Your cucumber plants are likely infected with bacterial wilt. The bacteria that causes this disease lives in the gut of cucumber beetles. Striped cucumber beetles (yellowish green with 3 long black stripes down the back) or spotted cucumber beetles (yellowish green with 12 black spots on the back) may spread the disease. Adult beetles are about 1-4 inch long. They transmit the disease to the plant through feeding wounds. The only control for bacterial wilt is to control the beetles. A floating row cover can exclude the beetles, but you'll need to remove the cover for a few hours each day or hand-pollinate flowers to get fruit set. Plant wilt-resistant varieties if possible. You can also spray vines with an insecticide to control beetles.