My potatoes have blight — should I cut the tops off, and can I still eat the potatoes?

Once blight is in the foliage there is nothing a home gardener can do to cure it. The whole question becomes how to save the tubers, and that turns on one fact: the spores wash down through the soil from the dying leaves. Removing the source and then waiting is what protects the crop underneath.

Safety

Step by step

1. Confirm it is blight.Dark brown or black blotches on leaves, often starting at the tips and margins, sometimes with a pale halo, spreading fast in warm humid weather, and a white mould ring on the leaf underside in the morning. Stems develop dark lesions. It moves through a row in days, not weeks.
2. Cut the haulm off at ground level immediately.Use shears or a knife and take everything down to a few centimetres. This removes the spore source before more of it washes into the soil, and it stops the infection running down the stems into the tubers.
3. Remove the cut foliage from the plot entirely.Bin it, burn it where that is permitted, or take it to a green waste facility that composts at high temperature. A garden heap will not reliably kill it, and infected material on the heap seeds next year's outbreak.
4. Wait two to three weeks before lifting.Two things happen in that time: spores on the soil surface die off, so lifting does not drag them across the tubers, and the skins set enough to handle storage. Lifting the same day is the commonest way people infect their own crop.
5. Lift on a dry day and leave the tubers on the surface for a few hours.Surface drying firms the skins and lets you see damage. Do not wash them — washing spreads infection and removes the protection dry soil gives.
6. Sort ruthlessly before anything goes into store.Cut a sample from any tuber you are unsure about. Sound flesh, right through, is fine. Reddish-brown or rust-coloured staining spreading in from the skin is tuber blight, and that tuber will rot and take its neighbours with it.
7. Store the sound crop in the dark, cool and dry, and check it weekly.Blight can be latent in a tuber that looked fine at lifting. Weekly checks in the first month catch a rot before it spreads through a sack.
8. Clear every last tuber from the ground.A missed tuber overwinters, sprouts next spring and is the classic source of an early blight outbreak. Go through the bed carefully, and again in spring when volunteers appear.

Tips

• If blight arrives late — say September on a maincrop — you may have most of your crop already. Cutting the haulm then costs you very little.
• Earlies are often lifted before blight becomes a problem at all, which is the strongest reason to grow them in a wet-summer garden.
• Blight-warning services and local allotment networks give a few days' notice of high-risk periods. Cutting haulm pre-emptively during a severe warning is a legitimate tactic on a crop that is nearly ready.
• The same organism attacks outdoor tomatoes. If your potatoes have it, check the tomatoes the same day.
• Do not save seed potatoes from a blighted crop, however good they look.

Common mistakes

• Lifting the crop the moment the tops are cut — Live spores are still on the soil surface and get smeared across every tuber you pull through it. Waiting a fortnight is what makes the difference between a store and a bin.
• Composting blighted haulm — Domestic heaps rarely reach a temperature that kills it, and infected material gives the disease a bridge into the following season.
• Storing borderline tubers 'to use first' — One rotting tuber liquefies and spoils a whole sack. Anything with stained flesh goes in the bin now, not into the store.
• Spraying once blight is visible — Protectant treatments work before infection, not after. Once lesions are established the foliage is lost and cutting is the answer.

If it doesn't work

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