The broad flat leaves of a zucchini plant might wilt for several reasons. Cultural reasons, such as how much water the plant receives, are easy things to control. More complicated issues include insect infestations and bacterial disease. Wilting is nearly always a sign of a deeper problem. However, in the very hottest part of summer, leaves may wilt slightly in the increased heat.
Zucchini plants wilt when over- or underwatered. Water the soil to a depth of two to three feet once per week. In the hottest months, watering frequency can increase to three times per week. As with most plants, you can judge the watering level by checking the soil with your hands. Soggy soil means too much water -- which may lead to zucchini wilt. Watering the leaves and not the soil can also trigger fungal and bacterial diseases that cause wilting.
Bacterial wilt causes leaves to droop and turn floppy. The leaves start to shrivel and die. Cutting the leaf stem produces a sticky, cloudy sap instead of the clear sap of a healthy plant. Unfortunately, a zucchini plant infected with bacterial wilt can't be saved. Pull up the entire plant, including roots, and destroy. Composting the plant risks exposing other zucchini plants to the infection in future.
In zucchini and other summer squashes, Phytophthora capsici symptoms include wilting. This fungal disease spreads throughout the plant and eventually kills it. Alongside wilting, the crown may look soggy and brown in color. The fungus appears as a dustlike white film on the zucchini fruit and causes brown dents and sores. If you notice wilting alongside any of these symptoms, you can use a range of fungicides, including products containing dimethomorph, zoxamide, cymoxanil or cyazofamid.
Beetles and Borers
The squash vine borer beetle produces larvae that attack the insides of zucchini stems. A plant infected with squash vine borers wilts rapidly and all over the plant. The bug produces a yellow dust around the bottom of the stems. In isolated outbreaks in one or two plants, slicing the stem and pulling out the bug may be enough to control the problem. Cucumber beetles are also responsible for spreading bacterial wilt. Use carbaryl or malathion to treat serious cases of either squash vine borer or cucumber beetle.