New Phytophthora root disease of common alder in Great Britain
A new and serious disease of common alder (Alnus glutinosa) caused by a Phytophthora species has recently been observed in southern Britain. Concerns about this disease were first raised in early summer 1993 in Kent. Further surveys conducted along waterways have showed that the disease is widespread in southern Britain and now affects more than 20.000 alders; in some areas up to 25 % of the trees are affected. However, the disease has not been found in Northern Britain. Symptoms are typical of Phytophthora root disease: leaves are abnormally small, yellow and sparse, and frequently fall prematurely. Dead roots can be found, and examination of the base of a stem with severe crown symptoms usually reveals the presence of strips of dead bark extending up from ground level. These are often marked externally by the production of a tarry or rusty exudate. Alder stems showing these symptoms will either die or suffer severe dieback. At present very few dead alders have been found near affected trees suggesting that it is a new disease. The causal agent is thought to be a form of Phytophthora cambivora but further studies are needed to clarify the issue. It produces water-borne spores which are playing an important role in the spread of the disease. Several factors can also facilitate spore dispersal, such as: inter-basin water transfer, movement of fish stocks, use of contaminated fishing equipment, wading birds etc. Chemical control does not appear feasible for environmental reasons, one possibility would be to find varieties of alder which are resistant to the disease.
Sources
Gibbs, J. (1994) Phytophthora root disease of common alder.
Research Information Note 258. Forestry Authority, Forestry Commission, Wrecclesham, Farnham, Surrey, GB, 4 p.