Maple powdery mildew

The disease known as maple powdery mildew is caused by the fungus Uncinula tulasnei (syn. Erysiphe tulasnei). This is an obligate parasite that specifically infects various species of maple (Acer). It is characterized by the formation of superficial mycelium on the surface of leaves, which derives nutrients directly from the host plant's epidermal cells.

The primary symptoms manifest as white, powdery patches covering the foliage during the summer months. As the infection progresses, the fungal growth thickens, often covering the entire leaf surface. Later in the season, small black structures known as cleistothecia develop within the white mats, serving as the sexual reproductive stage and allowing the fungus to overwinter.

Development of this fungus is favored by warm, humid weather conditions. High humidity levels, combined with poor air circulation within the tree canopy, create an ideal environment for conidia germination and rapid spread. Wind currents facilitate the dispersal of spores from infected trees to healthy ones, leading to outbreaks during late summer and early autumn.

While rarely fatal to mature trees, the disease causes significant physiological stress. The presence of mycelium blocks sunlight, hindering photosynthesis and reducing the tree's ability to store energy for the dormant season. Premature leaf drop is a common consequence, which diminishes the ornamental value and overall vigor of landscape maples and nursery stock.

Management and prevention focus on reducing the primary inoculum and maintaining tree health. Key strategies include:

• Sanitation by removing and destroying fallen leaves to eliminate overwintering sites.
• Pruning to improve air circulation within the canopy and reduce local humidity.
• Application of appropriate fungicides when infection pressure is high or when symptoms are first spotted.
• Ensuring proper irrigation and soil nutrition to strengthen the tree's natural resistance.