Growing cucurbits can be really frustrating for homeowners. Often our melon, cucumber, and squash plants are doing great but then they wilt overnight!
The most common cause of wilting on melon and cucumber is the cucurbit bacterial wilt. This is a bacterial disease that's transmitted by the striped and spotted cucumber beetles. The first symptoms of wilt are droopy leaves on a single vine or entire plant. This is easy to confuse with water or heat stress, except that infected plants do not recover and eventually die.
There's an easy way to determine if your plants are wilting due to bacterial wilt. First, check that the wilting vine is still attached to the plant. If it is, use a pocket knife to cut the vine and very slowly pull the ends apart. If you see sticky strands or bacterial ooze, there's a good chance this plant is infected with bacterial wilt. You can also cut the stem and touch one of the ends with the tip of your finger, slowly draw it away and observe if there is any strands are formed (see picture).
For more information view the Purdue University Pest Management Program video online.
Squash can also become infected with bacterial wilt. However, it's easy to confuse this disease with squash vine borer damage. The squash vine borer is a wrinkled, white caterpillar that feeds inside squash plant stems (usually near the soil line) from June through August. If your zucchini or pumpkin wilted, first check the base of the plant and look for small holes and frass (insect poop). If you cut the stem in half, you might be able to find one or more caterpillars inside the stem. If no borer or borer damage is found, this plant might be infected with bacterial wilt.
Watermelons do not get infected with bacterial wilt or squash vine borer. If watermelons are wilting, this might indicate that there's a fungal problem coming from the soil. Fusarium wilt of watermelon causes plants to wilt, and it may begin in one or more vines. Pull out a plant and look for any browning or discoloration at the base or on the roots. Using a sharp blade, cut the stem lengthwise and look for brown streaks on the inner tissues. Fusarium and other fungal infections typically cause browning of vascular tissues.
Bacterial ooze from wilt-infested plant.