Gardening Fast Facts: Blossom End Rot
While out harvesting your tomatoes this season, did you notice any fruits that had round, sunken, water-soaked spots on the bottom? This disorder is called blossom-end rot. As the name implies, it always occurs on the blossom end of the tomato (opposite from the stem end). The spot can grow large enough to cover the entire bottom half of the fruit. Blossom-end rot not only affects tomatoes, but can also occur on peppers, squash, and watermelons. The fruit may look like it's ready for the compost pile, but you can still eat it - just cut the rotted portion away.
Blossom-end rot is caused by a calcium deficiency in the developing fruit. This calcium deficiency is caused by slow plant growth and root damage, which is ultimately caused by several factors: fluctuations in soil moisture; rapid plant growth, followed by drought; excessive soil salts, and cultivating too close to the plant.
There are several things you can do to prevent these factors from causing the calcium deficiency. First, mulch your plants. This will help keep soil moisture levels at a uniform level. Second, use a rain gauge to make sure your plants are getting at least one inch of water per week. Third, don't use high-nitrogen fertilizer (like fresh manure); this will help prevent the rapid plant growth. Fourth, if your soil is salty, water plants thoroughly and deeply to help leach the excess through the soil. And fifth, don't cultivate near the base of the plants any deeper than one inch. (Mulching will keep the weeds away in addition to keeping the soil moisture uniform, so you may not need to cultivate at all.)
By following these preventive measures, you'll decrease the chance for blossom-end rot to form. But if you're still plagued by the problem, another thing to try is to spray the plants with a calcium chloride solution. Dilute calcium chloride in water at the rate of 5 tablespoons per gallon. Then, spray the solution on the plant's foliage until the leaves are thoroughly wet. Start your spray program when the first fruits have formed, and repeat weekly for three to four weeks.