The sudden appearance of white patches on tomatoes is often a sign that the fruit has reacted poorly to environmental stress, nutrient deficiencies, or insect activity, rather than a disease. The whiteness can manifest in two distinct ways: as a pale, external blister on the fruit’s surface, or as hard, fibrous, white tissue found only when the tomato is sliced open. Understanding the location and nature of the discoloration is the first step toward diagnosing the underlying cause and protecting the harvest.
External Causes: Sunscald and Heat Stress
The most common external cause of white spots is sunscald, which is essentially a sunburn on the fruit. Sunscald appears as large, pale, whitish-yellow, or grayish patches on the side of the tomato directly exposed to intense sunlight. This damage occurs when the fruit’s temperature rises significantly, often exceeding 86°F (30°C), causing the underlying tissue to become bleached and leathery. The affected area can eventually become blistered or sunken, making the fruit susceptible to secondary infections like black mold.
Tomatoes naturally rely on a canopy of leaves to shade the developing fruit from direct heat and UV light. Sunscald often occurs after a sudden loss of this protective foliage, perhaps due to over-pruning or a leaf-shedding disease. When shading is removed, the exposed fruit cannot tolerate the intense exposure, leading to rapid tissue breakdown confined to the outer skin and flesh.
Internal Causes: Physiological Ripening Disorders
Internal whiteness indicates a physiological ripening disorder that occurs while the fruit is developing on the vine. This issue is often diagnosed as “Internal Whitening” or “White Core,” where the fruit appears normal externally but contains tough, hard, white, fibrous tissue near the core or outer walls when cut open. A related issue is “Blotchy Ripening,” characterized by pale, uneven patches that remain green or yellow after the rest of the fruit has ripened. Both result from the fruit’s inability to properly synthesize the pigments and compounds needed for a full red color and soft texture.
These internal disorders are linked to nutrient imbalances and temperature fluctuations during the ripening phase. A deficiency in potassium (K+) is a primary suspect, as potassium is needed for proper sugar and pigment transport to the fruit. The problem is worsened by excessively high nighttime temperatures, which disrupt the plant’s metabolism and its ability to mobilize nutrients. Excessive nitrogen fertilization can also contribute by promoting lush leaf growth at the expense of fruit quality, complicating potassium supply.
Insect Damage and Edibility Concerns
A third cause of localized whiteness is the feeding activity of piercing and sucking insects, primarily stink bugs and leaf-footed bugs. These pests use their straw-like mouthparts to probe the fruit and extract internal juices. The damage manifests as small, cloudy, pin-prick lesions on the surface that correspond to hard, whitish, spongy, or corky spots immediately beneath the skin. This localized discoloration is caused by the plant tissue dying and callousing in response to the insect’s injected saliva and the physical puncture.
The fruit remains edible in nearly all cases—including sunscald, physiological disorders, and minor insect damage—once the white or damaged portions are cut away. While the flavor and texture of the remaining flesh may be compromised, the fruit is not toxic. However, sunscalded areas that have developed black mold or insect-damaged spots that have begun to decay should be discarded to avoid consuming secondary fungal or bacterial infections.
Long-Term Prevention Strategies
Preventing sunscald requires maintaining the plant’s natural defense system: its foliage. Gardeners should avoid aggressive pruning that exposes developing fruit to direct sun and may need to install a temporary shade cloth during periods of intense heat. Ensuring the plant remains healthy by managing defoliating diseases is also an effective preventative measure.
Minimizing Ripening Disorders
To minimize physiological ripening disorders, focus on providing consistent water and balanced nutrition, especially potassium, before the fruiting stage begins. Deep, consistent watering helps regulate soil and plant temperature, mitigating the stress that triggers internal whitening. Soil testing can confirm potassium levels, and applying a fertilizer with a higher middle number (indicating potassium) can help ensure the fruit receives the necessary nutrients for proper ripening.
Managing Pests
Managing pest damage involves Integrated Pest Management (IPM) techniques, such as removing weedy areas where stink bugs overwinter and using row covers early in the season to prevent the insects from laying eggs.