Black holes, spots, or sunken areas on developing tomatoes are a common problem for gardeners, signaling a disruption in the plant’s health. The visible damage can range from a purely physiological condition to an aggressive infectious disease or physical trauma. While the resulting appearance of a dark, compromised area may look similar, these symptoms are caused by three different issues requiring specific management strategies. Properly diagnosing the specific cause is the first step toward salvaging the rest of the harvest and preventing future losses.
Blossom End Rot
This condition represents a physiological disorder rather than an infection, manifesting as a large, dark, sunken spot found exclusively on the blossom end of the fruit, opposite the stem attachment point. The affected tissue initially appears water-soaked, turns brown, and eventually becomes a leathery, black, flattened area. This damage is irreversible on the affected fruit, which should be removed to redirect the plant’s energy toward healthy development.
Blossom end rot is caused by a localized calcium deficiency within the developing fruit tissue, which prevents the proper formation and integrity of cell walls. This deficiency usually results from the plant’s inability to transport the nutrient effectively due to inconsistent moisture levels. Calcium is only mobile in the plant through a steady flow of water, meaning that fluctuating between dry soil and overwatering, or periods of drought, halts the necessary uptake and distribution of the nutrient.
Stabilizing the moisture supply requires a consistent watering schedule that keeps the soil evenly moist but never waterlogged. Applying a two-inch layer of organic mulch can help regulate soil temperature and moisture, reducing stress on the plant. While consistent water is the primary fix, a supplemental application of a liquid calcium spray directly to the foliage and developing fruit can offer a temporary boost to aid in the recovery of subsequent fruit.
Infectious Black Spots
Black spots on tomatoes can also be the result of a pathogenic infection, often appearing anywhere on the fruit, including the sides, top, or near the stem. Fungal and bacterial diseases are facilitated by environmental conditions like high humidity, warm temperatures, and water splashing from the soil onto the foliage and fruit.
Early Blight, caused by Alternaria solani, creates dark, sunken lesions on the fruit that develop characteristic concentric rings, often described as target spots. These spots may be surrounded by a yellow halo and can spread across the fruit, causing extensive decay. Bacterial Spot, caused by Xanthomonas species, results in small, dark brown-to-black spots that are sometimes slightly raised or greasy in appearance.
Management requires improving air circulation and sanitation to reduce moisture that supports pathogen growth. Pruning the lower leaves prevents water splash from carrying soil-borne spores or bacteria up to the foliage and fruit. Applying a copper-based fungicide or bactericide can help control the spread of these infections, but crop rotation and destroying infected plant debris are necessary to prevent pathogens from overwintering in the soil.
Pest Damage and Entry Wounds
Physical damage caused by insects leads to black, rotten areas when a pest breaches the tomato’s skin, allowing secondary infections to take hold. This type of damage is often irregular in shape and can be found anywhere on the surface of the tomato.
The Tomato Fruitworm (Helicoverpa zea) chews a hole into the fruit, often near the stem or shoulder, and tunnels inside to feed. The resulting decay from the entry wound quickly turns the area black and soft. Stink bugs also cause damage by piercing the fruit with their mouthparts to suck out plant juices, leaving behind tiny, pinprick-sized wounds that become irregular, dark, sunken spots due to secondary rot.
Management involves removing and destroying any fruit showing signs of entry, which prevents the pests from completing their life cycle. Hand-picking visible pests like fruitworms can be effective in smaller gardens. Utilizing row covers early in the season can also help prevent adult moths and bugs from laying eggs on the plants, protecting the developing fruit.