Walnut anthracnose

Walnut anthracnose is a significant fungal disease caused by the pathogen Ophiognomonia leptostyla. It affects walnut trees globally, acting as a primary cause of premature leaf drop and fruit damage, which can severely compromise the vitality and yield potential of the orchard.

The host range is primarily limited to the Juglans genus, specifically targeting Persian walnuts (Juglans regia). The disease manifests across all aerial parts of the tree, including leaflets, leaf rachises, succulent shoots, and developing fruit husks.

Initial symptoms are characterized by small, circular, reddish-brown spots on the leaves. As the infection progresses, these spots darken and may coalesce, leading to necrotic areas. When fruit is attacked, the fungus causes sunken black lesions, often resulting in shriveled kernels and premature drop of the affected nuts.

The disease thrives in warm, humid conditions with frequent rainfall, which facilitates the germination of spores. The pathogen overwinters in diseased fallen leaves and infected twigs. During spring, airborne ascospores and rain-splashed conidia initiate the primary infection cycle on newly developing tissues.

Managing walnut anthracnose requires an integrated pest management approach to reduce inoculum levels and prevent secondary spread. Recommended practices include:

• Sanitation by removing and destroying fallen foliage in autumn;
• Pruning of infected branches to improve air circulation;
• Application of copper-based fungicides during bud break;
• Use of systemic fungicides during periods of high disease pressure;
• Selection of resistant walnut cultivars for new plantations.