The pathogen has been shown to cause disease in over 40 plant species, and has been recovered from more than 50 additional species. Tanoak (Lithocarpus densiflorus), various oaks (Quercus species) and bay laurel (Laurus nobilis) are major hosts in forests of the Pacific Northwest and California. Susceptible nursery crops include rhododendron (Rhododendron sp.), viburnum (Viburnum sp.), lilac (Syringa sp.), andromeda (Pieris sp.) and camellia (Camellia sp.).
The pathogen has caused extensive losses of oak and tanoak trees in forested areas in northern California coastal areas. Because oak species in eastern forests are known to be susceptible to this disease, federal and state regulations require inspection of west coast nurseries for the presence of P. ramorum prior to shipment of host nursery stock from California, Oregon, and Washington. Destruction of infected and potentially contaminated nursery stock has resulted in significant financial losses for affected nurseries. The disease could devastate forests of the eastern U.S. if it becomes established.
The disease is established in forests of northern California, and has been found in a forest in one Oregon county. The pathogen has also been detected in nurseries in Washington, Oregon and California. Shipment of contaminated nursery stock has resulted in nursery detections in numerous states, but the pathogen has not yet become established outside of the West Coast. Phytophthora ramorum was detected in Indiana for the first time in July 2006, on a viburnum that had been shipped to a retail outlet from an Oregon nursery. Surveys are conducted annually in Indiana nurseries that receive plant material from west coast sources.