Observing blackening tomato leaves is alarming, suggesting a serious problem. This tissue death, known as necrosis, is a common symptom caused by various factors, including aggressive pathogens, environmental stress, or nutrient imbalances. The specific visual cues accompanying the blackening are the most reliable way to diagnose the underlying cause and determine effective treatment.
Primary Fungal Diseases
Fungal pathogens frequently cause true black lesions on tomato foliage. Early Blight, caused by the fungus Alternaria solani, typically begins with small, dark lesions on older, lower leaves after fruit set. These spots expand up to a half-inch in diameter and often display distinct concentric rings, resembling a target or bullseye. The fungus thrives in moderate to warm temperatures (75–86°F), especially when humidity is high and leaves are wet.
Early Blight can also cause sunken, dark spots on the fruit, usually near the stem end. If the disease progresses unchecked, the leaves turn yellow, shrivel, and drop prematurely, leading to defoliation that exposes the developing fruit to sunscald. Management requires targeted fungicides, such as those containing chlorothalonil, applied preventatively or therapeutically at the first sign of infection.
A separate, more aggressive threat is Late Blight, caused by the oomycete Phytophthora infestans. Lesions appear as dark, water-soaked, or greasy patches that spread quickly across leaves and stems, often targeting younger growth. When humidity is high, a tell-tale sign is white, cottony growth on the underside of affected leaves, which are the spore-producing structures.
Late Blight favors cool, wet conditions, particularly nights in the 50s and 60s Fahrenheit with high moisture. The disease spreads rapidly via wind-borne spores, quickly enlarging lesions and causing entire leaves to shrivel and die, which can destroy a planting in a matter of days. Control involves immediate removal of infected plant material and applying specialized preventative fungicides.
Bacterial Infections and Pest-Related Blackening
Blackening can also signal bacterial pathogens, which require different management strategies than fungal diseases. Bacterial Spot and Bacterial Speck are common diseases producing small, dark lesions that lack the target-like rings of Early Blight. Bacterial Speck (Pseudomonas syringae pv. tomato) results in tiny, dark brown to black specks, often near leaf margins, sometimes surrounded by a narrow yellow halo.
Bacterial Spot (Xanthomonas species) presents as slightly larger, circular spots that initially appear water-soaked before turning dark brown. Both diseases favor warm, wet conditions and spread primarily by splashing water. Treatment relies on copper-based bactericides, which act as a protectant and must be applied before infection, as they do not cure existing spots.
Sooty Mold
Sooty Mold is a type of blackening often mistaken for a severe disease, but it is not an infection of the leaf tissue itself. This black, powdery fungus grows superficially on honeydew, a sugary residue excreted by sap-sucking insects like aphids, whiteflies, or scale insects. The mold causes indirect damage by blocking sunlight, reducing photosynthesis, and stunting growth.
To eliminate the black coating, control the underlying pest population producing the honeydew. Applying insecticidal soap or horticultural oil stops honeydew production, allowing the Sooty Mold to eventually dry up and flake off.
Abiotic Causes and Environmental Stress
Blackened foliage lacking distinct spots, rings, or fuzzy growth often points to an abiotic (non-living) environmental stressor. Cold or frost damage occurs when temperatures below 35°F cause plant cells to freeze and rupture. The affected leaf tissue, typically tips or edges, rapidly turns a dark, water-soaked black color as the cells die, an effect frequently seen when plants are set out too early in the spring.
Excessive fertilizer application can cause leaf edges to blacken and look scorched due to fertilizer burn. High concentrations of soluble salts, often from excessive nitrogen, draw water out of the leaf cells, leading to desiccation and necrosis at the margins. This nutrient toxicity is best addressed by flushing the soil with plain water to wash away the salt buildup.
A third major abiotic cause is exposure to herbicide drift, where airborne particles from nearby weed killer applications settle on the plants. Tomatoes are highly sensitive to growth regulator herbicides like 2,4-D and dicamba, which can drift long distances. While damage often manifests as distorted or curled leaves, severe exposure can lead to blackening and death of the foliage.
Integrated Management and Prevention
Preventing blackening leaves involves minimizing plant stress and interrupting pathogen and pest life cycles. Always water tomato plants at the base using drip irrigation or a soaker hose to avoid wetting the foliage, as leaf wetness is a primary requirement for spore germination. Applying organic mulch prevents soil-borne spores from splashing onto the lower leaves during watering or rain.
Adequate air circulation is also important. This is achieved by staking or caging plants to keep them off the ground and pruning the lowest leaves. Proper spacing allows air to flow freely, helping foliage dry quickly and reducing the high humidity favored by pathogens. Additionally, practicing a two-to-three-year crop rotation prevents pathogens from building up in the soil.