Cercosporella leaf spot of blackberry

Cercosporella rubi is a fungal pathogen belonging to the kingdom Fungi and the genus Cercosporella. It is specifically known to cause leaf spot disease in blackberry plants (genus Rubus). By infecting the foliage, the fungus reduces the plant's capacity for photosynthesis, which stunts growth and significantly impacts the total fruit yield and quality.

The disease is characterized by the appearance of circular to angular spots on the leaves. Initially, these spots are brownish or reddish-purple. As the fungus matures, the center of the spot turns light grey or white, surrounded by a dark brown margin. In severe cases, these spots merge, leading to premature leaf senescence, chlorosis, and extensive defoliation of the canes.

The life cycle of Cercosporella rubi is primarily saprophytic during the winter. The pathogen survives in leaf litter and infected cane debris as mycelium or conidia. When temperatures rise in the spring, the fungus begins sporulation. Spores are then dispersed through rain splashes and wind, landing on healthy foliage and initiating the primary infection cycle of the new season.

Optimal conditions for development involve prolonged periods of high humidity and moderate temperatures, typically between 18°C and 25°C. Thick, poorly pruned blackberry canopies are particularly susceptible, as they trap humidity and prevent rapid drying of the foliage after rain. This environment significantly increases the rate of spore germination and tissue colonization by the fungus.

Effective management strategies require a combination of cultural and chemical interventions:

• Removing and destroying infected plant debris to reduce the primary inoculum load.
• Pruning canes to maintain adequate air circulation within the canopy.
• Applying protective fungicides early in the season or when environmental conditions are favourable for infection.
• Ensuring proper water management to avoid excessive humidity near the foliage.