Willow scab

Pathogen and disease type
Willow scab is caused by the pathogenic fungus Fusicladium romellianum. It is a fungal disease that primarily targets the foliage and young shoots of willow trees. As a parasite, the fungus colonizes plant tissues, disrupting normal growth and photosynthesis through the development of necrotic lesions.

Susceptible plants
The primary hosts for this disease are various species of the genus Salix (willow). It is frequently observed in nursery settings, decorative landscape plantings, and natural willow populations, particularly those located in humid or riparian environments.

Symptoms and signs
The disease typically presents as dark, blackened spots on the leaves, which often grow together, causing the leaves to wither or curl. Under humid conditions, a velvety olive-green fungal sporulation layer can be seen on the undersides of the leaves. Severe infections may cause premature leaf drop and the characteristic "shepherd's crook" deformation of young shoots.

Conditions for development
Fusicladium romellianum thrives in cool, wet spring weather. High humidity is a critical factor for spore germination and infection. The fungus overwinters in fallen foliage and infected twig tissue, providing a reservoir of inoculum that spreads to healthy parts of the plant when favorable environmental conditions return in the spring.

Damage and control measures
The disease can significantly stunt the growth of young trees and reduce their overall vitality, making them susceptible to other stressors. Effective management strategies include:

• Removing and destroying infected leaves and twigs during the autumn sanitation cleanup.
• Pruning out infected branches to improve canopy airflow.
• Applying fungicides (such as copper-based products) early in the growing season to protect new growth.
• Maintaining healthy site conditions to prevent excess humidity around the trees.