My Cucumbers Are Turning Yellow on the Vine

Many vegetables make good home crops, with the cucumber plant (Cucumis sativus) near the top of the list. A vining plant that's usually productive and easy to grow, it's discouraging to see cucumbers develop but fail to turn green, or start out green and then become orange or yellow. This could be a natural occurrence, but might also signal a problem. Knowing the difference is the first step, followed by making some changes if yellowing is abnormal.

Although the majority of cucumbers are dark green, a few varieties are naturally yellow. A cultivar called "Lemon" is a good example. Like all cucumbers, it grows as an annual in all parts of the United States, but produces ball-shaped, 4-inch wide cucumbers that are bright yellow. Whether you start cucumber plants from seed or purchase seedlings at a nursery, check the label to identify the variety and the color of its cucumbers.

Leaving cucumbers on the vine too long can also cause them to turn from green to yellow or orange, and become overly large, a natural occurrence as the cucumber starts to fade. Leaving these older cucumbers on the vine can slow or stop production of new cucumbers, so it's best to remove them from the vine and discard them.

You might see yellowing of cucumbers on a plant if there's a problem with watering or fertilizing. The plant has shallow roots and needs adequate moisture, but if your season is especially wet or you water in excess, constantly soggy conditions can cause cucumbers to turn yellow. Cucumber plants do best when watered once weekly during dry spells, or whenever the top inch of soil feels dry to the touch.

Giving a cucumber plant adequate nutrition also helps ensure healthy green cucumbers; poor soil nutrients can cause yellowing of both leaves and cucumbers, which could also be small or stunted. To prevent this, mix 1 cup of a 10-10-10 granular fertilizer into each 10 feet of row at planting, then mix 1 tablespoon of fertilizer into the soil beside each plant when vines are 1 foot long.

Cucumber plants are susceptible to infection by the cucumber mosaic virus, an organism that causes mottled yellowing of the vine's leaves, along with small, stunted cucumbers that can also have yellow to white mottling. If this problem develops on a single plant, remove and destroy it, and watch other plants for similar signs.

The virus is spread by pests such as aphids, which are small greenish insects, so controlling these is important. Early in the season, cover the plants with floating row covers that you anchor to the ground, but remove these by early summer to allow pollination, which is essential for cucumber production. You can also control aphids by spraying plants until dripping wet with insecticidal soap, diluted at a rate of 5 tablespoons per gallon of water; spray on a windless day and repeat every two weeks as needed.

Several diseases can also cause yellow, wilted leaves, and small, sometimes yellow- or orange-mottled cucumbers. These include bacterial wilt, which is spread by striped cucumber beetles; control these by using floating row covers early, then hand-picking later in the season. Fungal problems can also cause poor plant health, and might lead to small, yellow or orange cucumbers. Fungal disorders are best controlled by spacing plants 8 to 15 inches apart and avoiding wetting the foliage when watering. Growing plants on trellises also improves air circulation, while keeping cucumbers off moist soil, which can cause them to yellow and soften.

If you experience disease problems in cucumbers, plant varieties bred for resistance -- such as "Marketmore 76" and "Fanfare" -- and rotate your planting spot every two years.