Finding perfectly ripened tomatoes beginning to decay while still attached to the vine is a common frustration for home gardeners. This premature fruit loss results from a range of issues, including infectious agents, environmental stress, and nutritional imbalances. Understanding the specific cause of the decay is the first step in protecting the harvest, as solutions for disease-driven rot differ significantly from those for physiological problems. Tomato decay is broadly categorized into infectious rot caused by pathogens or non-infectious issues resulting from external conditions or plant health problems.
Pathogens: Fungal and Bacterial Causes
The most destructive forms of tomato rot are caused by fungal and bacterial pathogens. These organisms thrive in moist, humid conditions and are often spread through splashing water, wind, or contaminated tools. The resulting decay is typically progressive and contagious, capable of rapidly moving through a garden.
Fungal diseases like Anthracnose frequently cause fruit rot, especially on ripe tomatoes. This disease initially appears as small, sunken, circular, water-soaked spots that can enlarge up to a half-inch in diameter. As lesions age, their centers may darken and show small, black fungal structures; under wet conditions, a salmon-colored mass of spores may become visible. Late Blight is another fungal issue, causing dark, rough, and firm lesions often starting near the stem end. These spots appear greasy or water-soaked, quickly turning brown and resulting in large, irregularly shaped areas of decay.
Bacterial pathogens also cause significant rot, with slightly different symptoms. Bacterial Speck (Pseudomonas syringae pv. tomato) results in tiny, slightly raised or flat black lesions less than one-sixteenth of an inch across. As the fruit grows, the surrounding tissue expands, causing the lesions to appear sunken or pitted, sometimes with a dark green or yellow halo. Bacterial Spot (Xanthomonas species) creates slightly larger, raised, dark brown blisters that later become scab-like and rough. Both diseases are often seed-borne and exploit entry points like microscopic wounds or natural openings on the fruit surface.
Environmental Factors and Physical Damage
While pathogens cause infectious rot, external environmental conditions often set the stage for invasion or cause damage that mimics decay. Inconsistent water availability is a primary trigger for fruit damage, leading to cracking or splitting. When a dry period is followed by a sudden influx of water, the fruit pulp expands faster than the skin, causing radial cracks (running from the stem end) or concentric cracks (encircling the stem).
These cracks are not infections, but they create open wounds that provide entry points for airborne fungal and bacterial spores. High humidity and poor airflow further encourage pathogen development by extending the duration of fruit surface wetness. Another common non-infectious issue is Sun Scald, which occurs when previously shaded fruit is suddenly exposed to intense, direct sunlight. The exposed area develops a bleached, whitish, or pale gray spot that feels thin and papery, resembling a sunburn. This damaged, dead tissue is unable to fend off secondary invaders, allowing opportunistic fungi and bacteria to colonize the area and cause soft, black rot.
Physiological Issues Often Confused with Rot
Blossom End Rot (BER) is a common problem frequently mistaken for infectious rot, but it is a physiological disorder. BER is related to the poor mobility of calcium within the developing fruit. Calcium is an immobile nutrient, meaning it moves primarily with the water flow to the parts of the plant that are transpiring the most, such as the leaves.
When water uptake is erratic (due to inconsistent watering, drought, or excessive moisture), calcium transport to the fruit is interrupted. This lack of calcium causes the cell walls in the fruit’s blossom end (the furthest point from the stem) to break down. BER presents as a distinct, dark brown to black, sunken, and leathery spot confined to the bottom of the fruit. Unlike infectious rot, it begins as a dry breakdown of tissue. Although the initial damage is not caused by a pathogen, the dead tissue is often invaded by secondary fungi and bacteria, causing a wet, soft decay that resembles true rot.