Culprit: Blossom end-rot
Blossom-end rot is an environmental disease that affects tomatoes, peppers, squash and watermelon. It is caused by lack of calcium in the fruit because the roots of the plant are having problems taking calcium from the soil. A small area at the bottom of the fruit can be affected or most of the fruit can rot. Mold can sometimes be seen growing on the damaged area.
Roots are damaged in five ways: Extreme fluctuations in soil moisture from very wet to very dry is first. Water often and evenly when the weather is dry and add mulch to keep the soil damp between waterings.
Too much nitrogen fertilizer or fresh manure will cause the plant to grow too fast, which tends to slow down later in the season. Very often this results in blossom-end rot early in the season and then it goes away on its own.
Excessive water in the soil from rain or irrigation drowns root hairs that the plant needs to take in the calcium. Plant these crops in well-drained soils, or cut back on watering.
Too much salt in the soil kills the roots. Plant in better soil or flush out the salts by applying more water, but not in poorly drained soils or else you will have the previous problem.
Lastly, roots can be damaged through weeding or cultivating the soil too close to the plant. Don’t go deeper than 1 inch within a foot or two of the stem.