Lilac phoma leaf spot

The disease is caused by the pathogenic fungus Boeremia lilacis, a member of the Coelomycetes group. This fungus acts as a necrotrophic parasite, infecting the host plant by secreting enzymes that break down plant tissues, resulting in distinct necrotic lesions on leaves and sometimes stems, which can severely impact the plant's physiological health.

The primary host for this pathogen is the common lilac (Syringa vulgaris) and related species within the Syringa genus. The fungus is host-specific, primarily targeting the foliage. By colonizing the leaf tissue, the pathogen disrupts normal photosynthesis and water balance, eventually leading to a reduction in the vigor of the infected shrub and potential premature senescence of leaves.

Symptoms typically begin as small, irregular, or circular brown to grayish-brown spots appearing on the foliage. As the disease progresses, these spots expand, often surrounded by a darker border. Under conditions of high humidity, the center of these lesions will display small black structures known as pycnidia, which are the fruiting bodies of the fungus that release spores for further infection.

Development and spread of the pathogen are highly dependent on environmental factors, particularly moisture. Frequent rainfall, dew, and high humidity levels are critical for the dispersal of spores and the germination process. The fungus survives the winter dormant phase as mycelium or pycnidia within fallen, infected leaves or on dormant twigs, serving as the primary source of infection in the following spring.

The economic and aesthetic impact of this disease is significant, as it ruins the ornamental appearance of the plants and stresses the shrubs.

• Sanitation: Remove and destroy all fallen leaves in autumn.
• Pruning: Improve airflow by thinning the shrub crown.
• Fungicides: Apply copper-based or systemic fungicides in early spring.
• Monitoring: Regularly inspect plants for early signs of lesions.