Phytophthora ramorum in North America

Two apparently unlinked events marked the beginning of one of the most devastating forest diseases known to man in recent times: Sudden Oak Death (SOD). In the mid 1990s, while a novel species of Phytophthora was being isolated from mildly diseased Rhododendron and Viburnum plants in German and Belgian nurseries, the first reports of inexplicable death of tanoaks (Notholithocarpus densiflorus) started to come in from concerned citizens on the coast of California. In 2000, red oaks such as Coast Live oak and black oak were also reported to be dying inexplicably, and in June of that year an undescribed Phytophthora species was isolated and quickly identified as the causal agent of the disease known as sudden oak death. As it turns out, the species from European nurseries and the one infecting California oaks and tanoaks turned out to be the same. This novel species was named Phytophthora ramorum a name that refers to the branch dieback it causes in rhododendron and other ornamental plants, but on oaks and tanoaks, the pathogenic activity of the organism is mostly concentrated on the cambial layer of the main trunk, which gets rapidly destroyed by the growth of the pathogen. P. ramorum was maybe the first aerial Phytophthora species described from temperate forests: the deciduous large sporangia become airborne and are responsible for the apparently aerial dispersal of the pathogen. The exotic nature of the pathogen was immediately postulated based on its limited geographic distribution around the San Francisco Bay Area, on the fact that both the disease and the causal agent had never been described before, and on the very high susceptibility of tanoaks and red oaks.