Characteristics of the Disease

There are four alder species native to Europe: the common alder (Alnus glutinosa), the grey alder (A. incana), the Italian alder (A. cordata) and the green alder (A. viridis). In general, members of the genus Alnus are pioneer species, able to colonise bare, open ground rapidly and with a great ability to tolerate wet sites. The roots have specialised nodules which fix atmospheric nitrogen as a result of a symbiotic association with the actinomycete Frankia. This nitrogen, fixed at rates of 60–400 kg h yr, is available to both the host tree and to the environment. Common alder in particular has considerable landscape value along waterways; it plays a vital role in riparian ecosystems and the root system helps to stabilise riverbanks.