Answer: What you're describing sounds like Sour Rot, especially if lesions are water-soaked, and may have a white scummy growth in the cracks. Lesions often occur near the stem end scar.
There are a number of fungi that can cause Sour Rot, including Pythium, Phytophtora, Rhizoctonia, and Alternaria.
To control, fruit should be kept off the soil (if possible), and surface wetting should be minimized. (Furrow or drip irrigation is preferred over sprinkler irrigation.)
As a last resort fungicide sprays may be used if you have a "zero tolerance" for defects in your tomatoes. Generally, however, correcting your watering practices is all that's required.
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