Answer: I can't diagnose the problem without examining the fruit but you could be seeing growth cracks that have calloused over. Two types of growth cracks affect 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.
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