By Carl Cantaluppi
N.C. Cooperative Extension Granville County
When we experience high temperatures, we can expect some problems with vegetable fruit set and quality. The ground around snap beans may be littered with aborted flowers. Flower and fruit abortion are common responses to heat stress as a result of the general failure of successful pollination and fertilization. The reproductive structures can become unreceptive, pollen can be killed, and the pollen tube may fail to form.
Many people are concerned most about their tomatoes and cucumbers. In general, cucumbers are more heat tolerant than tomatoes. Another problem can be a lack of female flowers, often noticed in summer squash. As the heat increases, the ratio of male to female flowers changes to more male flowers. When cooler weather returns, the plants will shift back to a more normal ratio.
Day temperatures over 90 degrees and night temperatures over 70 degrees reduce fruit set in tomatoes. As few as three hours at 104 degrees on two successive days may cause failure of fruit set.
Tomato problems related to heat stress on fruit set prior to the stress include sunscald and yellow shoulders. Sunscald occurs when previously covered fruit are exposed to direct sunlight from having poor leaf cover.
Solar yellowing (yellow shoulders) is a type of sunscald that occurs when lycopene (the red pigment in tomato) fails to develop in some varieties, leaving only the carotene (yellow pigment) to show at the shoulder where the dark green portion was. This occurs when tissue temperature rises above 86 degrees. Even with temperatures under 85 degrees, the surface temperature of exposed fruit can become high enough to inhibit normal red color development.
Symptoms of more severe sunscald are that the tissue starts off as a pale area of leathery dead tissue that becomes sunken, turning tan or light gray, later turning white, and dying. Many times, the dead tissue will turn black from fungi that are feeding on the dead tissue. This lethal sunscald occurs when tissue temperatures rise above 104 degrees. Damage usually occurs when fruits are suddenly exposed to sunlight.
This most frequently occurs after a harvest or a storm when leaves are moved around and fruit is exposed. Over pruning can also increase sunscald problems especially with fruit in the upper part of the plant. Good spray programs to ensure good foliage cover can reduce the problem.
Sunscald and yellow shoulders are most common in mid-summer when the sunlight is most intense and when temperatures exceed 85 degrees. Sunscald fruit is edible but not if the scalded area has turned black with a secondary fungus.
Another problem in high temperature tomatoes is internal white tissue. The white tissue is only noticeable when the fruit is cut. The hard, white areas tend to be in the vascular tissues in the outer walls of the fruit, although it can also appear in the center of the fruit and in the cross-walls. High temperatures during the ripening period seem to trigger the symptoms. Sometimes there is a relationship with low potassium levels, which can be a problem in sandy soils and only rarely in clay soils. Some varieties are more resistant to the problem, especially the very dark red varieties.
As always, and especially during hot temperatures, make sure that garden soils are wetted down to a depth of 6-8 inches. Poke a long-handled screwdriver into the soil until it becomes hard to push. Then pull it up and you can measure how deep the soil is wetted. Then water until you reach that depth. After gaining experience, you will know how long it takes to water your soil down to 6-8 inches, once the top of the soil starts drying out.
Problems with blossom end rot of tomatoes can be virtually eliminated by keeping the soil moist so the plant can take up calcium through the water stream. Calcium deficiency in the developing tomato fruit causes blossom end rot, which is worsened by dry soils not allowing the calcium to be in the soil solution to be taken up by plant roots.