Wilting in plants occurs when dehydration causes the cells in the leaves and stems to lose water. This water pressure, called turgor pressure, creates rigidity in leaves and stems. With loss of turgor pressure, the plant is unable to support itself. Leaves wilt and then become dry and die.
It is normal for cucumbers (_Cucumis sativus_) to wilt in the afternoon during hot weather, but wilting in the morning, dry foliage or dying leaves suggest a potential problem. Abnormal wilting in these annual vegetables can have several causes.
Before considering pests and diseases that cause cucumbers to wilt, assess the growing conditions of your plants to evaluate what might be causing your cucumbers to lose water.
Cucumbers have a shallow root system and require 1 to 1 1/2 inches of water per week. Plants can wilt and dry out during periods of low to no rainfall. Rather than watering lightly every day, cucumbers should receive a single watering to 6 inches deep every five to seven days. Using mulch also helps to keep the soil around your plants moist.
Somewhat counterintuitively, overwatering can cause similar symptoms to dehydration. Too much water damages the plant's roots and affects its intake of oxygen and nutrients. Cucumbers that receive too much water can wilt and die, just like plants that don't receive enough.
Correcting a soil drainage problem midseason can be difficult. Adding compost or mulch around your plants can begin to provide the organic material needed to amend the soil in the long term.
For future plantings, add 3 to 4 inches of organic matter such as compost or rotted manure for every 6 inches of clay soil. For every 6 inches of sandy or loam soil, add 1 to 2 inches of organic matter. Mix these amendments into the soil before planting your cucumbers. Planting cucumbers on mounds also helps to drain excess moisture away from plant roots.
During cool, wet weather -- air temperatures below 50 degrees Fahrenheit or soil temperatures below 62 F -- cucumber plants may become yellow and wilted, and the edges of the leaves may turn brown. Floating row covers -- light fabric sheets that can cover an entire row of plants -- can raise temperatures several degrees around the plant and help cold-sensitive plants like cucumbers to withstand cold snaps. Once the cool spell passes, cucumbers must be uncovered to allow pollinating insects access to the flowers. Otherwise, your plants will not produce fruit.
Pest damage to the vines and stems of cucumber plants can prevent water from reaching the leaves and cause plants to wilt.
Squash bugs can affect all cucurbit plants, including cucumbers. Adults are elongated, shield-shaped insects about an inch long, but you will generally notice the immature nymphs or eggs on your plants first. Nymphs have pale green or bluish gray bodies with dark-colored legs, and they typically cluster together. Squash bugs generally lay their eggs on the undersides of the leaves in the fork where leaf veins meet. Eggs are copper-colored and are generally laid in orderly rows.
Squash bugs feed on the stems of cucumber plants, draining the plant of water and nutrients and causing the leaves to wilt. You may also observe yellow spots or leaves browning and dying on the plant. Mature plants can withstand mild infestations, but serious infestations or infestations on young plants can kill your cucumbers.
Begin monitoring your garden in early June for squash bug activity, as the eggs and nymphs are easier to spot and destroy than the adults. Insecticides are rarely required for squash bug infestations. Instead, inspect plants every few days and drown nymphs and adults in a jar of warm, soapy water. Crush any eggs that you find. You can trap squash bugs by placing boards in your garden overnight. Adult squash bugs will hide under the boards and can be collected and destroyed in the morning.
Remove weeds and dead plant material from the area around your cucumbers to eliminate hiding places for squash bugs. At the end of the season, remove old cucumber vines to minimize sites for squash bugs to overwinter.
Squash Vine Borers
Cucumbers can also be afflicted by the squash vine borer, although this pest generally prefers squash and pumpkins. Adult insects -- wasp-like moths distinguished by their gray wings and orange bodies -- lay their eggs at the base of cucumber vines. When the larvae emerge, they burrow into the vines to feed, disrupting the passage of water into the leaves, which can cause wilting. Leaves served by that vine will eventually die. You can diagnose a problem with squash vine borers by observing whether there are holes at the base of the vine; the hole is often surrounded by an orange or green sawdustlike material called frass.
Begin monitoring plants in late June for the presence of adult squash vine borers. You can place a yellow-colored pan filled with water near your plants; yellow attracts adult borers, and the moths will drown in the water. If you observe adults, cover your plants with floating row covers for two weeks.
Once borer larvae have burrowed into a vine, the pest cannot be controlled. Promptly remove any dead borer-infested vines from the garden, since after emerging from the vine, larvae will overwinter in the soil and could reinfest your plants the following year.
Bacterial wilt is a disease spread to cucumbers by the cucumber beetle, a small yellow beetle with black stripes or spots. These beetles spread the disease from plant to plant as they feed. Leaves turn pale green and begin to wilt during the day, but they initially recover at night. Leaves then begin to turn yellow or brown around the edges, and wilting becomes more severe and begins to progress down the vine. Cucumbers wilt and die quickly after infection begins.
There is no treatment for bacterial wilt. Remove and bury infected plants as quickly as possible to prevent the disease from spreading to other plants.
Phytophthora blight can cause wilting leaves in cucumbers, although this disease tends to affect squash and pumpkin more often. The disease is caused by a fungus that grows best in warm weather following heavy rains or very wet conditions. In addition to wilting leaves, affected plants show yellow leaves and rotten spots on the leaves, vines and fruit. Phytophthora blight can spread quickly and kill an entire cucumber crop.
Fungicides are not very effective against Phytophthora blight, and the best strategies against the disease are preventative. Remove any infected vines and fruit immediately from your garden and bury them. Do not compost them or leave them near your garden, as spores can spread. Choose a well-draining site for growing cucumbers, control weeds in the area, do not work in the garden during wet conditions and water your cucumbers at the base of the plant rather than overhead.
Fusarium wilt, which is caused by a fungus, often appears first as wilting during the day followed by recovery at night. Growth may be stunted, leaves may yellow and show rotted lesions and a pink fungus may grow on vines. Fusarium wilt cannot be treated, and because the fungus can endure in the soil for years, future crops in the area may continue to be affected. In areas affected by Fusarium wilt, you should plant only wilt-resistant cultivars.