Maple powdery mildew

The first and most noticeable symptom is the appearance of a white or grayish powdery coating on both sides of the leaf blade. As the disease progresses, the coating thickens.

Tissues beneath the mycelium may show discoloration. Frequent symptoms include premature yellowing and curling of leaf edges, making affected areas look wilted.

As the infection progresses, the fungal growth spreads to leaf petioles and young shoots, which negatively impacts their normal growth and development.

In the late stages, tiny black dots — the fungus's fruiting bodies (cleistothecia) — become visible within the mycelial mat to the naked eye.

Severe infestation leads to deformation of the leaves. Foliage dries out and falls prematurely, significantly reducing the tree's ornamental value and overall vigor.

The causal agent of this disease is the ascomycete fungus Sawadaea nankinensis. It is a highly specialized parasite belonging to the powdery mildew group affecting woody plants.

The pathogen's life cycle is closely linked to the host plant's seasonal development. The fungus overwinters as cleistothecia on fallen leaves, ensuring the infection persists until spring.

In spring, ascospores are released and, under favorable conditions, settle on young leaves. Primary infection triggers vegetative reproduction through conidia.

The fungus develops primarily on the leaf surface, penetrating the epidermis via haustoria — specialized feeding organs that absorb nutrients directly from the plant.

This pathogen is highly host-specific, primarily affecting various maple species and causing characteristic aesthetic and physiological damage to the crown.

Disease development is activated during periods of moderately warm weather with high humidity. Sudden temperature fluctuations also contribute to the spread of conidia.

Dense plantings, where natural air circulation is restricted, create an ideal microclimate for spore germination and rapid infection accumulation.

Insufficient crown light exposure and excessive nitrogen fertilization, which promotes succulent shoot growth, increase susceptibility to this disease.

The most active spread of the infection occurs in the second half of summer when nighttime humidity levels become consistently high.

Weakened specimens growing under adverse urban environmental conditions are more prone to infection and struggle more to recover from pathogen impact.

The primary damage lies in the disruption of photosynthesis due to the blockage of the leaf surface by the mycelium, weakening the tree and reducing nutrient storage.

Premature leaf loss leads to decreased cold hardiness. The tree enters dormancy unprepared, increasing the risk of frost damage to shoots during winter.

Infection of young shoots slows down wood growth. Over the long term, this leads to plant depletion and loss of its structural and ornamental value.

In nursery conditions, the disease can cause significant mortality among young seedlings whose immune systems are not yet capable of resisting the pathogen.

Affected trees lose their aesthetic appeal, which is critical for parks and landscape design where maples are often used as focal or avenue trees.

Prevention begins with thorough removal and destruction of fallen leaves, which serve as the primary reservoir for the fungus.

Maintaining proper irrigation and fertilization is crucial, favoring phosphorus-potassium fertilizers to strengthen cell walls in leaf tissues.

Upon detecting initial symptoms, the crown should be treated with specialized fungicides authorized for use on ornamental trees.

• Sulfur-based preparations;
• Systemic fungicides of the triazole class;
• Biological control agents based on antagonistic bacteria.

Regular sanitary and thinning pruning of the crown improves airflow, significantly reducing the risk of recurring powdery mildew outbreaks.