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Ask a Master Gardener: Beware — heat can ruin your tomatoes

by Peter Sutter | July 31, 2022 at 3:15 a.m.

I don't think I have ever seen it so hot and dry, but then I probably say that every summer when it gets hot and dry.

David Trinklein, of University of Missouri Plant Science and Technology, had an article about the effects of heat on tomatoes. Here are some highlights.

Poor fruit set

High temperatures, especially if accompanied by low humidity, hinder tomato fruit set through failure of viable pollen to form and/or fertilization to occur. Temperatures above 90 degrees during the day and above 70 degrees at night usually result in poor flowering and reduced fruit set. In extreme cases, none of the flowers pollenate leading to a "blind" (fruitless) flower cluster. Research indicates night temperature likely is more critical than day temperature, with the optimal range for the former being 59-68 degrees.

Delay in ripening

Typically, it requires six to seven weeks following the fertilization of a flower for a tomato fruit to mature fully. During this period, the fruit goes through a number of developmental stages that ultimately result in a red (pink, yellow, orange, etc.) fruit that is ripe and ready to harvest. Mature, green tomatoes ripen most rapidly at temperatures 68-77 degrees. The greater the deviation from that temperature range, the slower the ripening process will be. Extremely stressful temperatures can virtually halt the process entirely.

Internal whitening (white core)

Under stressful conditions, tomato fruit often develop a tough, white core in their center. The white tissue might be expressed only in the area of the fruit just beneath the calyx or, in extreme cases, through the entire depth of the fruit. The internal walls of the fruit may also be pale in color and "corky" in appearance. Older varieties with five distinct cavities (locules) filled with seeds are especially prone to this disorder. Newer, "beef steak" types that have multiple locules tend to show white core less often.

Once again, excessive heat and improper fertility seem to be related to the formation of white core. Malnourished plants with poor foliage cover tend to bear fruit exposed to the sun, thus adding to the problem of temperature stress of the fruit. As was the case with yellow shoulder, insufficient tissue potassium levels have been associated with white core development.

Choosing newer varieties less prone to white core development, maintaining a fertility program that encourages good foliage cover and supplying ample amounts of potassium are best management practices for preventing the disorder.

Blossom-end rot

Blossom-end rot of tomatoes is a physiological disorder caused by a lack of calcium in the blossom end of the fruit. This disorder results in tomato fruit with brown or tan areas on their blossom (bottom) end. These areas start as small lesions and gradually develop to cover nearly the entire end of the fruit.

Although blossom-end rot is caused by a lack of calcium, it is a lack of water needed to uptake and translocate calcium that is most often responsible for its development. Since hot weather increases water loss (transpiration) from tomato plants, the incidence of blossom-end rot usually is greatest when temperatures are hot. Maintaining the proper soil pH, supplying tomato plants with adequate amounts of calcium and irrigating on a timely basis can prevent this tomato problem.

Fruit cracking

As its name implies, fruit cracking occurs when the epidermis of the fruit does not expand at the same rate as the fruit interior. The result is the formation of cracks. Radial fruit cracking is characterized by the cracks radiating outward from the stem of the fruit, while in concentric cracking they circle the stem. In both cases, cracking is most severe at the top of the fruit.

This disorder most often is associated with an irregular water supply to the plant. The rapid uptake of water by tomato fruit, after a rain or heavy irrigation, especially if the soil was somewhat dry, is an "ideal scenario" for the formation of fruit cracks. Fruits that grow too fast, or plants that are succulent due to high nitrogen levels or low potassium levels, are more likely to produce cracked fruit. Growers of heirloom tomatoes are all too familiar with this disorder, since most heirloom varieties are very susceptible. Plant breeders in recent years have made tremendous progress in developing hybrid varieties that resist fruit cracking. The use of these hybrids along with providing constant, adequate moisture usually eliminates this disorder.

Peter Sutter is a lifelong gardening enthusiast and a participant in the MU Extension's Callaway County Master Gardener Program.

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