Susceptibility of selected ericaceous ornamental host species to Phytophthora ramorum

We assessed disease reactions of 51 species or varieties of ericaceous ornamental hosts to two isolates of Phytophthora ramorum, the causal agent of sudden oak death. Inoculation was performed with an A2 mating type U.S. isolate from rhododendron and the R ramorum type culture of A1 mating type from Germany. For only one host were statistically significant differences in disease observed between the two isolates. Several different inoculation methods were compared. The 51 hosts tested varied widely in susceptibility, ranging from 0% to over 90% leaf area infected. Two cultivars of Vaccinium macrocarpon (cranberry) showed no disease, while three cultivars of Kalmia latifolia (mountain laurel) were all highly susceptible. The results indicate that many ornamental hosts grown in the United States are susceptible to P ramorum under artificial inoculation conditions. Inoculum density studies with two susceptible host species showed that R ramorum is capable of producing disease symptoms over sporangium concentrations ranging from 100 to 5,000 sporangia per ml. Mean numbers of chlamydospores forming in host tissue of 21 hosts ranged from 2 to over 900 chlamydospores per 6-mm-diameter leaf disk. Whether hosts showing susceptiblity under the experimental conditions used in this study would become infected with P. ramorum in the presence of inoculum under natural conditions is unknown.