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Phytophthora disease of alder

Alder (Alnus spp.)

Alders (genus Alnus) are deciduous trees and shrubs that thrive in damp cool sites.  There are considered to be around 35 species, with common characteristics of wind pollination, seed dispersal by wind and water, rapid colonisation of bare ground and a relatively short life span.  The male and female flowers are catkins, and the latter ripen to form woody cone-like structures.

Common alder (Alnus glutinosa) Alder catkins and cones Leaves of common alder

A special feature of alder is the way the roots bear nodules which contain a symbiotic micro-organism (Frankia) which can fix atmospheric nitrogen and make it available to the tree. This often makes alder the genus of choice for planting on various types of disturbed or nutrient poor soils. 

In Britain the only native species is the common alder (A. glutinosa).  It dominates wet woodlands and is abundant along streams and rivers, where its roots help to stabilise the banks:

Tangle of alder roots holding the undercut river bank Native riparian woodland

Other species of alder are native elsewhere in Europe: the grey alder (A. incana), the Italian alder (A. cordata) and the green alder (A. viridis).  Both the grey and Italian alder are planted widely in the UK.

Phytophthora in alder

Alder disease affecting common alderNow all these species are threatened by a lethal disease, which was first discovered in 1993 in Britain.   The causal agent is a previously unknown species of Phytophthora, P. alni, which is highly specific to alder.  The photograph shows affected trees on the River Lugg in Herefordshire where the disease has been present for more than a decade.

Research funding

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Forest Research work on alder disease has been jointly funded by the Forestry Commission and the Environment Agency.

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The European research effort (including Forest Research) has been co-ordinated through the EU Concerted Action FAIR 3615 Project - Phytophthora disease of alder in Europe: potential for damage, opportunities for limitation of pathogen spread, and for management and control.

For further information


Dr Joan Webber

or contact the Forest Research Tree Health Diagnostic and Advisory Service.