Mulch around your plants now, if you have not already done so. This will help to retain moisture during the dry months to come. But be sure not to mulch right up to the stems or trunks as it may cause stem rot. Try to water melon varieties and tomatoes from the ground, avoid the leaves. This will help to prevent mildew forming and killing the plants. I have been told that spraying with a diluted milk mixture will help prevent mildew, but I have not tried it myself.