Phytophthora disease of alder: what the agencies record

This page reports what the official source says. It is not legal advice: ask the office named here. It lists what the sources record about Phytophthora alni and does not diagnose any tree. The list of other diseases is on the tree diseases page, the reporting lines are in the United Kingdom report-a-pest guide, and the host tree is covered on the common alder page. A related root disease is on the Phytophthora lateralis page.

At a glance

Item	What the sources say
Pathogen	Phytophthora alni. Forest Research calls it an algae-like organism and one of the most important diseases of natural ecosystems to arise in Europe in the late 20th and early 21st centuries. The Forestry Commission note says molecular analysis showed it to be a hybrid between P. cambivora and a species close to P. fragariae, spreading across Europe as a hybrid swarm.
Names	The standard type is named Phytophthora alni subspecies alni. The variant hybrid types are named P. alni subspecies uniformis and subspecies multiformis (Forestry Commission note).
Hosts	All alder species. Forest Research names common alder (Alnus glutinosa), Italian alder (A. cordata), grey alder (A. incana) and green alder (A. viridis), and says it is not known to affect plants in any other genus. The Forestry Commission note says common alder is the most susceptible and grey alder the most resistant of the three it compared.
Status in the United Kingdom	Forest Research lists it as present in the United Kingdom and not reportable.
Status in EPPO lists	The EPPO Global Database shows P. alni subsp. alni on the A1 list of Kazakhstan and Armenia (added 2017) and of the Eurasian Economic Union (added 2016). It was on the EPPO Alert list from 1995 until 2001.

Symptoms as the sources state them

Crown. Forest Research lists small, yellow and sparse summer leaves, thin crowns, dead twigs and branches after several years of infection, and heavy cone production. The Forestry Commission note adds that leaves often fall early and leave the branches bare.
Stem base. Both sources describe bleeding at the base of the tree as tarry or rusty spots and streaks. The note says these can occur up to 2 to 3 metres from the ground and show that the bark beneath is dead.
Delay. The note says crown symptoms do not appear until the root collar has been largely girdled, so many years can pass between infection and visible disease. It also says bark killing often starts at the collar rather than in the roots.
Recovery. The note says some trees with severe crown symptoms recover when the lesions at the stem base stop developing, and that such trees may show basal cankers without tarry spots. It says it remains to be seen whether this is permanent.

Look-alike: alder dieback

The Forestry Commission note says Phytophthora disease should not be confused with alder dieback, which it describes as common in parts of Scotland. It says dieback kills only the aerial parts, with bark lesions on branches, dark brown stained wood and the fungus Valsa oxystroma, and not the root collar and roots as Phytophthora disease does.

How the sources say it spreads

The note says the free-swimming spores called zoospores probably disperse through river systems as well as soil, although the pathogen is rarely isolated from river water. It says the disease is less frequent in trees 1 to 10 metres from the water's edge, and that seedlings on flood plains can become infected after flooding. It records that the pathogen was found in rootstocks of alders from three of four commercial nurseries screened in Germany, and that disease in two young woodland plantations in the United Kingdom shared a nursery supplier that had imported plants from Belgium. It also records the disease on sites remote from watercourses, such as young plantations and orchard shelterbelts, and says the most likely origin there is infection in the nursery before planting. It says felling or winching out affected trees on riparian sites may spread the disease by bringing machinery on site and letting spores or fungal fragments reach healthy trees downstream.

Where it is recorded

Great Britain. Forest Research says it is widespread, first discovered in 1993, with the number of trees affected rising steadily and probably a third of alder trees affected. It names highest incidence in South-East England, heavy losses in the borders region of Wales and damage on Scottish river systems. The note's annual survey of riparian plots on rivers over 8 metres wide in southern England and east Wales found 4.3 per cent of assessed trees diseased or dead in 1994 and 15.3 per cent in 2003.
Europe. Forest Research reports particularly heavy losses in north-eastern France and Bavaria in southern Germany. The EPPO Global Database, last updated 2024-05-31, lists the subspecies alni as present in 19 European countries, including Austria, Belgium, France, Germany, Hungary, Ireland, Italy, the Netherlands, Poland, Spain, Sweden, Switzerland and the United Kingdom (England and Scotland), with restricted distribution recorded in Czechia, Latvia, the Netherlands, Spain and the United Kingdom. It marks Belarus as an absent, invalid record.
United States. EPPO lists the United States as present with few occurrences and Alaska as restricted. Forest Research says the subspecies uniformis has been found infecting red alder in Alaska.

What the agencies publish for management

Forest Research lists four points on its page: checking nursery stock for infection, biosecurity in nurseries, the high risk of planting alder on flood-prone river banks where the disease occurs, and coppicing as a way diseased trees can often regenerate. The 2004 note adds that time is not worth spending on trying to eliminate the fungus from a site by felling, reports nursery measures proposed by a European Union project, and mentions other flood-tolerant trees and natural regeneration from seed on remote riparian sites. It notes that fungicides applied before sale may suppress symptoms in infected plants. This page gives no product names, rates or timings. Who may apply pesticides is covered on the pesticide licences page.

Credit

The Forest Research material was read under the Open Government Licence v3.0 and is restated here in our own words.