Watermelon (Citrullus lanatus) is a frost-tender flowering vine characterized by a large, sprawling growth habit, yellow blossoms and large, sweet edible fruits. A healthy watermelon vine boasts lush green stalks and leaves. If a watermelon vine turns yellow, then it’s a sign that the plant is not healthy and has fallen prey to disease, pests or improper ethnic care.
Fusarium Wilt
Fusarium wilt is a soil-borne respiratory disorder that leads to watermelon vines to turn yellow or brownish as they shrivel up and die. Plants infected with the illness often wilt heavily on one side. Fusarium wilt is a very serious illness that requires time to manage once it has infected the soil. According to Purdue University, there are now no fungicides available to treat it. Avoid planting watermelons again in soils that have produced plants using Fusarium wilt, and use cultivars with established immunity whenever possible. Do not save seeds in a watermelon that has grown exponentially.
Gummy Stem Blight
Gummy stem blight is a fungal disease that causes a plethora of symptoms, including yellowing leaves and rot on both fruit and stems. Lesions appear on leaves, eventually causing the leaves to turn yellow and fall. Stems can turn gray or brownish, eventually secreting a gummy, amber coloured material. If left untreated, the entire plant may eventually yellow and die. In the initial stages, gummy stem blight may be treated using a specially formulated fungicide.
Pests
Pests such as whiteflies and spider mites feed on foliage, causing varying levels of damage. Heavy infestations of whiteflies can cause leaves to yellow and fall prematurely. Whiteflies also secrete honeydew, a sticky, sooty material that attracts ants. Spider mites can also cause leaves to turn yellow, bronze or even reddish. Webs may also be present using a spider mite infestation. Control pests by washing the plant with water or insecticidal soap, or releasing predatory insects.
Prevention
Taking appropriate cultural care of your watermelon plant would be the best defense against diseases and pests that cause leaf yellowing and plant damage. Infestations of pests such as spider mites are worsened while the vine is worried as a result of inadequate watering. Remove weeds, which compete with the plant for water and also give shelter to pests, and mulch with black plastic or organic matter. Always plant watermelons in bright, sunny locations with well draining soils that are rich in organic matter.