Why Are My Zucchini Leaves Curling?

Curling leaves on a zucchini can mean anything from a passing hot afternoon to a virus that ends the plant, so the symptom alone tells you little. What narrows it down is the detail around the curl: whether there are insects underneath, whether the new growth is distorted, and whether the leaves are mottled. Read those clues and the cause usually reveals itself. Let me take you through the likely suspects in order.

Aphids: look here first

The most common reason zucchini leaves cup and curl is aphids feeding on the soft new growth. These tiny sap-suckers cluster on the undersides of young leaves, and as they drain the sap the leaves pucker, curl and distort, often turning slightly yellow and going sticky with honeydew. Turn a curled leaf over: if you find clusters of small green, black or grey insects, you have your answer. A firm jet of water to dislodge them, followed by insecticidal soap on the undersides, clears them, and the new leaves emerge flat. Ants marching up the plant are a giveaway that aphids are present.

Mosaic virus: the serious one

If the curling comes with a mottled, blotchy yellow-and-green pattern on the leaves, puckering and distortion, stunted growth, and later bumpy or mottled fruit, suspect a mosaic virus. Viruses are spread by aphids as they feed, which is another reason to keep aphids in check. There is no cure for an infected plant — it should be removed and destroyed so it cannot become a source for the rest of your squash, and prevention rests on aphid control and resistant varieties. The distinguishing feature is the mosaic mottling alongside the curl.

Heat and water stress

Zucchini have broad, thin leaves that lose moisture quickly, and in fierce heat or dry soil they may cup or curl upward to reduce the surface exposed to the sun. If the curling appears during hot weather, the leaves are otherwise healthy, and there are no pests or mottling, this is simply a protective stress response. Keep the soil consistently moist, mulch generously to buffer it, and offer light afternoon shade during a heatwave, and the plant settles without harm.

Herbicide damage

If the new growth is narrow, strappy, twisted or fan-shaped and badly distorted, with no pests present, suspect herbicide exposure. Zucchini are sensitive to growth-regulator weedkillers, which can drift from a neighbour's lawn treatment or arrive hidden in straw, manure or compost made from sprayed material. Nothing reverses it; stop using any suspect mulch or compost, water well to dilute, and wait. Lightly affected plants often grow out of it and still crop.

Reading the curl

Put the clues together: curling young leaves with insects underneath equals aphids — wash and treat; curling with mottled mosaic colouring and distorted fruit equals virus — remove the plant; curling in heat with healthy, pest-free leaves equals stress — steady the water and shade; twisted strappy new growth with no pests equals herbicide — remove the source and wait. Check the undersides first, because aphids are the answer far more often than anything serious.