Why do my plant's leaves have bleached white patches?

Telling sun scorch from disease, chemical damage and frost, and the pattern that identifies each in a couple of minutes.

Difficulty: beginner
Time: 15 min
Read: 3 min

Short answer

Bleached, papery patches on the upper surface of leaves that face the sun are sun scorch. They appear suddenly after a bright day, often on a plant recently moved outdoors or watered in full sun. Scorch does not spread — if the patches are increasing, it is something else.

Leaf tissue can only handle so much light. A plant that has been indoors, in a greenhouse or in shade has leaves adapted to low light, and a single bright day can destroy them in hours.

Step by step

Check whether it is spreading.Scorch is done the moment it happens and does not enlarge afterwards. Patches that grow, spread to new leaves or develop a coloured margin are fungal or bacterial.
Check which surface and which side.Scorch is on the upper surface, and on the side facing the sun or the glass. Disease appears on both surfaces and in no relation to sun direction.
Think about the last few days.Recently moved outdoors, recently moved to a brighter window, first hot day of the year, or a greenhouse without shading. All point to scorch.
Rule out water droplets in sun.Watering in full sun on hairy or soft leaves can leave discrete round scorch spots. Water at the roots, and in the morning or evening.
Rule out weedkiller drift.Distorted, cupped or twisted growth alongside bleaching, especially on tomatoes, points to herbicide contamination rather than sun. Check what was sprayed nearby, or what was in any manure used.
Rule out frost.Frost damage appears overnight, affects the most exposed growth and often turns tissue translucent then brown, rather than white and papery.
Harden plants off properly.Two weeks of gradually increasing outdoor exposure before a plant goes out permanently. Skipping this is the single commonest cause of scorch.
Shade greenhouse plants in summer.Shade paint, netting or blinds from May to September. Glass concentrates light and heat and scorches plants that would be fine outside.
Remove the worst leaves and leave the rest.Badly damaged leaves can come off. Partially damaged ones still photosynthesise, and a plant that has just lost leaf area needs the ones it has.
Keep it watered while it recovers.A scorched plant has reduced leaf area and often damaged roots too if it dried out. Consistent watering matters more than feeding.

Tips

Scorch does not spread. That single test separates it from almost everything else.
Two weeks of hardening off prevents most of it.
Distorted, twisted growth with the bleaching means chemical damage, not sun.

Common mistakes

Moving a scorched plant into deep shade to recover — It swaps one stress for another and slows recovery. Bright indirect light and consistent watering is what it needs.
Spraying a fungicide on sun scorch — There is no organism involved. The damage is physical and already complete, and the spray itself can add to the stress.

If it doesn't work

Papery white patches on the sun side, not spreading

Cause: Sun scorch — Fix: Harden off properly, shade under glass, and keep it watered.

Round bleached spots scattered over the leaf

Cause: Water droplets acting as lenses in strong sun — Fix: Water at the roots, early or late in the day.

Bleaching with twisted, cupped growth

Cause: Herbicide contamination — Fix: Identify the source — spray drift, or contaminated manure or compost. Affected plants often do not recover.

Patches enlarging with a yellow halo

Cause: Fungal or bacterial leaf spot — Fix: Remove affected leaves, improve airflow, avoid overhead watering.

Questions people ask

Will scorched leaves recover?

No. The tissue is dead. The plant recovers by producing new leaves, so the test of whether you have fixed the cause is whether the new growth comes out clean.

Can plants be scorched through a window?

Yes, and it is common. Glass concentrates heat, and a houseplant moved to a south-facing sill in spring can be badly scorched within a day.

How do I stop it happening again?

Harden plants off over two weeks before they go outside, shade greenhouse glass from May, and water at the roots rather than over the foliage in bright sun.