Lilac powdery mildew

The causal agent of this disease is the ascomycete fungus Erysiphe syringae, which acts as an obligate parasite. The pathogen extracts nutrients from the host plant by inserting specialized structures known as haustoria into the epidermal cells. This fungal infection is one of the most persistent issues affecting lilac plants in nurseries and landscape gardens worldwide.

The primary host for this pathogen is the common lilac (Syringa vulgaris), though it may affect various cultivars of the genus. The fungus specializes in colonizing the foliage and tender stems, significantly impacting the aesthetic value of the shrubs throughout the growing season, particularly from mid-summer onwards.

Symptoms are identified by a characteristic white, powdery coating on the surface of leaves and shoots. As the infection progresses, the fungal mats grow denser and may turn grayish or light brown. By the end of the season, tiny, dark-colored fruiting bodies called chasmothecia appear within the mycelium, allowing the pathogen to overwinter in plant debris.

Favorable conditions for the development of Erysiphe syringae include high humidity, moderate temperatures, and poor air circulation. Overcrowded plantings or shaded locations that retain moisture for longer periods provide an ideal environment for spore germination and rapid spread throughout the shrubbery.

The impact of the disease is multifaceted: it disrupts photosynthesis, causes premature leaf drop, and hinders the formation of flower buds, leading to a decline in bloom quality the following year. Effective management strategies focus on both prevention and curative action:

• removal and destruction of infected leaf litter and twigs during autumn;
• pruning shrubs to improve airflow and reduce humidity within the foliage;
• application of systemic fungicides containing sulfur, tebuconazole, or azoxystrobin;
• maintaining plant health through balanced fertilization and proper irrigation;
• selecting resistant lilac cultivars when planting new garden beds.