This disease occurs over most of the United States. It affects cucumbers and muskmelons and, less commonly, pumpkins and squash. Individual leaves wilt during the heat of the day, but recover overnight initially. As the disease progresses, part or all of the vine wilts and dies. To test for the disease, cut a wilted stem near the base of the plant. Squeeze the sap out of the stem. If it is sticky and white and forms a thread when the tip of a knife is touched to it and drawn away, bacterial wilt is probably present. (This test works best with cucumbers.) The disease overwinters in the gut of cucumber beetles, and is spread to plants as the beetles feed.
Controlling cucumber beetles is the key to prevention. Look for varieties of cucumbers and squash that are resistant to cucumber beetles. Cover young plants with floating row covers. Knock, shake, or hand pick beetles off plants and out of flowers, and clean up plant debris to reduce the number of overwintering adults. Dust plants and flowers with a pyrethrum insecticide as directed by the product label.
Photo courtesy of Howard F. Schwartz, Colorado State University, Bugwood.org