Pestalosphaeria leaf spot

Pestalosphaeria leaf spot is a fungal disease caused by the pathogen Pestalosphaeria concentrica (often associated with Pestalotiopsis species). This fungus acts as a significant parasite, primarily affecting woody ornamental plants, causing necrotic lesions on the foliage.

The disease is most commonly found on Azaleas (Rhododendron spp.), though it can affect a variety of other broadleaf evergreens. The pathogen frequently attacks plants that have been weakened by environmental stressors, such as poor soil drainage, improper fertilization, or physical damage to the leaves or bark.

Symptoms begin as small, irregular or circular spots on the leaf surface that eventually expand and turn brown or grayish-tan. A characteristic feature is the development of tiny, black, pimple-like fruiting bodies known as pycnidia within the necrotic lesions, which release spores to spread the infection to adjacent leaves.

Development and spread of the pathogen are highly favored by warm, humid conditions and the presence of free water on leaf surfaces. Rain splashes and irrigation sprays are the primary modes of dispersal, allowing the spores to infect new tissue rapidly, especially in dense plantings where air circulation is restricted.

The harmful impact of the disease is reflected in premature defoliation, which severely reduces the ornamental value of the plant and limits photosynthesis. If left unmanaged, the infection can progress to cause dieback of twigs and branches, eventually threatening the long-term survival of the host plant.

Control and prevention measures include the following practices:

• Sanitation: Regularly remove and dispose of infected leaves and fallen debris to reduce the inoculum level.
• Cultural control: Improve air circulation through proper pruning and avoid overhead irrigation to keep foliage dry.
• Environment management: Ensure the site has adequate drainage to prevent waterlogged roots.
• Chemical control: Apply appropriate fungicides preventively during seasons with high humidity or frequent rainfall.