All alder species in Britain are threatened by a lethal disease first discovered in the country in 1993. The causal agent is a previously unknown species of Phytophthora, P. alni, which is highly specific to alder.
From a distance, diseased alders attract attention in mid to late summer because the leaves are abnormally small, yellow and sparse. They frequently fall prematurely, leaving the tree bare. Trees that have suffered from infection over several years will have dead twigs and branches in the crown. There may also be heavy production of cones, usually a sign of stress in an alder.
Although many trees die rapidly once the crown symptoms appear, this is not always the case. Sometimes, the disease can take a chronic form, and the trees may gradually deteriorate over many years with the loss of foliage and branch dieback increasing over time. In a single alder coppice stool with several stems, it is also not uncommon for one or more stems to be affected by the disease and even die, while other stems on the same stool may recover or appear to remain healthy.
Examination of the base of a tree with severe crown symptoms often reveals the presence of a bleeding exudate which takes the form of tarry or rusty spots, sometimes occurring up to 3 metres from ground level. These bleeds or spots indicate that the underlying inner bark (phloem) is dying or dead as a result of invasion by Phytophthora. If the inner bark underneath is freshly exposed with a knife, the recently killed tissue is reddish to purple brown and will often be marbled or mottled. It contrasts strongly with the creamy colour of the adjacent healthy inner bark.
As the tarry spots dry out and age, they may become less conspicuous. However, they can persist of many years, although they may be washed away from the base of the tree it is exposed to floodwater.
Phytophthora disease of alder is now widespread in the riparian ecosystems in the UK where alder commonly grows.
On average, the disease incidence is highest is southeast England. However, heavy losses are occurring in some of the large alder populations that occur along western rivers – for example, in the Marches and parts of Wales.
The disease is also causing damage to alders on Scottish river systems, including on the Rivers Avon, Dee, Deveron, Duirinish and Spey.
There is no requirement to report sightings of p alni.