Why Are the Tips of My Onion Leaves Turning Brown?

Brown tips on onion leaves are very common in summer and in many cases represent nothing more serious than the normal wear-and-tear of growing in hot, exposed conditions. However, tip browning can also signal pest activity — particularly thrips — and distinguishing between cosmetic weather-related browning and pest-caused damage determines whether any action is needed. Here is how to tell the difference and what to do in each case.

Heat and wind scorch

In hot, dry, or windy conditions, the tips of onion leaves — being the point furthest from the root system — are the first to lose moisture faster than it can be supplied. The result is a dry, tan-brown tip that does not progress much further down the leaf once conditions improve. This tip browning affects most or all plants in the bed roughly equally, is confined to the very tip of leaves, and the rest of the plant remains green and turgid. It is cosmetically annoying but causes no yield loss. Consistent watering at the base of the plants and a mulch to retain soil moisture reduces weather-related tip browning.

Thrips damage

Onion thrips (Thrips tabaci) are tiny, slender, straw-coloured insects barely 1.5 mm long that feed by rasping the leaf surface and sucking the sap. The distinctive damage pattern is silvery-white streaking and stippling on the leaves, with the tips browning and the silvery zones expanding toward the leaf base in heavy infestations. Look closely at the leaves — particularly into the leaf sheaths where thrips hide — and you may see the tiny insects moving. Thrips damage is most severe in hot, dry summers. Affected plants are weakened and in severe cases the bulbs can be smaller than expected. Organic sprays of insecticidal soap or pyrethrin applied to the leaf surface, including inside the leaf sheaths, can reduce populations effectively.

Calcium deficiency

Calcium deficiency occasionally causes browning and die-back of onion leaf tips in acidic soils with poor calcium availability. The browning typically starts at the very tip and has a slightly different texture to heat scorch — more papery and shrunken. If your soil is strongly acidic (below pH 6.0 for onions) and you have not added lime recently, a pH test and a lime application to raise pH to 6.5–7.0 improves calcium availability and generally improves onion health across the board. Foliar calcium sprays can also provide a quick correction in severe cases.

Downy mildew early stage

Downy mildew on onions (Peronospora destructor) can start with pale, slightly yellowed tips before the characteristic grey-purple sporulation develops in humid conditions. If tip browning is progressing rapidly down the leaves rather than staying confined to the tip, and conditions have been cool and wet, check for downy mildew. Remove and destroy affected leaves, improve airflow, and avoid overhead watering. Copper-based sprays can slow its spread on unaffected plants.