*This is an excerpt from The Alabama Vegetable Gardener, ANR-0479.

Nearly every vegetable grown in the home garden has its quirks and problems. A problem that is sometimes associated with cucumbers and, on very rare occasions squash, is bitter flavored fruit. It can be downright frustrating to have a seemingly perfect cucumber, only to bite into it and find it tastes so bitter that it can’t be eaten!

Cucurbitacin

The bitterness that is sometimes noticed in cucumbers is because of a substance known as cucurbitacin. Cucurbitacin is a toxin that is thought to play a role in cucumber disease resistance. The higher the concentration of this substance, the more intense the bitterness. A very high concentration of cucurbitacin can cause a memorable stomachache. Occurrence of cucurbitacin is a genetically controlled phenomenon, but it is usually triggered by some environmental stress on the plant.

The Culprit: Environmental Problems

There are several environmental factors that can cause stress on the plant. Wide temperature fluctuations for several days and nights, or possibly just very high daytime temperatures for a period of time, can contribute to bitterness. But, temperatures may be less important than some other factors. Low soil moisture (or uneven watering—very wet, then very dry), low fertility, and low pH are all possible contributors to stress that can lead to bitterness. Also, stress imposed by insect or disease infestations can lead to bitter fruit.

Bitter-Free

Selection of burpless or bitter-free slicer-type cucumber varieties such as Sweet Success, Sweet Slice, Marketmore, or Burpless can help reduce chances of coming up with bitter fruit. But, perhaps a more important way to reduce the chance of bitterness is to mulch around cucumber plants, fertilize and lime properly, provide 1 inch of water a week (depending on soil type), and control insects and diseases. If corrective measures are carried out after bitterness is noticed, the fruit that comes along later will sometimes have the normal cucumber flavor.

Squash Is Different

Bitterness in squash is a little different from that in cucumber. In squash the bitter flavor is because of genetic factors that are not influenced by environmental stress. Squash bitterness is much rarer than cucumber bitterness and has only been reported in zucchini and yellow straightneck varieties. Bitter squash can cause extreme stomach cramps and diarrhea, so if the first bite is bitter, don’t eat the rest.

Once bitter fruit is found, the subsequent fruit from that squash plant will be bitter. There is much that is unclear about this phenomenon in squash, and it is an area in which plant breeders and other researchers are interested.

For more information, see other excerpts from The Alabama Vegetable Gardener, ANR-0479.

Kerry Smith, Extension Home Horticulture Associate; Ayanava Majumdar, Extension Entomologist; Charles Mitchell, Extension Agronomist, Professor, Agronomy and Soils; John Everest, Visiting Professor, Agronomy and Soils; Edward Sikora, Extension Plant Pathologist, Professor, Entomology and Plant Pathology; Joseph Kemble, Extension Specialist, Professor, Horticulture; all with Auburn University; and Rufina Ward, Research Entomologist, Natural Resources and Environmental Sciences, Alabama A&M University.

Reviewed October 2021, The Alabama Vegetable Gardener, ANR-0479