The sun's rays are tough on our faces and arms, but we can wear sunscreen to reduce the damage to our skin. Tomatoes don't have that option. Exposed to bright sun, the fruit can heat up dramatically, reaching temperatures as much as 18 degrees warmer than the surrounding air.
Sunscald damage to your tomatoes depends on their stage of maturity, and the intensity and duration of the heat. The fruit is most susceptible when it is green or when the first pink color begins to show (called the breaker state).
If the fruit's temperature exceeds 86 degrees Fahrenheit on its sunny side, the flesh remains hard and will not ripen. At this temperature, the fruit can't produce the red pigment, lycopene, but still produces the yellow pigment, carotene. At a sizzling 104 degrees, the tomato stops producing carotene and the damaged area turns white. Damaged cells eventually collapse and the area may become sunken.
To prevent sunscald, maintain a healthy foliage cover. Keep plants well-watered and free of pests and diseases that can reduce foliage. Don't prune your tomato plants and place them densely enough that they can benefit from mutual shade. Keeping the soil cool with plenty of water and mulch helps, too.
If you let your plants sprawl on the ground, don't turn them to expose the underside of the fruit. That's a common cause of sunscald.
Summer heat can also cause cracking in tomatoes. Radial cracks are most common, starting near the fruit stem, and developing down the sides of the fruit wall. Concentric cracks sometimes appear as circles around the stem end of the fruit.
Cracking is most common during hot, rainy periods when temperatures are in the 90s, particularly following long dry spells. It is most severe on fruit that is ripening in full sun and plants that have been heavily pruned. Those heavily pruned or that have lost foliage due to insects, disease or weather will heat up and be most likely to crack.
To help prevent your tomatoes from cracking, use the same strategy as for sunscald. Fertilize plants judiciously and encourage good foliage cover. Mulch the soil around the plants and keep it consistently moist, using drip irrigation instead of overhead impact sprinklers.
Some tomato cultivars, such as "Beafsteak-type" tomatoes, are known for cracking, while others are more resistant. Cultivars like 'Early Girl,' 'Ultra Girl' and 'Springset' resist cracking.