Coryneum leaf spot

Coryneum leaf spot is a fungal disease caused by the pathogen Seimatosporium rhododendri. This condition specifically targets plants within the Ericaceae family, causing visible damage to foliage and potentially affecting the overall health and vigor of the specimen.

The primary hosts for this pathogen are species and cultivars of the genus Rhododendron, including deciduous and evergreen azaleas. Plants grown in humid environments or those lacking adequate space for airflow are significantly more susceptible to infection.

Symptoms typically manifest as circular, brown-to-grey spots appearing on the leaves. Over time, these spots develop a distinct dark border. If closely inspected, especially with a hand lens, small black fruiting bodies (pycnidia) can often be identified within the necrotic areas of the spots.

The disease thrives under conditions of high humidity, frequent rainfall, and stagnant air. These environmental factors facilitate the rapid production and spread of fungal spores, which can be transmitted through splashing water, overhead irrigation, or contaminated pruning tools during routine garden maintenance.

Effective disease management relies on both cultural practices and chemical control. Removing and destroying infected plant debris is essential to reduce the primary inoculum. Ensuring proper spacing and pruning to encourage air circulation helps minimize moisture on foliage. In cases of severe outbreaks, systemic fungicides or copper-based sprays are recommended to protect healthy foliage and curb the spread of the fungus.