In 1993, a new canker disease known as ‘Sudden Oak Death’ was observed on oaks and tanoaks in California, causing a high mortality rate among the trees. A few years later, a new disease caused by Phytophthora was identified in Germany and The Netherlands on Rhododendron species. The symptoms included black discoloration of shoot tips and brown spots on leaves (Fig. 1). Both sanitary problems were shown to be caused by a new Phytophthora species, Phytophthora ramorum. In Europe, the pathogen has been found mainly in nurseries on species of Rhododendron and Viburnum. Since it was first identified in 1997, however, other ornamental plants, notably Camellia, Hamamelis, Kalmia, Leucothoe, Pieris, Syringa, Taxus and Vaccinium, have also been identified as natural host plants. Phytophthora ramorum is an aerial pathogen which affects leaves and shoots. It is heterothallic. Until recently, only the mating type A1 occurred in Europe, while the mating type A2 was present in the USA. In 2003, one of the Belgian isolates was classified as an A2 mating type. Since November 2002, phytosanitary measures have been taken by EU Member States to prevent the introduction and spread of P. ramorum into EU countries (Commission Decision 2002/757/EC).
Objectives
Due to its expanding host range, its significant dissemination potential and the recent identification of the mating type A2 in Belgium, this study was undertaken to assess the importance of P. ramorum in Wallonia and to evaluate the ability of the fungus (mating types A1 and A2) to infect European forest trees.
Description of tasks
1) Occurrence of P. ramorum in Wallonia Since 2003, the laboratory has participated in the survey set up by the Belgian Plant Protection Service (Ministry of Agriculture) to detect P. ramorum in ornamental nurseries and parks. The fungus is detected by isolation on a selective medium and real-time polymerase chain reaction (PCR) using a method described by the European and Mediterranean Plant Protection Organization (EPPO document 04/10702). 2) Constitution of a collection of Belgian P. ramorum isolates and characterisation The isolates identified within the framework of this survey are maintained in a collection. A database has been created containing the information needed for their characterisation (origin, original host plant, date of collection). The mating type (A1 or A2) is determined by crossing with compatible isolates, while the PCR-restriction fragment length polymorphism (RFLP) technique described by Kroon et al. (Phytopathology 94, 613-620) is used to check the European origin of the isolates. 3) Susceptibility of forest trees to the A1 and A2 mating types The project involves conducting pathogenicity tests using the bark inoculation method on seedlings of Picea abies, Pseudotsuga menziesii, Abies nordmanniana, Fraxinus excelsior, Fagus sylvatica, Quercus robur, Q. petraea and Carpinus betulus in order to evaluate their susceptibility to the disease. Both mating types identified in Belgium (mating type A1 isolated from Rhododendron species and mating type A2 isolated from Viburnum bodnantense) are being evaluated. 4) Infection from soil As the sole European A2 mating type was isolated from roots of V. bodnantense (De Merlier et al., 2003), soil samples will be collected in nurseries from infected areas and evaluated for infection potential using a baiting method with Rhododendron leaves.
Expected results
The project should provide information on the spread of the disease in Wallonia, and the range of trees and ornamental species at risk.
