Blossom-End Rot Damaging Tomatoes and Peppers

If you've noticed brown blotches on the bottom of your tomatoes, squash, peppers, apples, apricots and other assorted fruits and vegetables, your not alone. A disorder known as blossom-end rot has been occurring in many gardens throughout Tucson.

The disorder causes the blossom-end, actually the bottom of the fruit, to develop a soft, brown or black sunken area. This area can be rather small, or can include upwards to half of the fruit. The disorder most often appears on immature fruit but can also appear on ripe fruit. The rot is not caused by a disease or insect, although a fungus can begin growing on the damaged tissue.

The cause of blossom-end rot is actually caused by a calcium deficiency in the forming fruit. But it's not a lack of calcium that's the real problem. We have plenty of calcium in our desert soils. In fact, we have way to much calcium, which causes our soils to be very alkaline.

The real problem behind blossom end rot is inconsistent watering. And more times than not, here in the desert, it's insufficient watering. If the soil drys between waterings or if the soil is kept moist then dry for a period of time the flow of calcium from the soil to the fruit is disrupted. This disruption, and resulting lack of necessary calcium causes blossom end rot to form.

In large commercial production of tomatoes and peppers, a form of calcium is often sprayed on plants during fruit formation. This effectively prevents the disorder. However, this is an impractical and unnecessary control measure for home gardeners. The key to preventing blossom-end rot in the home garden is to keep the soil consistently moist.

Be aware that when temperatures warm up rapidly, irrigation must be adjusted to meet the plants water needs. In cooler weather watering can be spaced two or three days apart. But when warm weather arrives, daily watering may be needed. And in hot weather, mid 90's and above, twice daily watering is often required to keep the soil moist. Using organic surface mulches is one of the best ways to keep prevent fluctuations in soil moisture. Forest mulch, straw, peat moss, or layers of newspaper placed over the soil and around plants will provide valuable insulation.

Along with blossom-end rot, another common problem of vegetables and fruits in June is blossom and fruit drop. High temperatures, high winds and a lack of soil moisture contribute to this problem. Some vegetables, such as tomatoes and snap beans will drop blossoms when day temperatures are above 95 degrees. Corn tassels are often killed when temperatures are above 100 degrees. Injury to the tassels will reduce or prevent formation of kernels on the ear.

Citrus trees will drop fruit in June. Some of this fruit drop is normal; a natural thinning of excess fruit. However, excessive fruit drop can be the result of insufficient watering. Apples and plums are also subject to this type of fruit drop.

June is the month that requires the most attention to water needs. Hot, dry conditions sap plants of moisture, stressing them and creating problems for the formation of flowers and fruit. Check soil moisture levels on a regular basis, and make watering adjustments as necessary.

Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
Material originally appeared in Arizona Daily Star gardening column, on June 21, 1998
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