Cylindrosporium leaf spot of ash

Cylindrosporium leaf spot of ash is a fungal disease caused by the pathogen Cylindrosporium orni (teleomorph: Mycosphaerella effigurata). It is a widespread condition affecting ash trees, leading to premature defoliation and weakening of the host plant throughout the growing season.

The fungus primarily targets the leaves of trees within the Fraxinus genus. While rarely lethal to mature, healthy trees, it can significantly impair the vitality of young saplings, urban landscape trees, and specimens in nurseries, making them more susceptible to secondary pests and environmental stressors.

Symptoms typically manifest as small, circular or irregular necrotic spots on the leaf blades. These spots are often yellow-brown initially, transitioning to dark brown or gray as the tissue dies. On the undersides of the leaves, the fungus produces acervuli (spore-bearing structures), which appear as tiny, dense clusters. Severe infections lead to extensive yellowing and early leaf drop.

Environmental conditions play a critical role in the disease cycle, with high humidity and frequent rainfall being the primary triggers for spore germination and dispersion. The pathogen survives the winter in fallen leaf litter, releasing spores in the spring that are wind-blown or splashed onto emerging foliage, initiating a new cycle of infection.

Effective control measures focus on sanitation and chemical intervention. Removing and destroying infected leaves in the autumn is essential to reduce the primary inoculum load. In severe cases, preventive spraying with copper-based fungicides or appropriate systemic treatments during the early stages of leaf development can significantly curb the spread of the disease.