Maple powdery mildew

Sawadaea nankinensis is a fungal species responsible for causing powdery mildew on various maple species. As a member of the Erysiphales order, this obligate parasite thrives by forming external mycelium on the surface of the leaves, extending haustoria into the plant cells to extract essential nutrients, which leads to the visible symptoms of the disease.

Host plants for this pathogen primarily include trees of the genus Acer. While it is often considered a cosmetic issue, in susceptible species and under favorable conditions, the infection can become quite severe, affecting both juvenile nursery plants and mature trees in urban forests and ornamental landscapes.

The primary clinical sign of the infection is the development of a white, powdery fungal growth on the leaves. As the colony matures, it produces cleistothecia—small, dark, fruit-like bodies that serve as the survival structure during the winter season. These structures are typically visible to the naked eye, appearing as scattered black dots on the white mycelial mat.

The disease cycle is strongly influenced by environmental factors such as ambient temperature and leaf moisture. Warm, humid weather is ideal for spore germination and mycelial spread. Poor air circulation within the canopy and densely packed plantings create micro-environments that significantly enhance the susceptibility of the maple trees to rapid infection.

Management strategies involve a multifaceted approach:

• Sanitation: Raking and removing fallen leaves to break the fungal life cycle.
• Cultural control: Pruning trees to increase air movement and light penetration.
• Chemical control: Utilizing sulfur-based or triazole-based fungicides when infection thresholds are reached.
• Monitoring: Regular inspections of maple foliage during the spring and summer months to detect early outbreaks.