Blossom End Rot of Tomatoes Blossom end rot is a common problem on tomatoes. It causes dark, leathery, sunken blotches on the bottoms of your beautiful, ripening tomato fruit. Blossom end rot is considered a nutritional disorder, since it results from a lack of calcium in growing fruits. Practically speaking, however, the cause is usually inadequate irrigation.
Soil moisture is essential for roots to absorb calcium.
Keep the plants well watered so calcium will be available. Also avoid great fluctuations in soil moisture which seem to aggravate the problem. Use an organic mulch around plants to help keep the soil evenly moist. If you use black plastic mulch, make sure it has enough holes for water to penetrate. WSU Extension Bulletin 422, Home Gardens, or call your WSU Cooperative Extension Office .