As hot temperatures set in, you can expect to see some problems with vegetable fruit set and quality.

“Flower and fruit abortion are common responses to heat stress, as a result of the general failure of successful pollination and fertilization,” says Carl Cantoluppi, Granville County Extension agent. “The reproductive structures can become unreceptive, pollen can be killed, and the pollen tube may fail to form.”

Heat stress is easy to see in tomatoes and summer squash. With rising temperatures, squash produce fewer female flowers, and the ratio of male-to-female flowers shifts to favor male flowers. When cooler weather returns this trend shifts back to a more normal ratio, thus producing more squash.

Fruit set decreases in tomato plants when daytime temperatures exceed 90 degrees and nighttime temperatures do not dip below 70 degrees. As few as three hours of 100-degree weather can cause fruit set to fail.

“Tomato problems related to heat stress on fruit set prior to the stress include sunscald and yellow shoulders,” says Cantoluppi. “Sunscald occurs when previously covered fruit are exposed to direct sunlight from having poor leaf cover.”

Sunscald and yellow shoulders are most common in midsummer, when sunlight is most intense and temperatures exceed 85 degrees. Fruit suffering from sunscald is edible, but not if the scalded area has turned black with a secondary fungus.

Another problem with tomatoes in hot temperatures is internal white tissue. Noticeable only when the fruit is cut, hard, white areas tend form in the vascular tissues in the outer walls of the fruit. They can also appear in the center of the fruit and in the cross-walls. High temperatures during a tomato’s ripening period seem to trigger the symptoms.

Problems with blossom end rot in tomatoes can be eliminated; keep the soil moist so the plant can take up calcium through the water stream. Calcium deficiency in developing tomato fruit causes blossom end rot It is worsened by dry soils that do not allow the plant’s roots to take up the calcium in the soil solution.

For more than 30 years, Toby Bost has been a resource to North Carolina gardeners and growers as an agricultural extension agent, a trainer for master gardeners, and an author. His books include The Successful Gardener Guide: North Carolina, North Carolina Gardener’s Guide, and The Carolina Gardener’s Guide. He can be reached through Our State magazine at gardening@ourstate.com

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