Why Are My Squash Leaves White?
White squash or courgette leaves may be naturally patterned, sun-scorched or covered with powdery mildew. The key is to look closely at the marks and at the plant's overall vigour. Powdery mildew is particularly common in late summer, but sensible watering, pruning and spacing can keep plants productive for longer.
First, identify whether the white is normal leaf colouring or a problem. Many squash and courgette varieties have silvery-white marbling along the veins from an early age. This pattern looks flat, stays in the leaf tissue and does not rub off. Powdery mildew, by contrast, appears as separate dusty patches that spread across the surface, often beginning on older leaves. Gently wipe a small patch with a finger: a powdery coating may smear or lift. Check both sides of leaves, stems and nearby plants. Also look for yellowing, curling or premature leaf death. Correct identification matters, because naturally mottled leaves need no treatment at all.
If the coating is powdery mildew, act promptly but do not panic. It commonly develops in warm, dry spells and during late summer when foliage is dense and plants are under strain. Remove the worst affected leaves using clean secateurs, cutting at the stalk base without damaging the crown. Put diseased material in household green waste if your local collection accepts it, rather than adding heavily affected leaves to a cool compost heap. Do not strip every leaf, as the plant needs foliage to feed developing fruit. Pick courgettes and squash regularly so the plant can direct energy to healthy leaves and existing fruit. Inspect remaining foliage twice weekly for new patches.
Improve airflow around the plant immediately. Clear weeds, pots and overhanging growth that trap humid, still air, and ensure sprawling stems are not piled into a dense mass. If plants were crowded at planting time, remove only the weakest or most diseased one rather than disturbing every root. Train stems gently along open ground or support them where the variety allows. Water the soil around the root zone, never the leaves, preferably in the morning so splashes dry quickly. A mulch of garden compost helps retain moisture and reduces soil splash. Avoid leaving foliage dry for long periods followed by sudden flooding, as uneven conditions make plants more vulnerable to stress.
Check whether the plant is getting enough water and food without being overindulged. Squash are thirsty, especially when fruiting, but constantly saturated ground can damage roots. Push a finger into the soil several centimetres from the stem: it should feel moist, not sodden or dusty. Water deeply when needed, allowing the surface to dry slightly between waterings. In pots, confirm that drainage holes are clear and that the container is large enough for the plant. Feed modestly according to the needs of fruiting plants, but do not overfeed nitrogen-rich materials, which encourage soft crowded foliage. Healthy, steadily growing plants usually tolerate mildew better than stressed ones.
Consider weather and site conditions, because mildew can be difficult to prevent completely in a typical British season. A sunny position with moving air is better than a damp, enclosed corner, yet strong dry wind can still leave plants thirsty. During prolonged dry weather, water consistently from a water butt or tap at the base. In a greenhouse or polytunnel, ventilate daily and avoid packing plants too closely together. Do not handle wet foliage unnecessarily, as this can spread other problems. If leaves are badly yellowed and mildew-covered late in the season, concentrate on harvesting sound fruit rather than trying to restore every leaf. Most established plants can still finish a useful crop.
Prevent a repeat next year by improving spacing and starting with vigorous plants. Sow in module trays or direct sow after the last spring frost, when the soil has warmed, then harden plants off carefully before planting. Mix well-rotted manure or garden compost into the bed beforehand and give each plant the spacing recommended for its type. Water newly planted squash at the base until roots establish. Rotate the planting area if possible, especially where mildew has been persistent. Choose a position with full sun and room for leaves to dry after rain. At season's end, clear old growth and compost only healthy material, then clean supports, pots and tools before reuse.
Frequently asked questions
Are white patches on courgette leaves always mildew?
No. Many courgettes naturally carry pale silver patterns, usually following the veins symmetrically. These markings are part of the leaf and do not look dusty. Powdery mildew forms irregular, flour-like patches that enlarge over time, often alongside yellowing or ageing leaves.
Will powdery mildew kill my squash plant?
It can weaken plants, especially if it arrives early and covers many leaves, but late-season mildew rarely means an immediate lost crop. Remove badly affected foliage, keep moisture even and harvest regularly. The aim is to preserve enough healthy leaf area for fruit to mature.
Should I water squash leaves to wash mildew off?
No. Wetting foliage does not cure powdery mildew and can encourage other leaf diseases or leave plants damp overnight. Water slowly onto the soil around the roots instead. Good spacing, morning watering and removal of heavily affected leaves are more useful practical measures.
General guidance for UK home gardens and allotments. Conditions vary by region, soil and season; the RHS or a local garden centre can give site-specific advice. Page updated 19 Sep 2026.