Willow powdery mildew

The disease known as willow powdery mildew is caused by the obligate biotrophic fungus Podosphaera major. This pathogen belongs to the Erysiphales order and spreads primarily via conidia, which are easily carried by wind to healthy tissues of susceptible host plants.

The primary host range includes various species and hybrids of the genus Salix (willows). While mature, healthy trees may tolerate minor infections, the disease is particularly detrimental to young nursery stock, ornamental willow plantings, and trees stressed by environmental factors, leading to stunted growth and reduced aesthetic quality.

Symptoms of Podosphaera major infection typically start as small, white, flour-like patches of mycelium on leaves and green stems. As the infection progresses, these patches expand and merge, eventually covering the entire surface of the foliage. Severe infections result in the yellowing of leaves, leaf curling, and eventual necrosis of the affected tissues.

Favorable conditions for the development of the pathogen include moderate temperatures and high humidity, specifically when combined with poor air circulation within the canopy. Warm days followed by cool, dew-heavy nights provide the perfect environmental trigger for the rapid germination and colonization of new host tissues by the fungal spores.

Management strategies for this fungal disease should focus on integrated pest management practices. Key interventions involve ensuring proper spacing between trees for adequate airflow, monitoring for initial symptoms during the spring, and applying specialized fungicides if the infection pressure reaches an economic or aesthetic threshold.

• Regular pruning of infected branches to reduce the pathogen reservoir.
• Application of sulfur-based fungicides for early season protection.
• Implementation of drip irrigation to keep foliage dry.
• Monitoring environmental data to predict and treat early outbreaks.