Present in United Kingdom
Scientific name of causal agent - Phytophthora alni (P. alni)
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. The number of trees affected has increased steadily since it was first discovered here in 1993, and an estimated 20% 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 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 in Alaska, and another new Phytophthora has been reported affecting alder trees in Australia.
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.
Alder trees dominate wet woodlands and are abundant along streams and rivers, where their roots (above) help to stabilise the banks and prevent erosion.
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 is resistant to wet conditions and it has traditonally 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.
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.
- 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.
- Phytophthora Disease of Alder, Forestry Commission Information Note (archived publication)
- Threats from Phytophthora
- About common alder trees (Alnus glutinosa)
- About Italian alder trees (A. cordata)
- About grey alder trees (A. incana)
- About green alder trees (A. viridis)
or contact our Tree Health Diagnostic and Advisory Service.