Blossom End Rot
Blossom end rot is a lack of calcium being supplied to the developing fruit. One cause of this disorder, which is most common, is fluctuation of soil moisture. When soil goes from being wet to dry and then wet again, the roots ability to uptake nutrients is reduced. Root damage, high soil ammonium levels or excessive amounts of water are a few more causes of blossom end rot. The paste tomato varieties are most susceptible.
Once you have developed blossom end rot you can prevent the problem on new tomatoes by spraying a solution of one teaspoon of calcium chloride in a gallon of water directly on to your leaves once a week. This will save any new fruit from developing the disorder as long as they receive treatment before they are about ½”. Even if the fruit is still healthy looking when it receives the calcium, it may still develop blossom end rot if the plant was not sprayed early enough. If you cannot find any calcium chloride, you can try a solution made of one gallon of water with three Tums dissolved in it. This is applied the same as the Calcium Chloride solution.