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Onion Problems

by John Harrison, author of Vegetable Growing Month by Month

Onions

Onions, especially onions grown from sets, are usually pretty trouble free. Seed grown onions, when directly sown, tend to have more problems but these are often as reliable as sets when started in modules and transplanted.

Onion Fly

The onion fly lays its eggs by the base of the onion which then hatch into maggots who eat away at the base of the onion and its roots. There's no chemical cure available to the gardener so preventing access by means of fleece is the only thing possible.

Usually only strikes at direct sown onion onions, being attracted by the scent of them when thinning, in similar fashion to the carrot root fly.

You can actually see the small maggots so identifying the problem is straightforward. The first sign is yellowing and drooping leaves, smaller onions will die fairly quickly and larger onions stop developing. Lift the onion and check the base carefully.

Bolting Onions

Bolting is a very common problem with onions. Bolting is when the onion suddenly puts up a central stalk that develops a seed head. Once started the only thing to do is to cut the stalk off an inch or two above the bulb. Use these first as they will not store well.

Bolting is caused by weather conditions usually, a cold spring followed by a hot summer seems to make it worse. Loose soil can also cause bolting. The plant roots are disturbed and, thinking it is starving, the onion reacts by trying to spread its seed.

I've read that seed sown onions are more susceptible to bolting but my experience is the opposite, sets giving more bolting problems. You can, however, buy heat treated onion sets that are resistant to bolting.

Onion White Rot

White rot is a serious problem with no real cure. The disease lives in the soil and causes a white mould to grow on the base of the plant, ruining it for storing or eating. There is no cure and worse still, the disease is very easily transferred from one patch of soil to another on your boots. If you have white rot, lift the onions and burn them. Do not compost.

You cannot grow on the same patch for 8 years, the disease will be waiting to strike. The only answer is to grow elsewhere or to construct a deep raised bed and fill with clean soil/compost and try to grow there. Be careful not to transfer the disease to the clean soil, just a dirty fork is enough.

I know of nobody who has successfully cleaned it out of the soil, once established, by drenching with Jeyes fluid, although I've heard it suggested.

Rusts, Smut, Mould, Downy Mildew and White Leaves

All of these are fungal diseases and once again caused by weather conditions being warm and humid. Plants are more susceptible when there is a high level of nitrogen in the soil. Try using a high potash fertiliser, like a tomato feed to change the nutrient balance. It can help but usually, once infected, it's too late.

Dithane 945 is effective against fungal diseases but it is not approved for use on onions, so I'm unable to recommend it.

Onions Rotting in Store

Grey mould on onions in store and general rotting is usually caused by the onions being insufficiently dried out prior to storing or damp storage conditions. Check regularly and discard rotting onions before the problem spreads to the rest.

Growing Onions & Shallots