Onion: lower leaves turning yellow

What it is likely to be on onion

1. Bulb ripening

   May be normalvery common on this plant

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Onions and shallots — harvesting
   • Royal Horticultural Society: Garlic — harvesting

   Checked 2026-08-13

   Onion, shallot and garlic foliage yellowing from the tips downward through midsummer, with the necks softening and the tops eventually folding over. This is the crop finishing, not failing. The bulb has stopped growing and is drawing the last of the leaf back into itself, and the yellowing is the signal to stop watering and lift — which makes it one of the few benign states where recognising it promptly actually changes the harvest.

   Points towards it

   • · ripening runs through the whole bed at once, because it is driven by day length rather than by anything local
   • · the outer and lowest leaves go first, each one corresponding to a skin the bulb is laying down
   • · nothing was done to it, which fits a crop arriving at maturity on schedule
   • · a bed or bay crop rather than an indoor one

   Argues against it

   • · ripening happens as the ground dries in midsummer; a saturated bed in July points at root disease instead
   • · the newest central leaves yellowing while the outer ones stay green is the wrong way round for ripening, and suggests the base of the plant is being attacked — this rules it out entirely.

   Check this next

   Check the calendar and the neck rather than the leaves. Onions and garlic ripening in June or July yellow from the leaf tips downward across the whole bed together, the neck just above the bulb goes soft so the tops flop, and there is a swollen bulb underneath — that combination is the harvest signal, and the correct response to it is to stop watering rather than to start.

   What to do now

   Stop watering and feeding, and lift within a fortnight of the tops going over rather than waiting for the leaves to finish. Dry the bulbs somewhere airy and under cover for two or three weeks until the necks are papery, and keep any thick-necked ones for the kitchen rather than the store, because those are the ones that rot first.

   ·source

   Sourced guidancehigh confidence

   • Royal Horticultural Society: Onions and shallots — harvesting and storing

   Checked 2026-08-13

2. Downy mildew

   Candidate 2occasional on this plant

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Onion downy mildew
   • Agriculture and Horticulture Development Board — AHDB: Alliums — foliar disease

   Checked 2026-08-13

   Pale yellow patches on the upper leaf surface with a greyish or purplish furry growth directly beneath them, the patches often bounded by the leaf veins into angular blocks. It is routinely confused with powdery mildew and behaves in the opposite way: downy mildew needs wet conditions, powdery mildew thrives in dry ones.

   Points towards it

   • · downy mildew needs prolonged leaf wetness, so it follows a wet spell or a crowded damp bed
   • · the older outer leaves are infected first and collapse from halfway up
   • · the yellowing is in defined oval zones along the leaf rather than a general fading
   • · a field and bed problem driven by weather

   Argues against it

   • · an evenly fading leaf with no patches on it is not downy mildew, which always works in defined zones
   • · a dry summer and an airy bed give the disease nothing to work with

   Check this next

   Go out early, while the dew is still on the crop, and look at the yellow zones through a hand lens. Onion downy mildew furs them with a fine grey-violet bloom that is visible only when the leaf is wet and has gone by lunchtime — pale oval patches with that bloom on them are the disease, and the same patches without it are more likely to be weather or ripening.

   What to do now

   Pull out and remove the worst plants to slow the spread, and use what is left promptly rather than trying to store it — infected bulbs do not keep. Give the following crop a wider spacing on a different bed, because the useful controls here are airflow and rotation rather than anything sprayed on.

   ·source

   Sourced guidancemoderate confidence

   • Royal Horticultural Society: Onion downy mildew — control

   Checked 2026-08-13

Narrow it down

This page lists everything at once, which is the right shape for reading and the wrong shape for deciding. The interactive tool asks one question at a time, starting with whichever would separate these causes fastest, and stops as soon as further answers would not help.

Opens with the plant and the symptom already set. Nothing you answer is stored or sent anywhere — the answers live in the address bar and nowhere else.

What this does not cover

These are the causes GrowingHQ holds rules for on onion — not every cause there is. Where several problems look alike, the honest answer is often that they cannot be told apart by eye, and a page that named one would be guessing. If none of the above fits, the general lower leaves turning yellow page lists causes across all plants, and the Onion profile lists the problems it is prone to.