Phytophthora disease of alder

Phytophthora alni is widespread in Britain and threatens all alder species.

Disease overview

Common alder (Alnus glutinosa)All alder (Alnus spp.) 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, named Phytophthora alni, which is highly specific to alder. The disease is now considered to be one of the most important diseases of natural ecosystems in Europe for the last twenty years.

Alder catkins and conesIn Britain the only native alder 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. Other species of alder are European natives including: the grey alder (A. incana), the Italian alder (A. cordata) and the green alder (A. viridis). Both grey and Italian alder are planted widely in the UK.

Leaves of common alderThis resource will help you to identify Phytophthora infections and explain how to manage the disease in alder trees. It will also explain the current distribution of Phytophthora Disease of Alder and outline the latest research into the species that causes the disease.


  • Summer leaves are small, yellow and sparse
  • Thin, sparse crown
  • Trees that have suffered infection for several years have dead twigs and branches in the crown
  • Heavy cone production
  • Bleeding visible as tarry or rusty spots at the base of a tree

Tangle of alder roots holding the undercut river bank 


Native riparian woodlandSurveys conducted by Forest Research have shown that Phytophthora disease of alder is now widespread in Britain. A series of plots have shown that the number of trees affected has increased steadily since the mid-1990s and an estimated 20% of trees are now affected.

  • Disease incidence in Britain is highest in southeast England
  • Heavy tree losses are occurring in alder populations in the borders region of Wales
  • Alders on Scottish river systems are suffering damage
  • Widespread in Europe
  • High losses in north eastern France and Bavaria in Germany

Phytophthora alni subsp uniformis recently found in Alaska and another new  alder infecting Phytophthora in Australia

Disease management

  • Evaluate nursery stock for infection before purchase
  • Ensure good practice in nurseries to prevent infection
  • Planting alder on river banks that are liable to flooding and where the disease occurs presents a high risk
  • Coppicing can regenerate diseased trees

Our research

Forest Research is assessing the impact of various Phytophthora species, including P. ramorum, P. kernoviae and P. lateralis. Our current research aims to:

  • Diagnose and detect Phytophthora pathogens
  • Understand the distribution and impact of recently introduced Phytophthora species
  • Assess the potential risk that Phytophthora poses to Britain
  • Identify the origins and entry pathways of recently introduced Phytophthora species
  • Determine how Phytophthora pathogens spread
  • Provide effective advice on management, containment and eradication

Related resources


Dr Joan Webber

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