Tomatoes rotting on the vine just as they approach ripeness is a frustrating experience for any gardener. This premature decay is typically the result of several distinct issues acting alone or in combination, and is often mistaken for a single problem. The fruit breakdown is not always caused by an infectious disease; environmental conditions or physical injury can weaken the fruit, allowing rot to set in. Understanding the specific cause, whether biological, environmental, or mechanical, is the first step toward salvaging a harvest.
Fungal and Bacterial Diseases
Infectious pathogens frequently cause premature tomato rot, spreading rapidly in warm, moist conditions. One common fungal culprit is Anthracnose (Colletotrichum), which targets ripening fruit. It appears as small, water-soaked, sunken spots that enlarge into dark, circular lesions with concentric rings.
The fungus overwinters in debris and soil, spreading to the fruit by splashing water, such as from rain or overhead irrigation. Another fungus, Phytophthora nicotianae, causes Buckeye Rot, typically infecting fruit where it touches the soil. This decay is characterized by brownish spots that grow into large lesions with distinct dark, concentric rings, resembling an eye.
Bacterial diseases also lead to early decay, often entering through tiny breaks in the skin. Bacterial Speck (Pseudomonas syringae) presents as small, dark spots that may appear slightly raised on mature fruit. These bacteria thrive in cool, wet weather and are easily spread by water splashing from the leaves.
Pathogens gain entry through natural openings, such as the stem scar, or through wounds caused by insects or mechanical damage. Once inside, these microorganisms rapidly consume the fruit tissue, leading to soft rot. High humidity and temperatures between 77 and 86 degrees Fahrenheit provide ideal conditions for these infections to flourish.
Environmental Stress and Water Management
Many instances of premature rot are physiological disorders caused by inconsistent care, not pathogens. Blossom End Rot (BER) is the most recognized, appearing as a dark, leathery, sunken spot at the bottom of the fruit. This disorder is caused by a localized deficiency of calcium in the fruit tissue during development. The deficiency is usually a transport issue related to uneven water availability, not a lack of calcium in the soil. When water supply fluctuates, the plant cannot efficiently move calcium to the fast-growing fruit cells, and the damaged tissue quickly softens as secondary fungi or bacteria colonize the area.
Rapid fruit expansion also causes decay by leading to splitting or cracking of the skin. This happens when a tomato plant absorbs a large amount of water suddenly, such as after heavy rain following a dry spell. The fruit pulp swells faster than the outer skin can stretch, resulting in cracks that compromise the protective barrier and provide an entry point for decay organisms. Sunscald can also damage the fruit’s skin, inviting rot. Sunscald appears as a white or yellowish, bleached area on the side exposed to intense sunlight, often occurring after leaf loss removes the natural shading canopy.
Physical Damage and Pest Infestation
Physical wounds inflicted by animals or insects act as open invitations for opportunistic rot-causing pathogens. Pests do not typically cause the decay themselves, but their feeding creates a breach that allows fungal spores and bacteria to enter the fruit flesh. For instance, pests like the Tomato Pinworm or the Cotton Bollworm tunnel into the fruit to feed, leaving behind channels that rapidly become infected. The resulting soft rot is a secondary infection that follows the initial physical damage. Even minor injuries, such as those caused by branches rubbing against the fruit or accidental nicks during pruning, can lead to rot by allowing common microorganisms to infiltrate the fruit before it reaches ripeness.