How to Manage Pests
UC Pest Management Guidelines
Leaf spot can cause symptoms on leaves and canes of blackberries. Leaf lesions are somewhat circular in shape, often having a brown or purple margin, and typically have a whitish center. Leaf spots are about 0.12 to 0.16 inch (3–4 mm) in diameter. Lesions on canes and petioles are similar to those on leaves but more elongate. Small, black pycnidia (fungus fruiting bodies of the Septoria stage) can be seen in the whitened area of the mature lesions. Symptoms appear late in the season although infection can occur multiple times throughout the season.
Leaf spot is found on erect and trailing blackberry, dewberry, olallieberry, and boysenberry. Conidia are disseminated by wind and water splash throughout the growing season, and infection increases with increased rainfall. For many years this pathogen was thought to be the same as the one that causes raspberry leaf spot on raspberry. It is now clear that the causal agent for leaf spot, Mycosphaerella rubi (anamorph Septoria rubi), does not infect raspberry. Raspberry leaf spot is caused by the pathogen Sphaerulina rubi (anamorph Cylindrosporium rubi) and is not common in California.The methods of control for the two diseases are very similar, however.
The key to managing leaf spot during the growing season is to increase air circulation within the planting and the row. This includes such practices as proper spacing, thinning to maintain proper cane density, and maintaining narrow rows. These practices lower canopy humidity and allow faster drying of foliage and canes, resulting in less infection. The most important management tool is dormant lime sulfur applications that serve to burn out old lesions and thus reduce sporulation.
The use of macrotunnels is very limiting to leaf spot infestations because of the near total lack of free moisture on the leaves and flowers of the plants.
The pathogen overwinters as mycelium and pycnidia in dead leaves and stems. To reduce sources of inoculum, it is important to prune out and remove from the site old fruiting canes, and to remove dead and damaged canes and leaf debris after harvest. Clean nursery stock limits introduction of the disease into the field.
Organically Acceptable Methods
UC IPM Pest Management Guidelines: Caneberries