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David Trinklein
University of Missouri
Division of Plant Sciences
1-87 Agriculture Building
Columbia, MO 65211
(573) 882-9631

Hot Weather Tomato Problems

Published: August 16, 2012

It’s now official; July, 2012 was the hottest July in U.S. history. Somehow I doubt this revelation comes as a surprise to anyone reading this article. The brutal, hot weather of this summer is a vivid reminder of how helpless we are when it comes to the weather. It also brought with it a number of problems for our garden plants. Since tomato remains the most popular home garden vegetable in the U.S., it is timely to review some of the problems of tomato production associated with hot weather.

Poor fruit set. The metamorphosis of a tomato flower into a fruit is contingent upon its pollination followed by the fertilization of its ovules. The latter ultimately will develop into seeds as the fruit matures. This remarkable series of events is sensitive to several environmental factors, with temperature playing an important role.

High temperatures, especially if accompanied by low humidity, hinder fruit set through failure of viable pollen to form and/or fertilization to occur. Temperatures above 90°F during the day and above 70°F at night usually result in poor flowering and reduced fruit set. Research indicates that night temperature likely is more critical than day temperature, with the optimal range for the former being 59 to 68°F.

It must be noted, however, that temperature cannot always be blamed for poor fruit set. A heavy fruit load combined with inadequate nutrition can reduce fruit set on flower clusters located on the middle-to-upper part of a tomato plant.

Delay in ripening. Typically, it requires between six and seven weeks following the pollination/fertilization of a flower for a tomato fruit to mature fully. During that 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 between 68 and 77°F. The greater the deviation from that temperature range, the slower the ripening process will be. Extremely stressful temperatures can virtually halt the process entirely.

Pigment (color) development in tomato fruit occurs in the very final stages of ripening and is temperature-sensitive. The red pigment lycopene and yellow pigment carotene are the two pigments that give a tomato fruit its color. Temperatures above 85°F tend to slow or even halt the production of these two pigments. Since 37% of the lycopene of a tomato fruit is contained by its skin, it is no wonder why tomatoes growing in excessively hot conditions produce poorly-colored fruit.

Yellow-shouldered fruit. “Yellow shoulder” is a physiological disorder characterized by areas at the top (shoulder) of the fruit remaining yellow and as the remainder of the fruit ripens and turns red. These yellow areas never ripen properly and the tissue below them is tough and poorly flavored. Tomato varieties that are green shouldered when immature are more likely to show the trait than varieties that have uniformly-green immature fruit. It appears that both temperature and nutrition are involved in the development of yellow shoulder.

As previously mentioned, high temperatures retard (or prevent) the production of the red pigment lycopene. Since the shoulder of a tomato fruit most often is exposed to the direct, warming rays of the sun, lycopene deficiency appears there first resulting in yellowish coloration. Additionally, it has been found that tomatoes exhibiting yellow shoulder most often are deficient in potassium. When plant tissue potassium levels drop from adequate (4-6%) to low (2-3%), yellow shoulder often ensues, especially if temperatures are high.

Therefore, choosing varieties with the uniform ripening trait, maintaining good foliage cover of the fruit and (especially) supply plans with ample amounts of potassium can greatly reduce or eliminate the incidence of yellow shoulder.

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 appear pale in color and “corky”. Older varieties with five distinct cavities (locules) filled with seeds are especially prone to this disorder. Newer, “beef steak” types have multiple locules and 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 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 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 entirely.

In closing, Mark Twain once stated, “Everyone talks about the weather but no one does anything about it.” Unfortunately for gardeners that statement still holds true and there is little we can do other than to keep our tomato plant vigorous, well watered and adequately fertilized. If foliage is sparse covering tomato plants with a material that partially blocks the sun such as cheese cloth or shade fabric is a tactic worth trying. As summer transitions into fall and temperatures start to moderate, a return to normal ripening for those fruit that our tomato plants did manage to set should occur.

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