Pilidium leaf spot of maple

Pathogen and disease type
Pilidium leaf spot, caused by the fungus Pilidium acerinum, is a fungal disease that primarily targets foliage. The pathogen infects the host tissue, causing localized cellular death and leading to the formation of characteristic necrotic spots. It is a common, though often underestimated, issue in temperate climate arboriculture.

Affected crops
The primary host for this fungus is the Maple genus (Acer). While different species show varying levels of susceptibility, the disease is frequently observed on maples in nurseries, urban parks, and woodland settings. Chronic infection can lead to canopy thinning and reduced overall tree vigor over several growing seasons.

Symptoms and signs
The infection manifests as circular or irregular brown spots scattered across the leaves. As the infection progresses, tiny black fruiting bodies known as pycnidia develop within these necrotic areas. In severe cases, the foliage may curl, turn yellow prematurely, and drop early, which interferes with the tree's photosynthetic capacity before dormancy.

Conditions for development
Pilidium acerinum thrives under conditions of high humidity and moderate temperatures, particularly during wet spring and summer periods. Poor airflow within the canopy or dense planting density significantly increases the risk of the pathogen spreading rapidly. The spores are easily dispersed by rain splash, wind currents, and even human activity within the canopy.

Protection and control
Management of this disease is largely focused on sanitation and cultural practices.

• Prune away and destroy heavily infected branches.
• Collect and remove leaf litter annually to eliminate overwintering inoculum.
• Ensure proper spacing during planting to promote leaf drying.
• Apply copper-based fungicides during the early leaf-out stage if historical infection rates are high.