Root and collar rot of alder

All species of the genus Phytophthora (Greek for “plant destroyer”) are plant pests and belong to a group of fungus-like organisms called oomycetes, also known as “water moulds”. Among the numerous Phytophthora species, three to date are known that exclusively attack alders: P. uniformis, P. ×multiformis, and P. ×alni. The latter is the most aggressive of the three pathogens and the main cause of a massive decline in alder stands in Europe. Until 2015, all three were considered variants of a single species.

All three species penetrate the tree either at the base of the trunk or via the root hairs, where they cause root rot. They then grow upwards on the trunk under the bark. When the bark is carefully peeled off, a tongue-shaped, reddish-brown staining is revealed underneath – the result of the death of the bark (necrosis). The tree responds to this with slime flux, i.e. the exuding of a tarry tree sap at the edges of the dying tissue. This causes orangey-brown to black rusty or tar-like spots in the lower trunk area.

As a result of the bark necrosis, the crown is no longer sufficiently supplied with minerals and water. Sometimes epicormic shoots also form on the trunk. These are probably an attempt by the tree to compensate for the damage in the crown area. Another stress reaction can be heavy flowering and cone production, also in young trees. The further the disease progresses, the more sparsely trees of all ages sprout in spring. The whole crown remains sparse, with its leaves small and yellowish throughout the growing season. First twigs die, then branches, and then, once the bark necrosis has spread all the way round the trunk, the tree itself finally succumbs.

The time it takes between infection and death may be a few months (in young trees), or also several years. However, infected trees are weakened even if the progress of the disease is slow, and they are thus susceptible to infestation by parasites that exploit weakness (e.g. wood-degrading fungi such as honey fungi [Armillaria]). As long as there is only slight damage to the bark, some trees can however recover from a Phytophthora infection by sealing off the destroyed tissue with callus growth, thus preventing the pathogen from spreading. Nevertheless, longitudinal cracks are often left at the base of the trunk.

All three parasites spread through water. Of central importance for the infection of further alders are the cells used for reproduction, called zoospores, which are released into the soil and surface or river water. These are not just transported passively by the current of the water, but can also swim actively like sperm thanks to their flagella. These allow them travel long distances in rivers, until they come across another alder.

They penetrate the tree usually either at the base of the trunk or via the root hairs. Flooding facilitates their penetration of the host plant as well as their spread, on the one hand because the roots may suffer injuries in the course of the inundation, which then serve as ports of entry for the pathogen, and on the other because it is also easier for the zoospores to reach natural openings such as aerial roots and lenticels on the trunk when the water level is higher.

Infections in alders far from the river are often due to the use of contaminated plant material from tree nurseries during afforestation measures. Infected soil particles may also be spread by vehicles deployed in water body maintenance or timber harvesting. Apart from timber transportation, the releasing of fish taken from infested waters may also contribute to the spread of the disease.

It is probable that all species of the alder genus (Alnus spp.) can be infected by all three of the pathogens mentioned. In North America, P. uniformis has so far mainly attacked the Oregon alder (A. rubra). In Europe, the grey alder (A. incana) and especially the black alder (A. glutinosa) are most frequently affected, and increasingly also the Italian alder (A. cordata), which is native to the south. In the case of the green alder (A. viridis, syn: A. alnobetula), infection has so far been proven almost only in the context of experiments.

Plants growing on clay loam soils are particularly at risk. Alder dieback often occurs along watercourses and in carrs. Often it is a case of locations that are either regularly affected by flooding or where the groundwater level is naturally high or fluctuates considerably. The pathogen is less common in nurseries or forest gardens, but when it does occur, it often goes unnoticed without targeted testing, because it is often years before infected young plants show symptoms visible to the naked eye.

Alder dieback not only leads to timber harvesting losses, but also reduces the biodiversity of watercourses. Thanks to a symbiosis with certain bacteria, alders can absorb atmospheric nitrogen and thus make it biologically usable. Their nutrient-rich foliage is an important food supply for the fauna in the habitat. The alders themselves also serve as a habitat for many organisms, providing refuges for fish, for example. The roots of the alder help to stabilise the river banks, which is particularly important for flood protection.