Alder powdery mildew

Taxonomic classification and nature: The pathogen Microsphaera alni (also classified as Erysiphe alni) belongs to the Kingdom Fungi, Phylum Ascomycota, and Order Erysiphales. It is an obligate parasite, meaning it requires living host tissue to complete its life cycle. It accesses nutrients from host cells through specialized structures known as haustoria.

Diseases and host plants: The primary disease caused by this fungus is known as powdery mildew. While it specifically affects alder (Alnus) species, it is also documented on birch, hornbeam, and hazel. The disease primarily manifests on the foliage, forming a dense, white, powdery layer on the surface of the leaves.

Biology and life cycle: The life cycle involves both asexual and sexual stages. During the growing season, conidia are produced for rapid spread. As the season progresses, the fungus produces cleistothecia—tiny, black, speck-like fruiting bodies. These allow the pathogen to survive harsh winters in fallen debris, initiating new infections the following spring.

Environmental conditions and impact: Disease development is favored by warm temperatures, high humidity, and poor air circulation within the tree canopy. The impact on host trees includes reduced photosynthetic efficiency, leaf curl, and premature defoliation. Repeated infections can significantly weaken tree health, making them susceptible to secondary pests and environmental stressors.

Control and management: Effective management requires an integrated approach. Cultural practices include removing and destroying fallen leaf litter to eliminate overwintering sites. In nursery or landscape settings, chemical intervention using sulfur-based products or systemic fungicides is recommended during the early stages of symptom appearance to prevent widespread outbreak.