Crown symptoms
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.
Sparse foliage on affected tree
Twig dieback on riverside tree
Alder stool with three dying and one healthy stem
Although many trees die rapidly once the crown symptoms appear, this is not invariably 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.
Bark symptoms
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.
Bleeding from dead and dying bark at base of infected stems of alder
Bark of infected alder stem cut away to show basal lesion
Basal bleeding lesions
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.
