Today we’re teaching you how to spray Stop Rot, a calcium additive for your tomatoes and watermelons. When your watermelons develop a black spot on the ends, that’s a sign of calcium deficiency. When you spray the product, you must be sure to wet the stem. It’s best to do this early or late in the day so that the plants don’t get scorched. Use on cucumbers, melons, peppers, and tomatoes.