Blossom end rot results when not enough calcium moves into the just-developing fruit. Any environmental condition that slows down transpiration will slow water and nutrient flow from roots to leaves and fruit. Here are some such environmental conditions:

• drought
• cool, cloudy weather
• root rot (water-logged soil)
• root damage (can happen during transplanting)

Some varieties of tomato are less susceptible to BER; try growing a few different varieties and notice which ones get it less. Then plant that variety (along with a few new varieties) next year. Eventually you will find your favorite.

Deep, rich soil will allow you to keep the soil moisture more consistent. Healthy soil has excellent soil structure so that the roots can access both water and air at the same time. If your soil is poor, BER will be more of a problem for you. If your soil is shallow, raise the row or bed to allow for good drainage and root aeration.

Here's a link to more information about Blossom End Rot: http://utahpests.usu.edu/plantdiseases/htm/vegetable/blossomrot

Good gardening!