Blossom end rot is common among tomatoes, particularly those that ripen in July and early August, according to Iowa State University Extension and Outreach. Blossom end rot appears as a brownish black spot on the bottom of the fruit and secondary organisms invade the spot, causing the fruit to rot. A calcium deficiency in the developing fruit is the cause for it. To reduce blossom end rot, it is recommended to water tomato plants on a weekly basis during dry weather, mulch the area around the plants and pick and discard fruit affected with blossom end rot. It can also be found on peppers, eggplants, summer squash, and zucchini. More information is available through the ISU Hortline at 515-294-3108.