Passalora leaf spot of poplar

Passalora leaf spot, caused by the fungus Passalora bolleana (previously known as Cercospora bolleana), is a common fungal disease that affects various species of poplar trees. The pathogen primarily targets the foliage, resulting in reduced photosynthetic capacity and overall tree stress.

The disease frequently attacks Populus alba and other poplar varieties, particularly in nurseries and plantations. Young trees are especially vulnerable to severe defoliation, which can hinder growth and make the trees more susceptible to secondary stressors or environmental impacts.

Symptoms of the infection manifest as small, brown to dark-brown spots on the leaf surface, often surrounded by a chlorotic halo. As the disease progresses, these spots expand and coalesce. On the underside of the leaves, a characteristic olivaceous to grey moldy growth develops, representing the fungal sporulation layer.

The development and spread of Passalora bolleana are closely linked to environmental conditions, particularly high humidity and warm temperatures. The fungus overwinters in fallen leaves, and spores are disseminated by wind and splashing rain, allowing the infection to spread rapidly throughout the canopy during the growing season.

Effective management and prevention of the disease require a combination of cultural and chemical control methods to limit the spread of the pathogen:

• Sanitation: Rake and destroy fallen leaves to reduce the primary inoculum source.
• Spacing: Ensure adequate spacing between trees to improve air circulation and reduce humidity levels within the canopy.
• Chemical Control: Apply appropriate fungicides, such as copper-based formulations, during periods conducive to infection.
• Monitoring: Regularly inspect foliage for early symptoms to implement timely control measures before the infection becomes severe.