Willow powdery mildew

Willow powdery mildew is a widespread fungal disease caused by the pathogen Erysiphe adunca. This organism acts as an obligate parasite, colonizing the surface of leaves and young shoots of various willow species (Salix spp.) by sending haustoria into the plant cells to extract nutrients.

The disease primarily impacts species within the Salix genus. While it is rarely fatal to mature, healthy trees, it poses a significant threat to young nursery stock, ornamental landscape plantings, and specimens in urban environments where air circulation is restricted.

Symptoms appear as a characteristic white or grayish-white floury coating on the leaf blades. As the infection progresses, the fungal growth thickens, and small dark fruiting bodies known as cleistothecia develop within the mycelium. Severe infestations cause leaf curling, yellowing, and premature defoliation of the tree.

The development of the disease is favored by warm days, cool nights, and high relative humidity. The spores (conidia) are spread efficiently by wind, rain splashes, and insects. The fungus survives the winter on fallen leaves and in the crevices of the bark, allowing it to reinfect the host plant the following spring.

Economic and aesthetic losses are mainly associated with reduced plant vigor and premature leaf loss, which weakens the tree before winter dormancy. Management and preventative strategies include:

• Removal and destruction of infected leaf litter during the autumn months to reduce inoculum.
• Proper spacing and pruning of branches to increase air circulation and light penetration.
• Application of appropriate fungicides containing sulfur or systemic active ingredients at the onset of infection.
• Maintaining plant health through adequate irrigation and balanced fertilization to enhance natural resistance.