AMES, Iowa ― Iowa State University Extension and Outreach horticulturists share information about bitter cucumbers, tomatoes that aren’t setting fruit and why red raspberries may be partly white. To have additional plant and garden questions answered, contact the ISU Hortline at 515-294-3108 or firstname.lastname@example.org.
Hot, dry conditions are usually responsible for bitterness in cucumbers in Iowa. Bitterness in cucumbers is caused by the compounds cucurbitacin B and cucurbitacin C. The cucurbitacins are normally found in the leaves, stems and roots of the cucumber plant. The cucurbitacins spread from the vegetative parts of the plant into the cucumber fruit when plants are under stress.
Bitterness does not accumulate uniformly in the cucumber fruit. The cucurbitacins are usually concentrated at the stem end of the fruit and just under the skin of the cucumber. Bitter cucumbers can sometimes be salvaged by cutting off the stem end and peeling the remainder of the fruit.
Cucumber varieties differ in their tendency to be bitter. Varieties that usually experience few problems with bitter fruit include Sweet Slice, Sweet Success, and Marketmore 76.
Extreme temperatures and dry conditions result in poor pollination and cause the flowers to drop from the plant without setting fruit. Blossom drop on tomatoes occurs when daytime high temperatures are above 90 F or low nighttime temperatures are above 75 F. Blossom drop is most common on the large-fruited tomato varieties. Cherry-type tomatoes set fruit over a wider temperature range. Fruit set also may be poor on peppers, eggplant and green beans in hot, dry weather.
Gardeners must wait for favorable temperatures for greater fruit set. Optimum fruit set on tomatoes typically occurs when daytime high temperatures range from 70 to 85 F. Water plants deeply once a week during dry weather. Hormone (“Blossom Set”) sprays do not prevent blossom drop due to high temperatures.
A raspberry fruit (berry) is composed of more than 50 drupelets. The white colored drupelets are likely due to sunscald or white drupelet disorder. Sunscald and white drupelet disorder are physiological disorders caused by sun exposure (solar injury) and excessive temperatures. Berries with full exposure to direct afternoon sun are most susceptible to sunscald and/or white drupelet disorder. However, high temperatures also appear to be involved, as berries shaded by the leaf canopy also may develop white drupelets.
Watering the raspberry planting on a weekly basis (in hot, dry weather) may reduce the incidence of sunscald and/or white drupelet disorder.
Raspberries with white drupelets aren’t very attractive, but are edible. Berries with brown (rotting) drupelets should be picked and discarded.