Seeing brown spots on developing tomatoes signals a problem with plant health or environment. Identifying the precise cause of these discolorations is the first step toward saving the harvest, as effective management depends on an accurate diagnosis. Brown spots are not caused by a single issue; they result from physiological disorders, infectious pathogens, or external physical damage. This guide helps differentiate between the various causes of spotting to determine the right course of action.
Spots Caused by Nutrient and Water Imbalances
The most common non-infectious cause of brown discoloration on tomato fruit is Blossom End Rot (BER). This physiological disorder is linked to the plant’s inability to transport sufficient calcium to the developing fruit, often resulting in tissue death. BER first appears as a small, water-soaked spot near the blossom scar, which is the bottom end of the tomato. The lesion quickly enlarges, becoming sunken, dark brown, or black, and develops a leathery texture.
Although BER is a localized calcium deficiency within the fruit tissue, it rarely stems from a lack of calcium in the soil itself. The problem is typically an issue of calcium mobility, which is governed by the plant’s water supply. Calcium is an immobile nutrient, meaning it moves through the plant only with water uptake and transpiration. Inconsistent soil moisture—caused by drought, overwatering, or high salt concentrations—disrupts this movement, preventing calcium from reaching the rapidly growing fruit.
The first tomatoes of the season are often the most susceptible to BER because the plant is undergoing rapid growth. This rapid growth demands a high volume of calcium that the roots may not supply quickly enough. Fluctuations in the water supply, even brief periods of drought, cause the developing fruit to experience a temporary calcium deficit. This deficit results in the characteristic tissue breakdown and the large, brown, sunken spots at the fruit’s base.
Identifying Infectious Fungal and Bacterial Spots
Infectious pathogens, including various fungi and bacteria, cause distinct brown spots on the foliage, stems, or fruit. Fungal diseases often thrive in warm, humid conditions, while bacterial diseases spread when water splashes from the soil onto the lower leaves. Differentiating these diseases relies heavily on the visual pattern and location of the lesions.
Early Blight, caused by the fungus Alternaria solani, typically begins on the oldest leaves closest to the soil. Lesions on the leaves are small, brownish-black spots that expand, characterized by concentric rings that create a distinctive “target-like” appearance. Fruit infection usually occurs near the stem attachment (calyx), forming sunken, dark, leathery spots that also display the bullseye pattern.
Late Blight, caused by the water mold Phytophthora infestans, is destructive and appears suddenly, favoring cool and wet weather. On leaves, it creates irregularly shaped, water-soaked patches that quickly turn dark brown or black. A fuzzy white growth is often visible on the underside of the leaf in damp conditions. Fruit symptoms are distinct, presenting as greasy, dark brown blotches that rapidly expand and can cause the entire fruit to rot.
Bacterial Spot, caused by various species of Xanthomonas bacteria, affects all aboveground parts of the plant. On the fruit, it manifests as small, dark brown-to-black, slightly raised or scabby spots. These lesions are initiated only on green, immature tomatoes, as the bacteria enter through tiny hairs on the young fruit surface. On leaves, bacterial spots are generally smaller than those caused by Early Blight and may be surrounded by a yellow halo, but they lack concentric rings.
Damage Resulting from Pests and Environmental Exposure
Brown spots can result from physical injury caused by excessive sunlight or feeding by certain insects, which leads to secondary discoloration. Sunscald occurs when fruit, particularly green fruit, is suddenly exposed to intense direct sunlight, often after foliage drops prematurely due to disease or pruning. The initial damage appears as a pale white or yellowish blotch on the side of the fruit facing the sun. This damaged tissue is prone to secondary infection by black mold or other organisms, causing the area to turn brown or black and become leathery or sunken.
Pest damage from insects like stink bugs creates discoloration when the insect pierces the fruit’s skin to feed on internal juices. On green tomatoes, the feeding site appears as a dark pinprick surrounded by a light-colored area. As the fruit ripens, this internal damage manifests externally as cloudy, yellowish, or golden spots just beneath the skin. The insect injects enzymes that liquefy the tissue, causing the internal flesh beneath the spot to become hard, whitish, and spongy.
Actionable Steps for Treatment and Future Prevention
Addressing physiological issues like Blossom End Rot requires managing the plant’s water supply and calcium availability. Establish a consistent, deep watering schedule to avoid extreme fluctuations in soil moisture, since brief periods of drought can induce the disorder. If a soil test indicates a calcium deficit or low pH, amending the soil with garden lime or gypsum several months before planting can correct the issue. For immediate relief, a calcium nitrate or calcium chloride drench applied directly to the soil can help, but foliar calcium sprays are ineffective.
Managing infectious diseases involves sanitation, cultural practices, and chemical intervention. For fungal or bacterial spots, immediately remove and dispose of all infected leaves and fruit to reduce the source of spores. Improve air circulation by staking plants and pruning the lower leaves, which are often the first infected by spores splashing up from the soil. Always water at the base of the plant using drip irrigation or a soaker hose to keep foliage dry, as moisture is necessary for spores to germinate and spread.
If the infection is widespread, fungicides or bactericides may be necessary, depending on the diagnosis. Copper-based sprays are effective against certain fungal diseases, such as Late Blight, and bacterial diseases like Bacterial Spot. For Early Blight, specific fungicides can be applied preventatively or at the first sign of symptoms. To prevent Sunscald, ensure the plant maintains adequate foliage cover to shade the fruit; if defoliation is unavoidable, use shade cloth.
For pest damage, such as that caused by stink bugs, chemical control may be necessary, focusing on the center of the plant where insects hide. Crop rotation is also an effective long-term strategy, as many disease spores can overwinter in the soil or on plant debris.