Effect of Environmental and Seasonal Factors on the Susceptibility of Different Rhododendron Species and Hybrids to Phytophthora ramorum

Although Rhododendron is the most important host of Phytophthora ramorum in Europe, there is little scientific information about the susceptibility levels of different Rhododendron species and cultivars. Increasing this knowledge would help nurseries in the management of the disease and could be used by plant protection services to target their inspections. In this study a total of 80 Rhododendron species and hybrids were screened for their susceptibility to P. ramorum using two detached leaf inoculation assays. Due to the variability in susceptibility for a given cultivar within and between years, multi-year data was deemed necessary to establish a reliable susceptibility ranking. The zoospore inoculation method involving nonwounded leaves was most informative. Using this method, a wide range in susceptibility to P. ramorum was demonstrated. A second objective of this study was to get a better handle on some of the internal and external factors (time of year, temperature, leaf age) that seem to effect the susceptibility level. Susceptibility was significantly lower during late fall and winter, and seems correlated with the physiological status of the plant. Leaf age mainly seems to affect susceptibility during the early stages of leaf maturity. In general, new leaves were more susceptible to pathogen development. However, young leaves of some cultivars seem covered by leaf hairs, which prevent the zoospores to reach the leaf surface. Environmental factors that affect stomatal regulation, such as temperature, also seemed to have an effect on the degree of symptom development.