Willow tar spot

Willow tar spot is a common fungal leaf disease caused by the ascomycete fungus Rhytisma salicinum. It is widely recognized by its distinctive appearance, which resembles droplets of black tar on the foliage, earning it the common name "tar spot."

The fungus primarily targets various species within the Salix (willow) genus. While it is rarely fatal to healthy trees, the disease significantly impairs the tree's photosynthetic capacity by killing leaf tissue, which can lead to premature leaf drop and overall vigor decline in heavily infested specimens.

The infection manifests as yellow, chlorotic spots on the leaf surface. As the fungus matures during the summer, these spots develop raised, shiny black stromata. By autumn, these hardened, ink-black structures become the defining feature of the disease, serving as the overwintering stage for the pathogen.

Disease development is heavily dependent on moisture and humidity. The fungus overwinters in fallen, infected leaves on the ground. In early spring, these leaves release spores that are carried by wind or rain splash to newly developing willow leaves, initiating the cycle of infection for the upcoming growing season.

Management strategies focus on sanitation to reduce the local inoculum load:

• Rake and remove all fallen leaves from under the tree canopy in autumn.
• Dispose of infected plant debris by burning or deep burial to prevent spore release.
• Ensure adequate tree spacing to improve air circulation, which helps keep the foliage dry.
• If necessary, apply copper-based fungicides in the early spring as leaves emerge to prevent infection.