Willow leaf spot

Linospora leaf spot is a fungal disease caused by the ascomycete Linospora capreae. This pathogen specifically targets trees within the Salix genus, leading to the development of necrotic spots on the foliage. While not always fatal to the tree, chronic infection significantly reduces the plant's photosynthetic capacity and overall health.

The host range of this fungus is limited to willow species. It is commonly observed on goat willow and other ornamental varieties. Trees planted in humid climates or crowded conditions are particularly susceptible, as these environments provide the ideal microclimate for the fungus to establish itself and proliferate during the growing season.

Symptoms appear during mid-summer as small, pale yellow spots on the leaves. As the infection progresses, these spots expand and turn dark brown or black. The presence of tiny, black, flask-shaped structures known as perithecia within the centers of these lesions is the most reliable sign of Linospora capreae. Severe infection leads to premature chlorosis and defoliation.

The environmental requirements for the spread of the disease are centered around moisture. Rain splashes and high humidity facilitate the dispersal of spores from fallen leaves to the canopy. The fungus overwinters primarily in the leaf litter, remaining dormant until spring temperatures rise and humidity levels permit the release of new spores to initiate a new infection cycle.

Effective management requires a combination of sanitation and chemical interventions. To control the disease, arborists and gardeners should:

• Remove and destroy all fallen leaves during the autumn to reduce inoculum.
• Prune branches to improve canopy ventilation and decrease local humidity.
• Apply copper-based or systemic fungicides if an infection is persistent.
• Monitor trees regularly during the spring for the first appearance of leaf spots.