Tomatoes are a popular garden crop, making the sudden appearance of black spots on developing fruit a frustrating experience. This discoloration is not a single problem but a symptom indicating one of several underlying issues. The blackening can be traced to three primary categories: a physiological disorder related to nutrient uptake, an infectious attack by a biological pathogen, or damage from environmental stress. Correctly identifying the specific cause is the first step toward effective mitigation, as the treatment for one problem will not resolve the others.
The Most Common Culprit: Blossom End Rot
The most frequent cause of blackening is Blossom End Rot (BER), which is a physiological disorder, not an infectious disease. It appears as a large, sunken, dark brown or black spot exclusively on the blossom end of the fruit. This tissue breakdown occurs because of a localized calcium deficiency in the rapidly developing fruit cells. Calcium is immobile within the plant, and its delivery depends entirely on the steady flow of water through the plant’s vascular system.
The disorder rarely stems from a true lack of calcium in the soil itself. Instead, the primary trigger is inconsistent soil moisture. When the soil swings between saturated and extremely dry, the plant cannot transport calcium steadily to the fruit, causing the cell walls at the fruit’s tip to collapse. High concentrations of other nutrients, such as nitrogen, potassium, or magnesium, can also interfere with the plant’s ability to absorb the available calcium. Once a fruit develops BER, the damage cannot be reversed, and the affected fruit should be removed to prevent secondary rots.
Addressing BER is largely a matter of water management and soil balance. Deep, consistent watering that keeps the soil evenly moist is far more effective than adding calcium supplements mid-season. While some gardeners use foliar calcium sprays for short-term relief, their effectiveness is debated since calcium does not readily move from leaves to fruit. Long-term prevention involves ensuring the soil pH is 6.5 to 6.8, which maximizes calcium uptake.
Identifying Fungal and Bacterial Diseases
Blackening can also be the result of pathogenic infections, which typically present symptoms distinct from Blossom End Rot. These diseases are spread by splashing water, wind, or contaminated debris, and often manifest on the sides or shoulders of the fruit, rather than just the blossom end.
Fungal diseases like Early Blight (Alternaria) often create dark, sunken spots on the fruit near the stem attachment. These lesions frequently feature visible concentric rings, giving them a “bulls-eye” appearance. The fungus thrives in warm, moist conditions and affects the lower leaves and stems first.
Late Blight (Phytophthora infestans) is highly aggressive, creating dark, water-soaked lesions that rapidly turn black or brown on the fruit’s shoulders. This pathogen is favored by cooler, highly humid weather and can quickly defoliate an entire plant.
Bacterial diseases, such as Bacterial Spot and Speck, cause blackening by creating small, localized lesions on the fruit surface. Bacterial Spot lesions are small, raised blisters that become scab-like and rough. Bacterial Speck results in tiny, slightly raised black dots that are more superficial. These spots do not penetrate deeply but can severely reduce marketability. Treating pathogenic issues requires immediate action, including pruning infected material to improve air circulation and applying approved protective sprays, such as copper-based products or fungicides.
Environmental and Stress-Related Factors
A third set of causes involves physical damage from environmental extremes, creating entry points for secondary black rots. This blackening is a consequence of tissue injury, not a nutrient disorder or primary pathogenic attack.
Cold damage, particularly from a sudden cold snap or light frost, can cause the fruit tissue to turn dark brown or black. Since tomato plants are sensitive to chill, temperatures sustained below 50°F can cause physiological stress. Actual freezing (32°F or lower) ruptures the fruit’s cells, leading to a dark, mushy collapse of the affected areas.
Sunscald is another common stress-related issue that eventually leads to blackening. It begins as a pale yellow or white patch on the side of the fruit exposed to intense sunlight, especially if protective foliage has been removed. The damaged tissue dies, becoming leathery and thin, making it highly susceptible to secondary invasion by opportunistic black fungi or bacteria. The resulting black spot is the secondary infection colonizing the sun-damaged area.
Physical injuries, such as growth cracks caused by erratic watering or wounds from insects, also provide easy entry for rot organisms. This leads to rapidly spreading black decay from the point of injury.
Integrated Prevention Strategies
Preventing blackening requires a holistic approach managing soil health, moisture, and the plant environment. Establishing deep, consistent watering is the single most effective cultural practice, mitigating both Blossom End Rot and the stress that leads to growth cracks. Aiming for one to two inches of water per week, delivered directly to the soil, helps maintain the steady flow of calcium to the developing fruit.
Mulching the soil surface with straw or wood chips helps regulate soil temperature and significantly reduces moisture evaporation, stabilizing the water supply. Mulch also prevents fungal spores from splashing onto the lower leaves and stems during watering or rain. Proper plant spacing is equally important, ensuring adequate air circulation to dry foliage quickly, which limits the spread of pathogens.
Proactive soil testing before planting helps to identify and correct pH imbalances and nutrient deficiencies. Tomatoes thrive best when the soil pH is slightly acidic to neutral, approximately 6.5. Additionally, practicing a three-year crop rotation breaks the life cycle of many soil-borne pathogens. Finally, immediate sanitation, including removing infected leaves and fruit at the first sign of discoloration, prevents the spread of disease spores and bacteria to healthy parts of the plant.