Zucchini and other summer squash can be grown in vegetable gardens throughout the United States, providing a good crop each year. Although the squash are considered easy to grow, the plants can suffer damage when not cared for properly. Yellow leaves are a sign of a potential problem in many plants, zucchini included. In some cases, overwatering may cause the problem. However, that is only one potential cause.
Zucchini need about 1 inch of water per week, the University of Illinois Extension reports. When rainfall does not meet these needs, the best technique for watering the squash is to provide one deep water per week. If you grow zucchini in a sandy soil, water more frequently but use less water with each irrigation, the University of Minnesota Extension recommends.
Too Much Water
Yellow leaves on a zucchini plant can be a sign of many problems, including too much water. Typically, leaves yellow as a result of wet soil following heavy rains when temperatures are cool or cold, the University of Minnesota Extension reports. Leaves also may appear dark green or brown, and many may wilt or die back, when temperatures drop.
Cold weather and too much moisture are among many potential causes for yellow leaves on a zucchini plant. Powdery mildew can also cause yellowing, typically on the plant's oldest leaves. Downy mildew can also cause yellowing, although typically in spots and splotches. Pests including the squash vine borer and squash bugs can make leaves yellow and wilt. Finally, plant viruses such as squash mosaic can lead to yellowing of leaves.
Treatment and Care
How to care for a zucchini plant with yellowing leaves depends on the cause. When the problem develops following a cold and rainy spell, the plant's health may improve without your aid as temperatures warm and the soil dries out. If the yellow leaves are pest related, you can remove visible pests and their eggs and larvae by hand when checking the plants; alternatively, try an insecticide applied at the base of the vines, suggests the Iowa State University Extension. Herbicides can help control mildew and other plant diseases.