Susceptibility Levels of Rhododendron Species and Hybrids to Phytophthora ramorum

The most important hosts of Phytophthora ramorum in Europe are cultivated Rhododendron species and hybrids. Since November 2002, EU emergency phytosanitary measures are being taken to prevent the introduction and spread of P. ramorum in Europe. Surveys at all commercial Rhododendron-growing premises are part of these measures and have led to P. ramorum findings at 2 to 5% of the inspected sites in several European countries. An eradication and quarantine program is initiated at those premises in case of positive findings, resulting in serious commercial damage for the grower. Belgium is one of the larger Rhododendron-growing areas in Europe, so growers are taking all possible preventive measures to avoid emergence of P. ramorum at their site. These measures mainly consist of sanitation, limiting the leaf wetness period, avoiding plant injuries, and preventive fungicide treatments. Using host resistance would be another possible tool, for which there is high interest among growers, but little scientific knowledge is available about the relative susceptibility of different Rhododendron cultivars and species. With the objective of increasing this knowledge we screened a total of 63 potential Rhododendron hosts (21 species and 42 hybrids) for their susceptibility to P. ramorum. Species were selected to represent the main subdivisions within the genus Rhododendron. Hybrids were selected based on their economic importance.