How do you water them? Cucumbers don't like to be watered from the top. (Spraying the water and letting it fall.) They like to be watered low, right on top of the soil. They also don't like to be over-watered. Do your leaves have lighter spots, almost like the sun is bleaching them? If so, they are likely being watered too much, and from above. The soil should be moist, but not wet. Have you noticed any brown bugs around, or other bugs? Stink bugs, and other bugs are killing crops all over the place. If you have stink bugs, go to a natural store and see if you can find Diatomaceous Earth. Squirt it on the leaves with a turkey baster. I don't know how hot it is where you are, but here it is too hot for cucumber plants. We have to plant them in March and harvest all the cukes before the hot summer. Place mulch or compost around the plants, to keep them cool.