Alder crust fungus

Alder crust fungus (Latin name Vuilleminia alni) is a specialized fungal pathogen that causes specific bark damage and decay on alder trees. As a parasitic organism, it colonizes the bark and cambium layers, eventually leading to localized cell death and disruption of essential tree functions.

The primary hosts of this fungus are various species of the genus Alnus, including grey and black alder. The pathogen is most commonly found in damp, riparian, or boggy forest environments, where the humidity levels are consistently high, supporting the fungal growth cycle on bark surfaces.

The symptoms of an infection are characterized by the development of flat, crust-like fruiting bodies that emerge through cracks in the bark. These growths are typically leathery in texture and may appear as elongated patches along the twigs and branches. As the infection progresses, the bark becomes brittle and may slough off, revealing underlying wood rot.

Environmental conditions play a critical role in the dissemination and development of the disease. High relative humidity and cool-to-moderate temperatures are essential for spore germination. The fungus typically enters the tree through wounds caused by frost, insect activity, or mechanical damage, spreading more rapidly in dense, poorly ventilated stands.

The economic and ecological impact of this disease involves the loss of tree vigor, reduced timber quality, and an increased risk of branch breakage in storms. Effective control and prevention strategies focus on:

• pruning infected branches;
• removing highly decayed trees;
• promoting forest health through proper thinning.