Stigmina leaf spot

The causative agent of this disease is the fungus Stigmina glomerulosa (syn. Wilsonomyces carpophilus), a common pathogen affecting various fruit and nut trees. It is a filamentous fungus that significantly impacts tree health by invading leaf tissues and potentially causing necrotic lesions on fruit hulls.

The primary host for this fungus is the walnut tree (Juglans regia). The pathogen is highly adapted to surviving in local environments, overwintering within infected twigs, buds, and fallen leaf debris. Once favorable conditions return, it releases spores that trigger new cycles of infection in the orchard.

Clinical signs of the disease start as small, circular, reddish-brown spots on the leaf surface, often surrounded by a darker border or a chlorotic halo. As the infection progresses, these spots may enlarge or coalesce, leading to tissue necrosis and the eventual formation of holes in the leaf (shot-hole symptom). Under humid conditions, a visible velvety spore mass appears at the center of the spots.

Environmental triggers are crucial for the spread of Stigmina glomerulosa. High humidity, frequent rainfall, and moderate temperatures provide the necessary conditions for fungal sporulation and spore germination. Orchards with poor air circulation or those neglected during the dormancy period are significantly more susceptible to severe outbreaks.

The economic impact of this disease is reflected in reduced photosynthetic area, premature leaf drop, and weakened tree vigor, which ultimately leads to smaller fruit size and lower yield. To manage the disease and maintain a healthy orchard, the following measures are recommended:

• Pruning and destroying infected branches to reduce inoculum levels.
• Sanitation practices, including removing fallen leaves during winter.
• Applying dormant sprays with copper-based fungicides to protect the tree.
• Timely application of systemic fungicides during the early leaf development stage if high infection pressure is present.