When cherry tomatoes develop dark discoloration, it signals a change that falls into one of three distinct categories: a natural characteristic of the variety, a physiological disorder caused by environmental stress, or an infectious disease. Understanding the specific cause of the blackening is the first step toward determining if the fruit is safe to eat and how to protect the rest of your harvest. The visual appearance and location of the dark spots provide important clues that can help you correctly diagnose the problem. The discoloration often indicates a need for an adjustment in care or immediate intervention.
Genetic Pigmentation Versus Damage
Some cherry tomato varieties are intentionally bred to exhibit deep purple or black coloration, which is a normal trait. Cultivars such as ‘Black Cherry’ or ‘Indigo Rose’ develop their dark skin hue due to a high concentration of anthocyanins. These are powerful antioxidants, the same pigments responsible for the color in blueberries and blackberries. This natural pigmentation often appears deep purple-black on the shoulders of the fruit, especially where the skin is exposed to direct sunlight.
The coloration is typically uniform and smooth, covering a large, consistent area as the fruit matures. Unnatural blackening, conversely, appears irregular, localized, and often sunken or leathery in texture. If the black color is an expected part of the ripening process, the spot will feel firm, and the interior of the fruit will remain healthy. Discoloration from damage or disorder will feel soft or present as a distinct lesion.
The Role of Calcium and Watering Stress
The most frequent non-infectious reason for black spots on cherry tomatoes is Blossom End Rot (BER). This physiological disorder results from a localized deficiency of calcium within the developing fruit tissue. BER is triggered by inconsistent water delivery to the plant’s roots, which interferes with the uptake and transport of calcium.
Calcium is transported through the plant via the xylem tissue, moving with the flow of water driven by transpiration. Tomato fruits transpire less than leaves, making them less competitive for available calcium. When water supply fluctuates—from drought, overwatering, or sudden heavy rain—the plant cannot continuously deliver enough calcium to the rapidly expanding cells at the fruit’s blossom end.
The symptom is identifiable by its location: a dark, water-soaked spot that enlarges into a sunken, leathery, brown-to-black patch exclusively on the bottom of the fruit, opposite the stem. This tissue death occurs because calcium is immobile once incorporated into the cell walls. BER often appears on the first fruits of the season during the plant’s most rapid growth phase. Affected fruit should be removed to redirect the plant’s energy toward healthy growth.
Black Spots Caused by Fungal Pathogens
If black spots are scattered across the fruit’s shoulders or sides, or if they are accompanied by dark lesions on the leaves, the cause is likely a fungal infection. Two common culprits are Anthracnose and Early Blight, both of which thrive in warm, humid conditions.
Anthracnose
Anthracnose, caused by fungi in the Colletotrichum genus, primarily attacks the fruit as it ripens. The infection begins as small, circular, water-soaked spots that become sunken and dark brown or black. As the lesions mature, they can develop dark, concentric rings and often produce pink or salmon-colored spore masses in moist weather. These spores are easily spread to other fruit by splashing water, making overhead irrigation a major risk factor.
Early Blight
Early Blight, caused by the fungus Alternaria solani, is distinguished by the characteristic “target spot” lesions it creates. On the fruit, these spots are typically dark, sunken, and located near the stem end, featuring prominent concentric rings that resemble a bullseye pattern. The fungus also causes dark, necrotic spots on the leaves and stems. These spots usually start on the older, lower foliage and move upward. The fungal diseases survive the winter in infected plant debris and soil, ready to reinfect the next season’s crop.
Immediate Steps to Protect Your Tomato Plants
To prevent the blackening of future cherry tomatoes, you must address both soil health and environmental management. Establishing a consistent watering routine is the single most effective action to prevent Blossom End Rot. Aim to keep the soil evenly moist, not waterlogged or dry, providing about 1 to 1.5 inches of water per week, especially during the fruiting period.
Applying a layer of organic mulch, such as straw or shredded leaves, around the base of the plant helps significantly by stabilizing soil moisture and temperature. Improving your soil’s ability to supply calcium involves ensuring the soil pH is in the optimal range of 6.0 to 7.0, which maximizes nutrient uptake. Avoid using high-nitrogen fertilizers, as excessive nitrogen promotes rapid leaf growth that outcompetes the fruit for calcium.
Managing fungal diseases requires strict sanitation and air circulation practices. Immediately remove and destroy any infected leaves or fruit showing black spots to reduce the source of fungal spores; do not compost this material. Prune the lower leaves and any dense foliage to improve air movement through the canopy, allowing leaves to dry quickly after rain or watering. If fungal symptoms persist, a preventative application of an approved fungicide, such as those containing copper or chlorothalonil, may be necessary to protect the remaining healthy fruit.