Agriculture and Rural Development of Alberta explains that yellow leaves on a cucumber plant may indicate exposure to warm, humid conditions with low light levels in the lower canopy of the plants. Yellow leaves may also indicate nitrogen deficiency, while yellow or necrotic tissue on the outside perimeter of the leaf may be caused by a rapid drop in temperature when day turns to night.
The department also explains that the yellow color may mean that the cucumber plant gets water too close to sunset, which creates high root pressure. Jackie Carroll explains to SFGate that potato leafhoppers feed upon cucumber plants by sucking out the sap from the seeds while leaving a toxin that turns the leaves yellow. She states that small pests, including aphids, whiteflies and spider mites, suck the sap from the leaves, which causes curling, yellowing and leaf distortion.
Several bacterial and fungal diseases, such as verticillium wilt and symptoms caused by the mosaic virus, may also lead to the yellowing of cucumber plants. Carroll further explains that nitrogen deficiency not only causes the plant to turn yellow, but also leads to stunted growth. He mentions that these plants may die if such conditions become severe enough.