Alder powdery mildew

Taxonomy and pathogen profile. Microsphaera penicillata (often synonymized with Erysiphe penicillata) is a fungus belonging to the Ascomycota division and the Erysiphales order. It is an obligate biotrophic pathogen, meaning it exclusively relies on living host tissue for nutrition. The fungus is highly adapted to its specific host, utilizing haustoria to absorb nutrients directly from the epidermal cells of the plant.

Host range and diseases. This pathogen is the primary cause of powdery mildew on various species of alder (Alnus). The infection manifests as a white, cotton-like mycelial mat on the leaf surface. As the infection progresses, the fungal growth becomes denser and develops characteristic small black fruiting bodies known as chasmothecia (or cleistothecia), which contain the sexual spores.

Biology and life cycle. The life cycle involves both asexual and sexual stages. During the growing season, the fungus produces conidia, which are easily wind-dispersed and allow for rapid secondary infections across the canopy. The sexual stage occurs towards the end of the season, resulting in chasmothecia that enable the fungus to overwinter on fallen leaves, ensuring its survival until the next spring.

Environmental conditions and impact. The pathogen thrives in environments with high humidity and moderate temperatures, often exacerbated by stagnant air in dense forest stands or ornamental plantings. The physiological impact is significant: the fungus disrupts photosynthesis, causing premature chlorosis and leaf drop. This reduces the tree's vigor, slows annual growth, and may lead to increased susceptibility to secondary stresses.

Protection and management strategies. Integrated disease management is key to suppressing Microsphaera penicillata outbreaks:

• Sanitation: collecting and destroying fallen leaves to reduce the primary inoculum load.
• Cultural practices: pruning trees to improve canopy air circulation and reduce humidity levels.
• Fungicide application: using sulfur-based or systemic azole fungicides during early disease stages.
• Site selection: avoiding high-humidity areas for susceptible alder cultivars.