Blossom end rot is a physiological condition of tomatoes, peppers and eggplant that can occur when there is not enough calcium in the fruit that is developing. This deficiency can occur from drought stress or too many fluctuations in the water supply when the fruit is developing that somehow makes the calcium unavailable to the fruit. On a tomato it looks like a brown patch on the bottom that continues to enlarge and eventually sink or rot.
It seems like early in the growing season, I have had this problem on many plants and have heard many others say they have a problem with it. Harry Ellmore of Ellmore’s Garden Center, Hamilton, Virginia, told me a remedy he uses and advises his customers to do also. He grows beautiful plants and has a reputation for quality.
He said to mix 1 Tablespoon of Epsom Salts (magnesium sulphate) in 1 gallon of water and water the tomato at the base of the plant with this mixture. You may not need the entire gallon. I expect it depends upon how big your plants are. Don’t pour over the plant, make sure you pour at the base. You can repeat this procedure twice a month. Hope you have a great tomato crop.
Photo from Ohio State University Extension website