Raspberry leaf spot

The fungal pathogen Sphaerulina rubi is the primary cause of white leaf spot or raspberry leaf spot. This Ascomycete fungus is a serious threat to small fruit production, specifically affecting various species within the Rubus genus and significantly impacting overall plant vigor.

Raspberries and blackberries are the main hosts for this disease. It thrives in regions with temperate climates, where high humidity and moderate temperatures provide the ideal environment for the fungus to establish and spread across commercial plantations and garden berry patches.

Clinical signs of the disease include the formation of circular lesions on leaves and canes. These spots typically start as small, brownish-purple blemishes that gradually enlarge and develop a white or grey center with a distinct dark margin. Over time, tiny black pycnidia develop in the centers of these spots, which serve as dispersal structures for spores.

The development of the fungus is highly dependent on environmental conditions, particularly moisture levels. Rain splash and wind are the primary vectors for spore dissemination from overwintered plant debris to healthy foliage. Warm, humid weather during the spring and summer accelerates the life cycle and potential for widespread infection.

The economic impact of the disease is significant, resulting in premature defoliation and severe weakening of the canes, which reduces fruit yield and cold hardiness. Effective management strategies involve a combination of rigorous sanitation, such as removing infected canes, and the timely application of approved protective fungicides to minimize infection pressure.

• Pruning and destroying infected plant tissue after harvest.
• Maintaining proper row spacing to improve air circulation.
• Applying preventive fungicide sprays during the growing season.
• Selecting resistant varieties where available.