The black rotting spots on your tomatoes are known as blossom end rot. It's caused by a lack of calcium when the tomato begins it's development. I put crushed egg shells in the planting hole. However, lack of water will also cause this as the plant cannot take up the calcium it needs. Be sure to keep the soil moist at all times. The spots are probably caused by a fungal disease. Take couple of close up pictures of the spots in to your local independent garden center for and Id and a recommendation of the proper fungicide. Best And Happy Yardening, Nancy.