Elm black spot

Elm black spot is a fungal disease caused by the pathogen Dothidella ulmi (also referred to as Plagiostoma ulmi). This organism specifically targets the foliage of elm trees (genus Ulmus), often causing significant aesthetic damage and premature defoliation during the growing season.

The primary symptom involves the development of small, black, slightly raised spots on the upper surfaces of elm leaves. As the infection progresses throughout the summer, these spots may enlarge and cluster together, creating a distinct tar-like appearance that gives the disease its common name.

The pathogen thrives in warm and humid conditions, which facilitate the release and spread of spores. The fungus overwinters on fallen, infected leaves on the ground. In the spring, spores are released and carried by wind or splashing rain to infect the new, developing foliage of healthy trees.

While the disease is rarely fatal to established, healthy elm trees, it can lead to premature leaf drop, which reduces the tree's vigor. Repeated infections year after year can weaken the tree's overall structure, making it more susceptible to other biotic and abiotic stresses in the environment.

Effective management focuses on sanitation and cultural practices.

• Removing and destroying all fallen leaves at the end of the season is the most critical step.
• Pruning branches to improve air circulation within the canopy helps keep the foliage dry.
• In severe cases, protective fungicide applications during early spring can prevent initial infections.