Blossom end rot can be caused when calcium and/or water levels in the root zone are low. In our soils, generally calcium is plentiful and low water levels are the cause of the deficiency in the fruit. When water in the soil is in short supply, the plant takes up insufficient amounts of calcium to meet the plants demand. Since calcium is transported in the xylem with the water, during conditions when water is in short supply, most of the water is transported to the leaf and very small amounts are transported to the flowers. This causes a localized deficiency which is expressed as the development of a brown leathery patch on the blossom end of the fruit. This just happens to be the area where fruit growth is most rapid. What conditions cause blossom end rot. There are several. These include low soil moisture levels (water stress), high temperatures (high water demand by the plant), high salinity levels in soils (less available water for plant use), low humidity levels (greater transpiration by leaf so less water and calcium goes to the fruits) and rapid plant growth rates (high demand for calcium) to meet cell wall requirements. Cultivar differences also exist. Small fruited tomatoes are less prone to the disorder than large fruited types. This is due to the rate of cell growth and demand for calcium in the fruits.

How can it be controlled? First try to maintain more even soil moisture levels. Don't let the soil fluctuate greatly from wet to dry and then wet again. Second, maintain more even growth rates. Excess nitrogen in the soil accelerates growth and makes the plants more susceptible to blossom end rot. If salts are a problem, see what can be done to leach them out before growing tomato or water with a less salty water source so the salt levels of the soil does not build up. Finally, after you have done all of these other things, try growing a smaller fruited cultivar, thin the number of fruits on a cluster to 2-3 and spray the plant periodically with calcium nitrate or calcium chloride (4 grams per liter of water) starting about a month after planting. Do every two weeks through the main flowering period.