Cucumber leaves often turn yellow because they are damaged by the potato leafhopper. The small leafhoppers pull sap from cucumber leaves and inject a toxin into the leaves, causing them to turn yellow or brown and appear burned.
Leafhoppers also feed on weeds around cucumber plants. Regular weeding helps control the population. Leafhoppers can be sprayed off with a garden hose early in the day before they become active. Significant leafhopper damage may require treatment with an insecticide applied to both sides of the cucumber leaves. If cucumber plants are damaged, it is important to check nearby plants because leafhoppers also feed on other vegetables, fruits and flowers.