Concentric fruit cracking of tomato. (credit: University of Minnesota)

July is that time of year when many gardeners start to reap the "fruits of their labor" with regard to their home garden tomatoes. To the dismay of many, the results of their toil are not what they had hoped for from the standpoint of both yield and fruit quality. Let's take a closer look at a number of the problems many tomato growers experience.

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 tomato 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. In extreme cases, none of the flowers pollenate leading to a "blind" (fruitless) flower cluster. Research indicates that night temperature likely is more critical than day temperature, with the optimal range for the former being 59 to 68°F.

However, it is important to note 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.

'Blind' flower cluster where no fruit has set. (credit: University of Maryland)

Delay in ripening

Typically, it requires between six and 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 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. Lycopene (a red pigment) and carotene (a yellow pigment) 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 shoulder

Yellow shoulder is a physiological disorder characterized by areas at the top (shoulder) of the fruit remaining yellow 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 this disorder 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.

Severe yellow shoulder of tomato. (credit: University of Delaware)

Blotchy-ripening

Blotchy-ripening is characterized by parts of the fruit surface remaining green, gray, or yellow and not ripening fully. Gray wall is another term used to describe the disorder. Blotchy ripening is somewhat similar to yellow shoulder in appearance but scattered irregularly over the surface of the entire tomato fruit.

Inadequate potassium supplied to maturing fruit has been linked to the disorder. Additionally, compacted soil, above-optimal nitrogen application rates, excessive application of potassium uptake competitors, and excessively large or dense foliage canopies all contribute to blotchy ripening. High temperatures along with high relative humidity and low light levels seem to provide the ideal environment for the disorder to develop.

Blotchy ripening or gray wall of tomato. (credit: University of Kentucky)

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.

Internal whitening or "white core" of tomato. (credit: University of Maryland)

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 entirely.

Blossom-end rot of tomato. (credit: Texas A&M University)

Gold fleck

This superficial disorder is characterized by the presence of a multitude of very tiny gold or yellow specks uniformly spread across the epidermis of the tomato fruit. When gold fleck is very heavy, the fruit takes on a yellowish-orange hue instead of being red. Feeding damage by certain insects such as thrips and mites can also mimic gold fleck.

There are several theories relative to the cause of gold fleck; most involve the elements potassium and calcium, making it a nutritional problem. In short, the potassium-to-calcium ratio of tomatoes is known to influence gold fleck severity. Gardeners who fertilize tomatoes heavily with calcium nitrate without supplying additional potassium are more likely to experience gold fleck than those who rotate calcium nitrate with a fertilizer high in potassium such as potassium nitrate. Fortunately, just as beauty is only skin deep, so is gold fleck. The disorder does not affect the internal tissue of the tomato fruit nor alter its flavor

Severe case of gold fleck. ( credit: University of Maryland)

Sun scald

Sun scald appears as a whitish yellow spot on the tomato fruit where it is directly exposed to the sun. The disorder tends to worsen during hot weather. Some varieties are more prone to show the disorder than are others. Sun scald is especially problematic when tomatoes suddenly loose part of their foliage to a disease such as blight. Prevention strategies include planting tolerant varieties, maintaining an adequate foliage cover through proper nutrition and preventing foliage diseases.

Sunscald of tomato fruit. (credit Missouri Botanical Gardens)

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.

Radial fruit cracking of tomato. (credit: Missouri Botanical Gardens)

In closing, in spite of everything that can go wrong during the course of growing a tomato crop, it remains America's most popular home garden vegetable. The culinary experience of eating a homegrown, vine-ripened tomato is hard to put into words. That, in addition to the fact that most tomato growers are optimists, is what keeps gardeners determined to improve their production skills each year.