Start with a rub and a flip. White, powdery spots that smudge off on your finger, starting as small circles and spreading, are powdery mildew. Hundreds of tiny pale dots, like fine stippling, with fine webbing on the leaf undersides, are spider mites. Silvery or bleached streaks with tiny black, varnish-like specks are thrips. Larger bleached or papery patches on the sunniest leaves, with no powder and no insects, point to sunscald or spray burn. Silver-gray patches running evenly along the veins of squash and zucchini leaves are often a normal trait of the variety.
Powdery mildew is the most common cause. It grows on the leaf surface and, unlike most leaf diseases, does not need wet leaves; moderate temperatures (60 to 80°F) and shade favor it. Each species attacks specific plants, so mildew on squash won't spread to your tomatoes. It is common on squash, cucumbers, melons, and peas. Remove the worst leaves, plants for airflow, and grow resistant varieties next time. A potassium bicarbonate or sulfur product labelled for powdery mildew can slow spread at the first spots. Don't use sulfur at about 85°F or above or within 2 weeks of an oil spray, and check that the label lists your cr.
Spider mites are tiny and easiest to see with a hand lens, or by tapping a leaf over white paper. Damage starts as pale stippling and can turn leaves bronze; it is worst in hot, dusty weather on water-stressed plants. Spray leaf undersides with water daily, keep plants watered, and keep dust down. Thrips are slender insects about 1/20 inch long, found on flowers and leaf undersides; they scar the leaf surface, leaving silvery patches with black specks of excrement. For either pest, insecticidal soap on the undersides can help; don't spray above 90°F or on drought-stressed plants, and test a few leaves first.
Sunscald and spray burn don't spread. moved outdoors without , or leaves suddenly exposed to strong sun, can develop bleached white or tan patches on the most exposed leaves. Soaps, oils, and sulfur sprayed in heat or on stressed plants can leave pale or brown spots in the pattern of the spray droplets. New leaves come in clean in both cases. On squash, silvering that spreads over the whole leaf, rather than staying in an even pattern along the veins, points to spider mites or, in warm regions, silverleaf whitefly.
Prevention depends on the cause. Check leaf undersides weekly once weather turns hot, water deeply so plants don't go into drought stress, and keep paths damp or to cut dust. Plant mildew-resistant varieties of squash, cucumbers, melons, and peas, and space plants so air moves through them. Harden off transplants over 7 to 10 days before setting them out. Spray only in the cool of the morning or evening, and test any new product on a few leaves first.
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.
- ZucchiniThe summer squash that turns a garden into a produce stand — if you can keep the vine borers away.
- CucumberA thirsty vine that wants warm soil, steady water, and something to climb.
- PumpkinA sprawling vine that needs room to breathe and soil warm enough to germinate without rotting.
- CantaloupeThe summer melon that rewards steady heat and almost nothing else.
- PeaA cool-season crop that rewards early sowing and quits when summer arrives.
- Powdery MildewWhite powdery spots on leaves that spread into a floury coating — common when warm, dry days follow cool nights.
- Spider MiteStippled, bronzed leaves with fine webbing on undersides; damage intensifies in hot, dry weather.
- ThripsSilver-streaked or stippled leaves with distorted growth; tiny slender insects visible on flowers and undersides.
- Early BlightDark spots with concentric rings on lower leaves that yellow and drop, working up the plant.
- Leaf MinerPale winding tunnels or blotches inside leaves, with tiny fly larvae visible inside the mine.
- Why do my squash leaves look silvery or bleached?Silvery patches along the veins of squash leaves are often a normal, harmless trait of the variety; silvering that spreads over the whole leaf can point to spider mites or silverleaf whitefly, and white dusty spots that rub off are powdery mildew.
- Why do my squash, cucumber, or melon leaves have a white powdery coating on them?White powdery coating on cucurbit leaves is powdery mildew — a fungal disease that spreads in warm, dry conditions with moderate humidity and typically appears in mid to late season.