Phytopthora ramorum in Belgium: 2002 survey results and research efforts

Phytophthora ramorum is a new and aggressive Phytophthora species that causes leaf blight and dieback symptoms on Viburnum and Rhododendron plants in Europe. A variant of this fungus is responsible for Sudden Oak Death (SOD) in California and Oregon. In Europe, problems so far are mostly restricted to nursery plants of Rhododendron and Viburnum while in the US, the fungus has been isolated from over 20 host species and is responsible for massive killing of oak trees (mostly Quercus agrifolia and Lithocarpus densiflorus) in forest and park settings. The potential for infection of native tree species in Europe and the recent detection of the fungus in nurseries of several European countries has lead to the implementation of EU emergency phytosanitary measures. As a result, most European countries have conducted surveys and are doing research as part of risk assessment efforts. The first part of this paper focuses on the plant diagnoses of the 2002 survey of P. ramorum in Belgian nurseries. The data from the survey indicates P. ramorum is present in Belgium at similar rates as in the neighbouring countries, in an apparent random distribution. The second part of this paper describes research results relating to the in vitro effect of oomycete fungicides on P. ramorum, Rhododendron cultivar susceptibility, the determination of the leaf infection site, and pathogen survival. Some fungicides had excellent in vitro activity against P. ramorum and should be tested further on plants. Use of host resistance as a control strategy may be limited as little difference in cultivar sensitivity was observed. Infection studies showed that wounds and the lower sides of the leaves are most susceptible to infection. Once the pathogen gets inside, it can survive well on detached leaves, especially when they are kept cool and moist. These data can contribute to management decisions of P. ramorum at the level of nurseries as well as the government.