The sight of ripening tomatoes suddenly marred by decay is disheartening for any gardener. Tomato rot is the breakdown of fruit tissue before harvest, stemming from various causes. Understanding the specific nature of the rot is the first step toward effective prevention and management. This decay is often a symptom of underlying environmental stress, pathogen activity, or physical damage. Identifying the root cause helps protect the rest of the harvest.
Physiological Stressors and Environmental Damage
Many forms of tomato rot are not caused by infectious agents but by the plant’s inability to manage resources under external pressure. Blossom End Rot (BER) is a common non-infectious issue, appearing as a dark, sunken, leathery patch on the bottom of the fruit. This disorder is a localized calcium deficiency within the developing fruit tissue. Since calcium mobility depends on a consistent water supply, inconsistent watering is the primary trigger for BER, even when soil calcium levels are adequate.
Environmental factors can also directly damage the fruit, creating pathways for opportunistic decay. Sunscald occurs when tomatoes, especially those exposed after leaf loss, are scorched by intense sunlight and heat. The damaged area initially appears as a pale, whitish patch facing the sun, which later becomes sunken and dry. This injured tissue is susceptible to colonization by secondary fungi and bacteria, leading to rot.
Another physical disorder is fruit cracking or splitting, which happens when a dry period is followed by a sudden influx of water. Rapid water uptake causes the fruit to expand faster than the skin can accommodate, resulting in concentric or radial cracks, usually near the stem end. These open wounds serve as entry points for airborne or water-splashed decay organisms. Maintaining consistent soil moisture through regular watering and mulching prevents both BER and cracking.
Infectious Diseases Causing Fruit Rot
Rot that progresses rapidly and spreads between plants is often caused by living pathogens, primarily fungi and bacteria. Anthracnose, caused by Colletotrichum fungi, is a common ripe rot favoring warm, wet conditions, often appearing on fruit closest to the soil. It presents as small, circular, sunken, water-soaked spots that can grow up to half an inch in diameter and may develop concentric rings. Under humid conditions, the center of the lesion may excrete a mass of salmon-pink spores.
Late Blight, caused by the oomycete Phytophthora infestans, thrives in cool, wet weather with high humidity. Fruit lesions typically begin on the shoulders of the tomato as dark brown or golden, firm, and sunken patches. The distinguishing feature is the presence of a white, fuzzy growth visible around the edges of the fruit and leaf lesions, indicating active sporulation. This disease progresses quickly, turning the fruit into a mottled, leathery mass.
Southern Blight, caused by the fungus Athelia rolfsii, is distinguished by its primary infection site. While the main damage is a girdling stem rot near the soil line, fruit touching infested soil can develop soft, water-soaked lesions that quickly collapse. The presence of tiny, spherical, tan to reddish-brown structures called sclerotia, which resemble mustard seeds, on the white fungal mat at the stem base or on the infected fruit is a definitive sign of this pathogen. Poor air circulation and overhead watering accelerate the spread of these diseases by splashing spores onto developing fruit.
Insect Activity That Leads to Rot
Insects rarely cause complete rot directly but act as vectors or initial agents of injury, opening the door for infectious pathogens. The damage they inflict compromises the fruit’s natural protective barrier, the skin. Tomato fruitworms, the larval stage of a moth, bore deep into the fruit, usually near the stem end, to feed. This boring creates a tunnel immediately colonized by opportunistic decay fungi and bacteria, resulting in internal rot. The fruit may appear fine externally until the rot is well-established.
Stink bugs damage fruit by piercing the skin with their mouthparts to suck out plant juices. While the immediate damage appears as tiny pale or yellowish spots, the puncture site is a microscopic entry point for pathogens. The resulting rot is a secondary infection, but the initial insect feeding is the necessary precursor. Managing these pests is an indirect but effective strategy for preventing rot.