Lasiodiplodia nut canker

The most prominent symptom is the dieback of individual branches or entire scaffold limbs. Leaves on the affected branches wilt rapidly, turn dark brown, and drop prematurely.

Characteristic depressed lesions or cankers appear on the bark. These lesions often show a dark, necrotic center and can expand significantly if the infection is not halted.

Gumming or sap exudation is a common indicator of the disease. The sticky substance oozing from the bark cracks serves as a focal point for the fungus to develop its spore-producing structures.

Upon closer inspection of the dead bark, small black protruding dots—the pycnidia—can be observed, especially during periods of high humidity when the fungus is actively sporulating.

• Branch dieback starting from the canopy downwards.
• Dark, sunken cankers on the main trunk and limbs.
• Exudation of gum at the infection sites.

The causative agent of this disease is the fungus Lasiodiplodia regiae, which acts as a specialized parasite affecting walnut wood. It primarily attacks the vascular system and cortical tissues of the host tree.

This pathogen demonstrates strong survival capabilities, overwintering as mycelium or pycnidia within infected branches, bark tissues, and fallen organic debris left in the orchard.

The fungus produces conidia within its fruiting bodies, known as pycnidia. These spores are disseminated throughout the growing season by wind, splashing rain, and insect vectors.

Entry points for the infection are typically mechanical wounds, pruning cuts, frost cracks, or sites damaged by wood-boring insects, where the fungus begins its colonization process.

Lasiodiplodia regiae is biologically noted for its rapid colonization rate, effectively disrupting water and nutrient transport by breaking down the plant's cambium layers.

High humidity and moderate to warm temperatures are the primary environmental drivers for Lasiodiplodia regiae. Frequent spring rains significantly enhance spore germination on bark surfaces.

Orchards with high tree density, which limits air circulation, provide an ideal microclimate for the disease. Stagnant air keeps the bark damp for longer, facilitating fungal penetration.

Trees that are stressed due to poor irrigation, nutrient deficiencies, or soil compaction are far more susceptible to infection compared to vigorous, well-maintained walnut trees.

Wood-boring insects act as primary carriers, not only creating the wounds necessary for fungal entry but also transporting spores directly into the tree's internal vascular system.

The fungus thrives within a temperature range of +20°C to +28°C, making late spring and summer the most critical windows for disease progression and rapid colonization.

The disease is highly destructive as it can lead to the total collapse of the tree. The gradual loss of the canopy limits the tree's ability to produce energy, eventually killing it.

Nut production is severely affected as the tree's vascular system fails. This results in poor quality nuts, reduced kernel size, and a significant decrease in overall yield for the grower.

Infested trees lose structural integrity, making them hazardous. Large limbs may snap during high winds or under the weight of the crop, creating risks for orchard workers and equipment.

Economic impact is high, covering not only the loss of potential harvest but also the labor-intensive costs of pruning, sanitizing, and eventually removing and replacing dead trees.

If left unmanaged, the infection can spread across the entire orchard, turning a productive plantation into a high-maintenance site with declining viability.

Rigorous sanitation is the cornerstone of control. Infected branches must be pruned at least 15–20 cm below the visible necrosis, with tools disinfected after every cut to prevent cross-contamination.

All pruned materials must be removed from the orchard and destroyed by burning. Leaving infected wood in the field provides a reservoir for continued inoculum production.

Prophylactic applications of copper-based fungicides are recommended. These should be applied during the dormant season and throughout the growing season to protect healthy bark.

Active maintenance of tree health is essential. This includes sealing large pruning wounds with protective coatings and managing wood-boring pests to limit the availability of infection entry points.

Ensuring balanced fertilization helps the trees maintain their natural defenses. Strong, healthy trees are significantly more resilient against Lasiodiplodia regiae attacks.