The sight of purple-brown blotches on the upper portion of blackberry leaves, followed by powdery spores on the lower portion of the leaf, are early indicators of blackberry disease, also called leaf rust fungus. If left untreated, the disease will progress into a heavy infestation, causing leaves to become discolored and fall. However, a complete infestation can be pre-empted or lessened by properly following chemical or cultural control practices to control blackberry disease.
Reduce or eliminate overhead watering, which can enhance disease growth, lead to moisture stress and impede chemical treatments. Use drip irrigation tubing for water in lieu of overhead watering.
Keep weed growth in check. Apply a 2-inch layer of pine bark mulch around the base of the plants to keep the planting area as weed-free as possible.
Cut infected canes with pruning shears as soon as they are detected. Destroy the canes after removing from the bush.
Use a disinfectant to clean the pruning shears after cutting to prevent contaminating other canes or plants.
Apply Triclopyr, a foliar herbicide, according the manufacturer’s directions. (The herbicide is absorbed through the leaves and is transferred to the bush’s roots.)
Apply Metsulfuron-methyl according to the manufacturer’s directions. Use this herbicide when Triclopyr is not suitable, such as in urban areas and near crops.