Why Are the Leaves of My Cucumber Plant Turning Yellow?

Cucumbers (Cucumis sativus) are favorites in the home vegetable garden due to their versatility and flavor. Unfortunately, cucumbers and other cucurbits are prone to a number of diseases and pests. Several of these cause the leaves to yellow; proper identification of the culprit is necessary to curb the problem.

Basket with cucumbers
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Healthy cucumbers produce large leaves, long vines and plump fruits.

General Care

Healthy plants are less susceptible to pests and diseases, although not immune. The ideal site for cucumbers is one with full sun and an extremely fertile, well-drained soil with plenty of organic matter. Planting too early can cause issues, as well, as drastic changes in temperature from day to night can cause issues with plant production.

Arthropod Pests

Whiteflies and spider mites can cause the leaves of your cucumber vines to yellow, albeit differently.

Whiteflies congregate on the undersides of leaves. The tops of leaves will become yellowish while a sooty mold will be underneath. When the cucumber vine is disturbed, these tiny, white insects disperse into a cloud of flying insects. Removing infested leaves can help curb their population and damage.

Spider mites, on the other hand, cause yellow stippling of the leaves that can eventually result in completely yellow, then bronze, leaves. You may notice web-like materials on the undersides of leaves.

In the case of either pest, spray a ready-to-use insecticidal soap over the entire plant, including the undersides of leaves. Insecticidal soaps must come in contact with the pests to be effective. Repeat every other week as needed; some insecticidal soaps are safe enough to use up to the day of harvest.

Cucumber beetle infestations create other problems that create yellowing in plants like fusarium wilt. The green-yellow spotted cucumber beetle has a black head with black spots, while the striped cucumber beetle has black stripes on its wings. Both beetles attack the cucumber's foliage, runners and immature fruit, leaving holes or a skeletonized appearance. Remove the beetles by hand-picking and dumping into a container of soapy water.

Fungal Diseases

A few fungal diseases of cucumber can result in yellow leaves.

Fusarium wilt, a soil-borne fungal disease which can be carried by cucumber beetles, causes plants to become stunted and yellow; the runners die slowly. Fungicides are not effective against this disease. Dig up and destroy any infected plants. In future years, rotate crops and select disease-resistant cucumber varieties.

Downy mildew, another fungal disease, is caused by poor air circulation, wet and humid conditions, too much nitrogen fertilizer, weeds in the garden and lack of proper sunlight. Symptoms first appear as irregularly shaped yellow spots on the upper part of leaves and a powdery mold on the undersides. Thin out leaves and plants to improve air circulation to avoid the problem, avoid overhead irrigation and remove infected plants to reduce the risk of spread. When removing infected plants from the garden, be sure not to spread the disease through handling healthy plants or through infected tools. Sterilize your tools by wiping them off with alcohol before using them on unaffected plants and wash your hands. Many varieties of cucumbers are resistant to this common disease.

Nutritional and Environmental Issues

Cucumbers are heavy feeders, meaning they may need more fertilizer or organic matter in the soil than other veggies. Pale, yellowing leaves may be a sign of nitrogen deficiency. If this is the case, apply 1 tablespoon of high-nitrogen fertilizer, such as 33-0-0, around each cucumber hill one week after blooms develop; repeat in three weeks. Water thoroughly after fertilizing.

Another issue may be low levels of sunlight reaching certain leaves. If the lower leaves are yellowing, consider clipping a few of the upper leaves on the vine to allow more sunlight and air circulation to this part of the plant.