Walnut scab

Walnut scab is a destructive fungal disease caused by Fusicladium levieri. This pathogen belongs to the Ascomycota phylum and is known for causing significant damage to the foliage, shoots, and fruit of walnut trees worldwide. It thrives in humid environments, often causing severe defoliation and fruit loss.

The primary host for this fungus is the English walnut (Juglans regia). The disease impacts the plant's physiological processes by reducing the surface area available for photosynthesis and invading the developing fruit. In cases of severe infestation, the disease can lead to premature fruit drop and weakened tree growth.

Symptoms initially appear as small, olive-brown spots on the underside of young leaves, which eventually expand and turn black. Similar lesions develop on petioles and young shoots. On the fruits, the scab manifests as black, sunken spots that may coalesce, causing the husks to rot and the kernels to become shriveled or non-viable.

The development of Fusicladium levieri is highly dependent on environmental conditions, particularly moisture. Rainfall, high humidity, and moderate temperatures are ideal for spore germination and dispersion. The fungus overwinters in dead leaves and infected branches, releasing spores in the spring when conditions are suitable for infection.

Effective management requires a combination of cultural and chemical methods to break the disease cycle.

• Sanitize the orchard by removing and burning fallen leaves and infected twigs.
• Ensure proper tree spacing and pruning for better air circulation and sunlight penetration.
• Apply copper-based fungicides at the dormant stage and during bud break.
• Monitor the orchard closely during wet spring months for early signs of infection.
• Choose resistant walnut cultivars whenever possible to minimize disease impact.