Why are my squash plants wilting?

Start by looking at the plant again early the next morning. Big squash leaves lose water faster than roots can replace it on hot afternoons, and a plant in moist soil that droops at midday but stands up again by evening or morning is usually reacting to heat. A plant that is still wilted in the morning, with moist soil, has something blocking water inside the stem. Early bacterial wilt can also recover overnight, so recheck for a few days. Then work through the clues below, starting at the base of the main stem, where the most common cause leaves its mark.

Squash vine borer is the usual reason a healthy zucchini or pumpkin collapses in early summer. The adult is a wasp-like moth that lays eggs near the base of stems; the larvae, whitish with a brown head and up to about 1 inch long, tunnel inside and cut off the plant's water. Look for holes near the soil line with green-to-orange, sawdust-like frass pushed out of them. One runner or the whole plant may wilt suddenly. Butternut squash tends to be less affected than zucchini, acorn squash and pumpkins.

Squash bugs cause a slower, patchier decline. Adults are flat, gray-brown to nearly black and about 5/8 inch long; nymphs are smaller and gray, often in groups. Both suck sap, leaving yellow spots that turn brown, and heavy feeding can wilt and kill whole runners. Turn leaves over and look for copper-colored egg clusters, often in the V where two veins meet. If you find bugs but no frass and no holes in the stem, squash bugs are the more likely cause, and they hide under boards and mulch during the day.

Bacterial wilt is less common in squash than in cucumbers and melons, but it happens. Leaves turn dull green and wilt during the day, recover at night at first, then the wilt moves down the vine until it dies. Striped and spotted cucumber beetles spread it. To check, cut a wilted stem near the base, press the cut ends together and pull them slowly apart; sticky strands of ooze point to bacterial wilt. University of Minnesota Extension also notes that cold, wet spells, with soil below about 62°F, can wilt squash and turn leaf edges olive green to brown.

Act on what you find. For vine borers, slit the stem lengthwise above the entry hole with a clean knife, remove the larva, and mound moist soil over the wound and over vine nodes so they can root. For squash bugs, crush egg clusters, hand-pick bugs in the morning, and trap them under boards laid beside the plants; insecticidal soap or neem oil can help on young nymphs if sprayed in the evening, away from open flowers, and not above 90°F. Pull plants with bacterial wilt; they can't be cured. For heat, water deeply in the morning and mulch.

Next season, cover young plants with row cover from transplanting until the first female flowers open, then remove it so bees can pollinate. Wrapping the bottom 6 inches of stems in foil can block borer egg-laying. A second, later planting can miss the main borer flight; your extension office can tell you when moths are active in your area. Clear spent vines right after harvest to remove squash bug hiding places, and keep thick mulch away from the stems.

Before using any product, read the whole label. It must list the crop you’re treating, and its directions on amount, timing, protective gear, bees and days before harvest are legal requirements. If this page and the label disagree, follow the label.