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Zucchini leaves curl most often because of sap-sucking pests like aphids and squash bugs, mosaic virus, powdery mildew, heat and water stress, or herbicide drift. Diagnose it by first checking the leaf undersides for insects and their eggs, then looking for the mottled light-and-dark pattern of mosaic virus or the white dusty coating of powdery mildew, and finally considering weather and any nearby lawn spraying. Fix it by clearing pests, watering consistently, improving airflow, and keeping zucchini away from weed killers. Fast-growing zucchini often outgrows minor damage.
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Read the curling zucchini leaves first
Zucchini leaves are big and naturally patterned, so look closely before deciding. This table sorts the likely causes.
| What you see | Pattern | Likely cause |
|---|---|---|
| New leaves cupped and distorted, sticky residue | Insects clustered on undersides | Aphids |
| Curling with wilting, bronze egg clusters underneath | Shield-shaped bugs at the base | Squash bugs |
| Leaves mottled light and dark green, curled, puckered | Spreads, growth stunted | Mosaic virus |
| White dusty coating, then curling and yellowing | Warm days, humid nights, crowded plants | Powdery mildew |
| Leaves curl down and wilt in the sun | Worst in hot afternoons, recovers by evening | Heat and water stress |
| Strap-like, crinkled, distorted new growth | Sudden, plant-wide | Herbicide drift |
Aphids and sap-sucking pests
Aphids are a leading cause of curling zucchini leaves. They gather in colonies on the undersides of leaves and on tender new growth, piercing the tissue to feed, which distorts and cups the developing leaves. Their giveaways are the small green, black, or white insects themselves, sticky honeydew, and sometimes black sooty mold growing on that honeydew. Critically, aphids transmit mosaic viruses as they feed, so clearing them protects the plant from a far worse problem. Blast them off with a firm spray of water, then treat the leaf undersides with insecticidal soap, repeating every 3 to 5 days until they are gone. Encourage ladybugs and lacewings, which prey heavily on aphids, and avoid over-fertilizing with nitrogen, which produces the soft growth aphids love.
Squash bugs
Squash bugs are a signature zucchini pest and a common cause of curling, wilting leaves. The adults are flat, shield-shaped, gray-brown bugs that hide at the base of the plant and under leaves, and they lay distinctive bronze-colored egg clusters on the leaf undersides, usually in the angle of the veins. As they feed they inject a toxin that causes leaves to curl, wilt, yellow, and eventually die back, and a heavy infestation can kill a plant. Check the undersides and the crown regularly. Crush the egg clusters, hand-pick the adults into soapy water in the morning when they are sluggish, and remove garden debris where they overwinter. Row covers early in the season keep them off young plants until flowering, when covers must come off for pollination.
Mosaic virus
Several mosaic viruses infect zucchini and cause leaves to curl, pucker, and take on a distinctive mottled pattern of light and dark green, often with stunted growth and misshapen fruit. Aphids and cucumber beetles spread these viruses as they feed, which is why pest control is the first line of defense. Mosaic virus is easy to confuse with a nutrient issue, but the mottling is patchy and irregular rather than the uniform pale color of a deficiency, and it comes with distortion and stunting. There is no cure. Remove and destroy infected plants promptly to stop the virus spreading to healthy squash, cucumbers and melons nearby, disinfect your tools and hands, and control the aphids and beetles that carry it. Resistant varieties are the best long-term protection.
Powdery mildew
Powdery mildew is nearly universal on zucchini by late summer. It starts as a white, dusty coating on the leaf surfaces and, as it advances, the affected leaves curl, yellow, dry out, and die. It thrives in warm days paired with humid nights and in crowded plantings where air does not move. Improve spacing so air circulates, water at the base rather than overhead to keep foliage dry, and remove the worst-affected leaves. Resistant varieties make a real difference in humid regions. While mildew rarely kills an established plant outright, it weakens it and shortens the productive season, so acting early — at the first white patches — keeps the plant producing longer. Do not confuse the natural silvery-white blotches along zucchini veins with the dusty mildew coating.
Heat and water stress that curls zucchini leaves
Zucchini has large leaves that lose a lot of moisture, so on hot afternoons above 90F the plant often curls and wilts dramatically to conserve water, recovering by evening. This is a temporary reflex, not damage, provided the soil is moist. The mistake is to see midday wilt and overwater — check the soil first. Zucchini wants deep, consistent moisture: water when the top inch of soil is dry, aiming to keep the root zone evenly damp, and mulch to hold moisture and keep roots cooler. In a heat wave, light afternoon shade helps. Wide swings between drought and flood stress the plant and worsen curling, so steady watering is the goal. If the plant wilts and does not recover overnight even with moist soil, look instead at squash bugs or squash vine borer.
Herbicide drift
Squash are sensitive to growth-regulator herbicides, and drift is an overlooked cause of curling. Products containing 2,4-D or dicamba cause zucchini leaves to curl, cup, and crinkle, with distorted, strap-like new growth. The exposure can drift on the breeze from a neighbor spraying their lawn, or come in hidden in contaminated compost, mulch, manure, or straw made from herbicide-treated hay — the chemical persists through composting and is a frequent surprise source. There is no treatment. Mild exposure may grow out over a few weeks as clean new leaves emerge, while heavy exposure stunts or kills the plant. Prevent it by keeping the garden away from lawn-spraying zones and sourcing compost, manure and straw only from herbicide-free suppliers.
Preventing zucchini leaf curl
- Check undersides and the crown weekly — catch aphids and squash bug eggs before they distort growth.
- Control the insect vectors — aphids and cucumber beetles spread the mosaic virus that curls leaves.
- Space plants for airflow — and water at the base to slow powdery mildew.
- Water deeply and evenly, mulch — steady moisture prevents heat curl; check soil before overwatering midday wilt.
- Keep zucchini away from lawn chemicals — use only herbicide-free compost, manure and straw.
- Remove and destroy virus-infected plants — and choose resistant varieties for mildew and mosaic.
Related: why is my zucchini turning yellow and why are my squash plants turning yellow, plus the Seeds & Vegetable Growing hub and how we rank.