In 1993, a severe decline of riparian alders (Alnus spp.) was observed in southern Britain. Affected trees displayed crown dieback (Fig. 1) and inner bark lesions at the stem base, sometimes with tar-like or rusty exudations (Fig. 2). A high level of mortality was observed among trees in some regions of England. A Phytophthora species resembling P. cambivora in its gametangial and sporangial morphology, but presenting homothalism (self-fertility) rather than outcrossing, as well as a high frequency of zygotic abortion, was identified as the causal agent. Molecular studies showed that it comprises a range of heteroploid isolates generated by hybridisation between P. cambivora and another as yet unknown Phytophthora species. Since its first identification, the disease has been recorded in several European countries. In Belgium, ‘alder Phytophthora’ was isolated for the first time in 1999 from a stem lesion on a common alder (Alnus glutinosa).
Objectives
Riparian alders are very common along streams and rivers in Europe. Being tolerant of periodic flooding, they are well adapted to wet sites. They also play an important ecological role by protecting soils (bank stabilisation) and improving river water quality (water filtration and purification). Due to the rapid spread of the fungus and its lethal effect on infected trees, the project was established to evaluate the importance of the disease in Wallonia and to characterize the pathosystem in order to recommend control measures for river bank management.
Description of tasks
1)Characterisation of alder Phytophthora isolates collected in Belgium The objective of this study is to compare the genetic and phenotypic traits of Belgian alder Phytophthora isolates with those of reference isolates (variant and standard) from other countries. 2)Development of a PCR method for the specific detection of alder Phytophthora The project includes developing polymerase chain reaction (PCR) primers for the specific detection of alder Phytophthora from infected plant tissues, soil or river water using sequence-characterised amplification region (SCAR) technology. 3)Impact of environmental factors on the production of sporangia and zoospores Emphasis will be placed on the role played by various environmental factors, including temperature, pollution and light, on the production of sporangia and zoospores and in the mechanism of infection. 4)Selection of resistant genotypes Pathogenicity tests on selected genotypes (or ecotypes) will be conducted in order to identify potential resistant plant genotypes which could be used in highly infected areas.
Expected results
The project should provide information on the environmental conditions that favour the disease. This information will be used to establish protocols aimed at preventing the spread of the disease in healthy areas and minimizing its effects in areas where the fungus is well established.
Contribution
1)Characterisation of alder Phytophthora isolates found in Belgium on the basis of morphological features, molecular grouping (using RAPD and RAMS techniques) and aggressiveness. 2)Design of PCR primers for the specific detection of alder Phytophthora. 3)Evaluation of the various environmental factors influencing the production of sporangia and zoospores. 4)Development of pathogenicity tests that could be used routinely for the selection of plants that are tolerant or resistant to the disease.
Partners
The main partners are the Walloon Agricultural Research Centre and the Agricultural University of Gembloux (Forestry Resources and Natural Environments Management Unit). The latter is in charge of implementing a disease survey in southern Belgium along all classified rivers (184 inventory units, 1012 alder trees described), providing a complete description of the inventory units according to a predefined form and creating a database to establish the potential correlation between environmental factors and the occurrence of the disease.
