If you see this on the base of your ripening tomatoes, it's a condition called Blossom End Rot, caused by calcium deficiency. As tomatoes begin to set, calcium is important in order for the fruit to develop properly. A lack of calcium results in a brown, water-soaked spot on the end of the fruit where the blossom once was. As the tomato grows, this spot darkens, eventually becoming leathery and black. Calcium deficiencies can be solved in several ways. First, make sure you're watering your tomato plants consistently. Over watering followed by under-watering reduces the plant's ability to absorb calcium in the amounts needed to ripen fruit.Avoid overhead watering. Water at the soil line, slowly and deeply. Build a moat around each plant and slowly fill the moat with water. Be careful when you're working around your tomato plants, to avoid damaging the root system. Happy Frog Fruit and Flower fertilizer with a higher middle number (Phosphate) than first number (Nitrogen). fertilizer also contains Calcium and Magnesium supplements and mycorrhizae. Adding mycorrhizae will increase root strength and improve nutrient uptake. separately with Root Rally. Apply a liquid calcium directly on the leaves and fruit, using Fertilome's Yield Booster. Products such as Cal-Mag and Down To Earth™ Oyster Shell applied to the soil around your tomato plants will help increase available Calcium. Blossom End Rot does not make the fruit inedible. You may cut the spots off the harvested fruit and eat the remainder. Blossom End Rot cannot be reversed on a tomato once it's started, but taking these steps will help minimize the damage to your crop this season.