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Present in United Kingdom

Not reportable

Scientific name of causal agent – Phytophthora alni (P. alni)

P. alni cankers 2 Barnaby Wylder FC.JPG

Phytophthora disease of alder is a disease of alder trees (trees in the Alnus genus) caused by an algae-like organism called Phytophthora alni (P. alni). It is considered to be one of the most important diseases of natural ecosystems to arise in Europe in the late 20th and early 21st centuries.


Phytophthora alni is widespread in Great Britain. A record of all the mapped findings of P. alni where its identity has been confirmed in the laboratory is available, although the frequency with which visible symptoms can be found on alder indicates it is even more widespread than the map suggests. The number of trees affected has increased steadily since it was first discovered here in 1993, and probably a third of our alder trees are now affected. More specifically:

  • disease incidence is highest in South-East England;
  • heavy tree losses are also occurring in alder populations in the borders’ region of Wales; and
  • alders on Scottish river systems are suffering damage.

The disease is also widespread in Europe, with particularly heavy losses of alder trees reported from north-eastern France, and Bavaria in southern Germany.

A sub-species of the organism, called P. alni subsp. uniformis, has been found infecting red alder in Alaska.

Susceptible species

P. alni can infect all species of alder, including the UK’s native common or black alder (Alnus glutinosa) and the other two species widely planted here, which are Italian alder (A. cordata) and grey alder (A. incana). Green alder (A. viridis), another species native to continental Europe, but less often used here, is also susceptible.

It appears to be highly specific to alder species, and is not known to affect plants in any other genus.

The threat

Alder trees by stream 2 1041104.012big.width-500.jpg

Alder trees dominate wet woodlands and are abundant along streams and rivers (above), where their roots (below) help to stabilise the banks and prevent erosion.

Alder roots 1019697big.width-500.jpg

They provide valuable habitat and food sources for a wide variety of plant and animal life, including nesting sites among the roots for otters.

Alder trees are a natural choice for use in flood-mitigation schemes, and the nitrogen-fixing nodules on their roots make them useful for soil conditioning and fertility enhancement on wasteland and brownfield sites.

Alder timber tolerates wet conditions very well, and it has traditionally been used for products where that quality is essential, such as piles, boats and water pipes. It coppices well and makes good charcoal, and modern uses include veneer, pulp and plywood.

Any significant loss of alder trees would put these benefits at risk.

Identification and symptoms

Affected trees can display some of the following symptoms.

  • Small, yellow and sparse summer leaves.
  • Thin and sparse crowns.
  • Dead twigs and branches in the crowns of trees which have suffered infection for several years.
  • Heavy cone production
  • Bleeding at the base of the tree, visible as tarry or rusty spots and streaks (below).

P alni cankers 1 Barnaby Wylder FC.JPG

Management and control

We advise the following measures.

  • Evaluate nursery stock for infection before buying alder plants.
  • Ensure good biosecurity practice in nurseries to prevent infection.
  • Be aware that planting alder on river banks that are liable to flooding and where the disease already occurs presents a high risk.
  • Coppice diseased trees, which can often regenerate them.

Our research

Forest Research has been assessing the impact of various Phytophthora species, including P. ramorum, P. kernoviae and P. lateralis. Our research aimed 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; and
  • provide effective advice on management, containment and eradication.


Dr Joan Webber

or contact our Tree Health Diagnostic and Advisory Service.

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