Chive Pests and Diseases: Thrips, Rust, Mildew and Winter Rot

Silvery streaks mean thrips

Thrips are the most common chive pest and they arrive in hot, dry weather. They are barely visible, slim yellow-brown insects less than a sixteenth of an inch long, and they rasp at the leaf surface and drink the sap. The damage shows as fine silvery or whitish streaking running along the length of the leaf, sometimes with tiny black specks of excrement on it.

The clump does not die from thrips but it stops looking edible and the growth slows. The best response is also the simplest: shear the whole clump to the base and remove the cut leaves from the garden. That takes most of the population away with the foliage, and the regrowth comes through clean if you keep the plant watered, because thrips build up fastest on drought-stressed plants.

1. Cut and remove, do not compost Bin or burn the infested leaves rather than composting them near the bed. A heap of thrips-covered leaves next to the clump repopulates it in a fortnight.
2. Water more, not less Thrips numbers climb in hot dry conditions. Keeping the clump watered through a heatwave is the most effective control available.
3. Hose the regrowth A firm jet of water over the new leaves every few days knocks the survivors off. Do it in the morning so the foliage dries.

Silvery streaks along the leaf are thrips. Orange raised spots are rust. Fuzzy gray patches are downy mildew. The three look nothing alike once you have seen each one.

Rust and downy mildew

Rust shows as small raised orange or rusty-brown pustules on the leaves, most often in a humid summer or on a crowded clump with no air movement. Downy mildew is different: pale, washed-out patches on the leaf that carry a fuzzy gray or violet growth in damp weather, and the leaf collapses from that point. Both are fungal, both thrive in still, wet foliage, and both are made much worse by an overcrowded clump.

Shearing the clump is the standard response to almost every foliar problem on chives. Unlike a shrub, the plant loses nothing by being cut to the base and regrows in three weeks.

Allium leaf miner and onion fly

Chives are alliums, so the pests that attack onions and leeks will use them. Allium leaf miner leaves neat rows of small white dots down the leaf where the adult fly has fed, and the larvae then tunnel down into the base, leaving brown streaks and small brown pupae in the bulbs. Onion fly maggots do a similar job underground and the clump goes yellow and pulls away easily.

Neither is common on chives compared with onions, but if you grow alliums nearby the chives can act as a reservoir. Insect-proof mesh over the clump during the two main flight periods, in spring and again in late summer, is the only reliable prevention. Remove and destroy any affected clump rather than leaving it to complete the cycle.

1. Check the base, not just the leaves Pull a few leaves apart at the crown and look for brown streaking or small rice-grain pupae. Damage on the leaves alone may be superficial; damage at the base is not.
2. Do not replant alliums in the same spot If a clump has been lost to leaf miner or onion fly, move the replacement to fresh ground. The pupae overwinter in the soil where the plant stood.
3. Cover during flight periods Fine mesh laid over the clump from early spring and again from late summer stops the adults laying. Weight the edges down; these flies walk under loose netting.

Aphids, slugs and the things that are not really a problem

Aphids occasionally colonise chive flower stems, clustering just below the head. They rarely do real damage and are washed off with a jet of water. Slugs and snails will graze new spring shoots on a clump coming out of dormancy, leaving ragged, chewed leaf tips, but an established clump grows away from them quickly.

The strong smell that chives carry does deter some pests, which is why they are often planted near roses and carrots, but do not rely on it. Companion planting reduces pressure; it does not remove it. Judge each clump on what you can actually see on its leaves.

Chive leaves are hollow and easily kinked. Bent, browned leaves after a windy week are mechanical damage, not disease. Cut them off and ignore them.

The clump that does not come back in spring

The most serious chive failure is not a pest at all. A clump that dies back in autumn as usual and then never reappears has almost always rotted at the base over winter, in soil that stayed saturated. Dig where it was and you will find soft, slimy bulbs instead of firm white ones, and often no roots at all.

The other cause is age. A clump left undivided for five or six years can be dead in the middle and so weak round the edge that a hard winter finishes it. Lift the remains, check whether any outer bulbs are still firm, and if they are, replant those in better-drained ground. If everything is soft, the clump is gone and the ground needs improving before you plant another.

Recording problems so you see the pattern

Note the date you first see each problem and which clump it was on. Thrips, rust and leaf miner all arrive at roughly the same point in the season each year, and a record of first appearance turns them from surprises into things you can shear or cover for in advance.

Record what you did as well, and whether the regrowth came back clean. Chives give unusually fast feedback because a sheared clump is replaced within a month, so one season of notes is enough to tell you which responses work in your garden and which are wasted effort.