Blossom end rot occurs on tomatoes when there is not enough calcium in the developing fruit. Cells at the blossom end of green tomato fruit are damaged creating an area with dark, sunken leathery tissue. The unaffected portion of ripe fruit can safely be eaten if secondary decay has not occurred.
Most soils in San Diego County contain enough calcium. In some situations tomatoes may benefit from an application of gypsum (calcium sulfate) prior to planting. Adequate irrigation and good cultural practices can help prevent blossom end rot. Select varieties resistant to soil diseases and nematodes. Avoid planting beefsteak varieties if blossom end rot has been a problem since they often develop the disorder. Plant tomatoes in good garden soil where they will receive sun all day. Mix in a complete fertilizer high in phosphorus prior to planting and do not over fertilize with nitrogen during the growing season. Excess nitrogen stimulates lush vegetative growth which competes with developing fruit for available calcium.
Irrigate a large area around each plant to encourage the development of a large root system. Wet the soil at least a foot deep when the top few inches of soil begins to dry out. Tomatoes grown in containers usually require more frequent irrigation than those grown in ground beds. For best results use a large container such as a half oak barrel and make sure there are several holes in the bottom for drainage.
By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension, August 2012