa Cruz County. The symptoms observed on the rhododendrons were identical to that described in Europe. A Phytophthora was isolated from the leaves of a single rhododendron. This isolate matched the morphology of the sudden oak death Phytophthora . Molecular markers confirmed the DNA signature is again identical to the new species responsible for the epidemic mortality of oaks in California and the European isolates from rhododendrons.