Willow scab

The causal agent of willow scab is the ascomycete fungus Venturia martianoffiana (often associated with Pollaccia saliciperda). This pathogen acts as a serious parasite on various willow species (Salix), leading to severe necrotic lesions on young shoots and foliage. The fungus survives the winter primarily as mycelium in infected twigs and leaf debris, releasing spores in early spring.

The host range is predominantly limited to the genus Salix. Willows planted in nurseries, plantations, and ornamental landscape settings are at the highest risk, especially when the planting density is high or the environment lacks proper air circulation. Younger trees are particularly vulnerable to extensive shoot blight.

Initial symptoms include dark, necrotic spots appearing on the leaves shortly after bud break. These spots expand rapidly, often following the veins. On infected twigs, the fungus causes blackening, wilting, and characteristic curling. During humid periods, a dark, velvety layer of conidia becomes visible on the affected tissue, facilitating secondary infection spreads.

The disease development is heavily dependent on moisture. Frequent spring rains, high humidity, and moderate temperatures provide ideal conditions for the germination of conidia and the infection of fresh growth. The susceptibility of the plant is highest during the early stages of shoot development in the spring season.

The economic and aesthetic damage of willow scab is significant. It causes premature defoliation, shoot dieback, and stunted growth, which severely degrades the health and decorative value of the trees. Persistent annual infections can lead to the death of major branches and even the entire tree if the pathogen is not controlled over several seasons.

Effective management requires a combination of cultural and chemical practices. Pruning and destroying infected branches during the dormant season is essential to reduce the primary inoculum load. During the growing season, applications of fungicides (such as copper-based or systemic products) are recommended, starting from the bud-burst stage to protect the vulnerable new growth.