Answer: If you feel you have given your plants all the conditions for good growing -- plenty of sunlight, water, and nutrients, and a rich soil with neutral pH -- then the wilting is likely caused by a disease called bacterial wilt. The disease is spread by cucumber beetles as they feed. To test for bacteria, cut a wilted stem near the base of the plant and squeeze out the sap -- look for a milky white substance. Touch the tip of a knife to the ooze, and if, as you withdraw the knife, it pulls out in a fine thread, the plants have a bacterial infection. There is no chemical control for this type of wilt; the best bet is to control the beetles, or better yet, grow resistant varieties.
Also, borers can invade the stems, causing plants to wilt. If borers are the culprit, you'll find small holes and sawdust-like frass on the vines near the soil line.
If your cucumber beetle problem is severe, you might consider growing your cucumbers under "all-season" lightweight fabric row covers to exclude the pest insects. If you choose to use row covers, also choose cucumber varieties that do not require pollination to set fruit. Otherwise, you'll have to pollinate the flowers with a small artist's paintbrush.
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