Willow tar spot

Willow tar spot is a fungal disease caused by the pathogen Rhytisma filamentosum. It primarily affects various willow species (Salix), manifesting as distinct, dark lesions on the foliage. This pathogen is part of the broader group of tar spot fungi that typically target deciduous trees during the growing season.

The primary symptom of this infection is the development of yellow to light-colored blotches on the upper leaf surface. As the fungus matures, these spots develop raised, shiny black centers that resemble drops of tar. This distinct appearance is a diagnostic feature used to identify the presence of the pathogen on willow leaves.

Environmental conditions play a critical role in the spread of Rhytisma filamentosum. The fungus overwinters in fallen leaves as sclerotia or developing apothecia. In the spring, spores are discharged into the air during wet periods, landing on young leaves to initiate the infection cycle when moisture levels are high.

While often considered a cosmetic issue for many trees, severe infestations can lead to premature leaf drop. This early defoliation reduces the tree's ability to produce sufficient energy through photosynthesis, potentially resulting in reduced growth, lowered vigor, and increased susceptibility to other environmental stressors during the winter months.

Effective management focuses on sanitation to reduce the local inoculum load. Because the fungus survives in infected leaf litter, the most vital control measure is the thorough removal and destruction of all fallen leaves during the autumn season.

• Sanitary pruning of dead branches
• Ensuring proper spacing for air circulation
• Application of protective fungicides in high-pressure nurseries