Cucumbers are a warm-weather crop that can be successfully grown in a greenhouse or outdoors in the home garden. Cucumbers can fall victim to myriad pests and diseases that can cause the plants to die before producing any fruit. If you your cucumber plant has turned yellow and stopped growing, diagnosing the problem and treating it correctly will help your plant recover and prevent the problem from spreading to throughout your garden.
A nitrogen deficiency in your soil causes your cucumber plant's leaves to turn first pale and then yellow. A nitrogen deficiency also stunts the growth of your plant, and any new growth will be undersized. Remedy the deficiency by applying a nitrogen-rich fertiliser to your cucumber plant immediately and at regular intervals thereafter.
Cucumber Mosaic Virus
Cucumber mosaic virus is transmitted to cucumber and other squash plants when aphids and cucumber beetles feed off of plants. Symptoms of the disease include yellow, spotted, down-turned leaves; smaller leaf size; and stunted plant growth. Fruit from an infected cucumber plant will likely be discoloured and have a bitter flavour. To prevent cucumber mosaic virus, control aphids and beetles with insecticidal soap or by hosing off your cucumber plants each night with a strong stream of water. Plant cucumber varieties that have been bred to resist the disease, and remove any weeds from around the garden that could transmit the disease. Once a plant is infected with the virus, there is no treatment; such a plant must be removed to prevent the virus from spreading to other plants.
According to the Washington State University Hortsense website, spider mites are small eight-legged bugs that feed on the underside of leaves. A spider mite infestation can leave your cucumber plant with yellow-speckled leaves. If an infestation is bad enough, leaves turn completely yellow and hinder the productivity of your plant. To identify a spider mite infestation, look at the underside of your leaves for tiny yellow, green or red bugs. Also look for webbing on the underside of the leaves, a clear indicator of spider mites. Spray infected plants with a strong stream of water to remove the mites. You can also treat affected plants with insecticidal soap.
Angular Leaf Spot
If you tend to water your cucumber plants from above, you could have a problem with angular leaf spot. Angular leaf spot occurs when bacteria spread to your cucumber plant through splashing water. Signs of angular leaf rot include a yellow to brown, angular-shaped speckling on leaves, stems, vines and fruit. To avoid the bacteria, always water your cucumber plant at the roots, rotate your crops so that the same plant isn't placed in the same plot for at least two years, and wash your hands with soap and water before and after handling plants. Fixed copper products can also reduce the spread of the disease.