Blackberries are a rewarding fruit crop, but brown spots appearing on the leaves signal that the plant is under stress. This stress can range from minor environmental issues to serious pathogenic infections. Addressing this discoloration requires accurately diagnosing the underlying cause, which usually falls into two main categories: living pathogens or non-living environmental and nutritional problems. Understanding the cause is the first step toward implementing the correct cultural practices and targeted treatments to ensure a healthy, productive blackberry patch.
Pathogenic Diseases Causing Brown Spots
Fungal diseases, which thrive in moist, humid conditions, are the most frequent cause of distinct brown spots on blackberry leaves. The two most common culprits are Septoria Leaf Spot and Anthracnose, both spread by water. Distinguishing them relies on observing the specific lesion pattern.
Septoria Leaf Spot, caused by the fungus Sphaerulina westendorpii, is recognizable by its characteristic “frog-eye” lesions. These spots begin as small, purplish areas that mature into lesions with a whitish-gray or tan center surrounded by a dark purple or brown border. The fungus overwinters on old, fallen leaves and infected canes, releasing spores splashed onto new growth by rain or overhead irrigation.
Anthracnose, caused by the fungus Elsinoe necator, also presents as leaf spots, but they are typically smaller and less uniformly circular than Septoria. The spots start as small, purplish areas that develop gray centers with raised purple margins. As the infection progresses, the dead tissue may drop out, creating a distinctive “shot-hole” appearance. This pathogen also infects the canes, creating sunken, canker-like lesions that can girdle and kill the cane.
Environmental Factors and Nutrient Stress
Brown spots that are not perfectly circular or uniform may be caused by non-living issues, such as extreme weather or nutrient deficiencies. These abiotic disorders often create irregular or marginal browning patterns, which helps separate them visually from fungal infections.
Sun Scald
Sun scald, or sunburn, occurs when leaves are exposed to intense sunlight and high temperatures. This appears as large, irregular patches of bleached white or brown discoloration, usually on the most exposed foliage at the top of the canopy. This damage is not a disease and will not spread, but it indicates a need for better canopy management.
Water Stress
Water stress, either too much or too little, can lead to browning and necrosis. Under-watering causes leaves to curl, develop marginal scorch, and turn brown as the plant conserves moisture. Conversely, over-watering leads to root rot, preventing the roots from absorbing water and nutrients, resulting in similar symptoms of browning, yellowing, and leaf drop.
Potassium Deficiency
Potassium deficiency is a common nutrient issue that manifests as browning on the foliage. Since potassium is a mobile nutrient, symptoms appear first on the older, lower leaves. This deficiency is characterized by scorching or necrosis along the leaf margins, often accompanied by chlorosis (yellowing) between the veins. The resulting brown edges are a distinct pattern, unlike the centralized spots created by fungal pathogens.
Essential Cultural Practices for Prevention
Managing the growing environment is the most effective approach to preventing brown spots, whether pathogenic or abiotic. The primary goal is to reduce humidity around the canes and eliminate sources of infection.
Sanitation
Proper sanitation involves removing and destroying all old fruiting canes immediately after harvest. Since Septoria and Anthracnose overwinter on dead cane tissue and fallen leaves, this removal eliminates the primary source of fungal spores for the following season. Discard this material away from the blackberry patch, rather than composting it.
Air Circulation
Improving air circulation within the plant canopy reduces the prolonged moisture that fungi require for infection. This is achieved through appropriate plant spacing, typically 2 to 4 feet between plants and 6 feet between rows. Regular pruning and thinning of the canes further opens the canopy, allowing sunlight and air to penetrate and dry the foliage quickly.
Watering
The method of watering directly impacts disease spread, as fungal spores are dispersed by splashing water. Switching from overhead sprinklers to a drip irrigation system is highly recommended to keep the foliage dry. If hand watering is necessary, apply water directly to the soil in the morning so any accidental wetting of the leaves can dry completely before nightfall.
Specific Remedial Treatments
When cultural practices are insufficient to control a severe infection or correct a nutrient imbalance, targeted interventions are necessary. These treatments are most effective when correctly timed and matched to the specific problem.
Fungal Infection Treatment
For confirmed fungal infections, a preventative fungicide application is the first chemical line of defense. The most effective timing is the delayed dormant spray, applied in late winter or early spring just before new growth begins. Products containing copper (such as copper hydroxide) or lime-sulfur are commonly used at this stage to eradicate overwintering spores on the dormant canes.
If infections persist during the growing season, foliar fungicides can be applied, requiring strict adherence to product labels and pre-harvest intervals. To prevent resistant fungal strains, growers should rotate between fungicides with different modes of action. Applying these products when young canes are 8 to 10 inches tall and again 10 to 14 days later is recommended for early season protection.
Correcting Potassium Deficiency
To correct a potassium deficiency, a soil test should first confirm the nutrient imbalance. Remediation involves applying targeted fertilizers that are high in potassium and low in chloride, as blackberries are sensitive to high salt levels. Potassium sulfate is often preferred and can be applied as a slow-release granular fertilizer to the soil surface. This targeted application ensures the deficiency is addressed without creating an excess of other nutrients.