Irrigate plants thoroughly and often enough to maintain a constant water supply without water-logging the plants. Tomato plants require 1.0 to 1.5 inches of water per week during growth and fruiting depending on soil type and weather conditions. Extreme fluctuations in soil moisture can cause an increase of blossom-end rot. Preventing moisture stress is important to control blossom-end rot, especially during fruit set and fruit enlargement. Foliar sprays can be used to help prevent the problem on young developing fruit before signs of blossom end rot occurs.
Calcium chloride is suggested only for tomatoes.
Calcium nitrate can also be used as a spray treatment. Calcium chloride may burn the plant if sprayed during the hotter part of the day. Do not spray plants under stress conditions. Soil-applied treatments and prevention by cultural practices are generally preferred over sprays. Apply sprays every 7 to 10 days until 3 or 4 applications have been made.