This is the time of year, in many small towns, when you can't leave your car parked with the windows rolled down or you might come back to find your front seat full of zucchini or cucumbers! Both vegetables are members of the Cucurbit family, which also includes pumpkins, melons, squash and gourds. A common problem found in zucchini and cucumber is bitterness, which can be very frustrating to the gardener who finds the vegetables too bitter to eat!
All cucurbits produce a group of chemicals called cucurbitacins, which cause the vegetables to taste bitter, and the higher the concentration of cucurbitacin the more bitter the vegetable will taste. In commercially cultivated cucumbers and zucchini, they are normally in such low concentrations that they cannot be tasted. These chemicals provide other attributes to the cucurbits, such as the musky scent of cantaloupe.
Mild bitterness is fairly common in cucumbers resulting from higher levels of cucurbitacin triggered by environmental stress, like high temperatures, wide temperature swings or too little water. Uneven watering practices (too wet followed by too dry), low soil fertility and low soil pH are also possible stress factors. Over mature or improperly stored cucurbits may also develop a mild bitterness, which is often not severe enough to prevent gardeners from eating them.
However, occasionally a gardener will find a zucchini growing in their garden that is extremely bitter, as was the case recently for one Dodge country gardener. Eating these vegetables caused severe stomach cramps and diarrhea that lasted for several days. These symptoms are similar to twenty-two cases of human poisoning by bitter zucchini reported in Australia from 1981-1982, and in Alabama and California in 1984. The variety of zucchini grown in Dodge County was 'Black Beauty' and the variety implicated in Australia was 'Blackjack.' Very small amounts (3 grams) of the bitter zucchini were ingested.
Of 12 zucchini grown in the Dodge county garden only 1 plant produced very bitter fruit. Since all plants in the garden originated from one seed packet, were planted in the same location and received the same amount of water, simple environmental stresses could not be the culprit. Unlike cucumbers, extreme bitterness in zucchini and summer squash is not influenced by the environment, but is completely controlled genetically by one dominant gene.
How do these plants with extremely bitter fruits happen? Large amounts of cucurbitacins are still present in wild cucurbits, such as buffalo gourd, making them inedible to humans and most animals. Since cucurbits are pollinated by insects, especially bees, cross-pollination of cultivated plants by wild cucurbits (in the form of weeds growing near a seed production field) can cause problems if seeds from those plants are saved and planted in the garden the following year. Fruits from the parent plant would not be affected at all, and would not exhibit bitterness. Only the seeds resulting from the cross-pollination with a wild cucurbit or gourd would carry the gene for extreme bitterness. Plants grown from these seeds could express the dominant gene for fruit bitterness. Rarely, mutations in seed-production fields could also result in seeds that carry the dominant bitterness gene.
What should you do if you find extremely bitter zucchini in your garden? Well, you're unlucky since these plants are rare, but don't be discouraged. Don't eat the zucchini and don't give them to your neighbors! Discard them. Finally, don't save seed from plants that produced extremely bitter fruits. If you like to save your own seed, be sure to save fruit only from flowers that have been isolated to ensure that pollen came only from other domesticated, non-bitter squash and not from gourds or wild cucurbits.