Answer: Without seeing the fruit I can't tell you for sure, but it sounds like either catfacing or growth cracks.
Catfacing describes the presence of deep indentations in the blossom end of the fruit. In some cases, the fruit itself is misshapen, becoming kidney-shaped or otherwise distorted. This damage occurs when temperatures drop below 50 degrees F during flowering and fruit set, resulting in poor pollination. In some cases, excess heat, 2,4-D injury, and erratic soil moisture can lead to catfacing. High nitrogen has also been shown to aggravate this disorder. Catfacing is cultivar specific and appears more frequently on older cultivars. Large-fruited cultivars are most susceptible.
Two types of growth cracks affect the stem end of tomatoes: concentric and radial. Concentric cracking produces circular cracks around the stem end of the fruit. Radial cracks spread outward from the stem scar. These cracks typically appear as the fruit matures. Growth cracks often appear when conditions drastically change the rate of growth, such as wide fluctuations in temperature and moisture. Dry weather followed by heavy rains causes radial cracking in many tomato cultivars. Cultivars vary in their ability to withstand cracking depending on the strength and "stretchability" of the skin. Very susceptible cultivars will crack while still green, those that are somewhat resistant often don?t crack until the fruit has reached the breaker stage. The earlier the fruit cracks, the deeper the cracks become. High nitrogen and low potassium are often implicated in fruit cracking and therefore, proper plant nutrition and adequate, regular irrigation will reduce the likelihood of growth cracks.
Sound like what's happening to your tomatoes?
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