Blossom-end Rot on Tomatoes

Stan Roark

This time of year I get many phone calls about problems on tomato plants. The most common problem this year has been blossom-end rot. Blossom-end rot (BER) is not a disease but a physiological disorder caused by a calcium deficiency. It starts with a dry brown region the size of a dime near the blossom-end of the fruit. The condition then enlarges as the tomato grows, turns black and may become covered with one of several molds.

Calcium is an essential element for plant growth. The most common source of calcium is the lime that gardeners use to adjust the pH of garden soil. Soil testing is the best way to determine the amount of calcium in your garden. If your garden lacks calcium, then it is likely that your tomatoes will develop some blossom-end rot.

However, blossom-end rot often occurs in gardens that have plenty of lime. Why is this? Calcium is not a highly mobile element in plants. Even brief changes in soil moisture levels can interrupt the flow of calcium into a tomato plant. This means that blossom-end rot can also be caused by drought, root damage from improper cultivation, too much water (dilutes the amount of calcium), too much fertilizer at one time and sometimes the variety of tomato being grown.

Prevention: To control blossom-end rot you should:
1. Keep the pH of the soil at 6.0 to 6.5. Apply lime 2-4 months before planting.
2. Apply fertilizers in the required amounts – according to soil test results.
3. Use mulches to conserve and even out moisture levels.
4. Supply 1.5 inches of water weekly during fruit development either from rainfall or supplementary watering.

Cure: There is a short term cure for this problem. Plants can be sprayed with a calcium nitrate or calcium chloride solution at the rate of 4 level tablespoons per gallon of water. Be careful with the calcium chloride since it can burn plants when temperatures are above 85 degrees. You should spray 2 or 3 times each week. Spraying calcium is not a substitute for a good moisture and fertility program.

For more information see our website www.aces.edu and extension publication ANR-1059.