Tomatoes require a minimum of six to eight hours of direct sunlight daily for a good harvest. While necessary for healthy growth, intense, prolonged sunlight and high temperatures can damage both the fruit and the foliage. This overexposure leads to a physiological disorder that reduces the quality and yield of your plants, often exacerbated during heatwaves or when plants are suddenly exposed to strong light.
The Mechanism of Sunscald and Overexposure
The damage tomatoes experience from excessive sun is called sunscald, a physiological condition, not a disease. Sunscald occurs when fruit tissue is exposed to high light intensity and extreme heat. The fruit’s inability to dissipate heat causes cellular damage, leading to photo-oxidative stress. This stress destroys chlorophyll and pigments, causing affected areas to lighten in color. Damage is common when shaded fruit is suddenly exposed, such as after heavy pruning or significant foliage loss.
Identifying Signs of Sun Damage on Tomatoes
Sun damage symptoms appear on both the fruit and the leaves. On the fruit, sunscald initially presents as pale, whitish, or yellowish blotches on the side directly facing the sun, usually on the shoulders. As the damage progresses, the affected area flattens, develops a leathery texture, and may blister. The scorched area eventually turns grayish-white or brown and takes on a thin, paper-like appearance. This damaged tissue does not ripen normally and is highly susceptible to secondary infections from fungi or bacteria, which can cause the fruit to rot. Sun damage on foliage, called sun scorch, appears as scorched, dry, and crispy brown edges. In severe cases, the leaf tissue may become bleached white or yellow before turning brown and dying.
Practical Methods for Plant Protection
Protecting tomatoes requires preventative measures to mitigate the effects of high heat and sun.
Pruning and Shade
One effective preventative step is ensuring proper pruning practices leave sufficient leaf cover to act as natural shade for the developing fruit. Avoid excessive removal of leaves, especially those that overhang or shade clusters of fruit.
Temporary Shade Cloth
For high-heat periods, implementing temporary shade is recommended. A shade cloth with 30% to 50% coverage can be suspended over the plants during the hottest hours of the day, typically between 11 a.m. and 3 p.m. This reduces the intensity of direct light while still allowing enough sun for photosynthesis and growth.
Watering and Mulching
Consistent and deep watering helps plants cope with heat stress, though it will not reverse existing sunscald. Watering in the early morning allows the plants to take up moisture before the heat of the day, helping them maintain turgor and overall health. Applying a layer of mulch around the base of the plant helps regulate soil temperature and conserve moisture, which minimizes overall heat stress.