Potato Blight: How to Recognise and Beat It

Potato blight is a fast-moving fungal-like disease, caused by the organism Phytophthora infestans, that strikes in warm, wet UK summers and can flatten a healthy bed of potatoes in under a week. You can't cure it once it takes hold — but if you cut off and remove the foliage the moment you spot it, the tubers underground are usually safe to lift and eat. Acting fast is everything.

It is the same disease that caused the Irish potato famine, and it remains the single most common reason a UK potato crop fails. The good news: with a bit of recognition and a few simple habits, most beginners can dodge the worst of it or rescue a crop that's already infected.

How to recognise it

Blight moves quickly, so learning the early signs is the most useful thing you can do. Check your plants every few days from midsummer onwards, especially after a spell of muggy, damp weather.

Look for these signs, roughly in the order they appear:

• Dark brown or black patches on the leaves. These usually start at the leaf tips and edges, often with a slightly water-soaked, yellowing halo around them. They are not neat spots — they spread and merge into large dead areas.
• White, fuzzy growth on the leaf undersides. In damp or humid conditions, turn an affected leaf over and you may see a fine white mould around the edge of the brown patch. This is the disease producing spores, and it's a sure sign of blight rather than ordinary leaf damage.
• Brown or black lesions on the stems. As it advances, the disease attacks the stems too, leaving dark streaks and weak points. Stem infection is bad news — it means the whole plant is collapsing.
• Rapid collapse and a foul smell. Within days, the foliage can turn from healthy green to a slimy, blackened, sour-smelling mess. A bed that looked fine last week can rot down to nothing.
• Tuber rot. If spores wash down into the soil, the potatoes themselves develop reddish-brown, rusty-looking patches under the skin and a firm rot that spreads inward. Affected tubers often go soft and smell rancid in storage.

Why and when it strikes

Blight needs warmth and moisture to spread. The spores travel on the wind and in rain splash, and they germinate when conditions are right — typically when temperatures stay above about 10°C and humidity stays high for two days or more. Forecasters and growers call these stretches "blight periods" (you may also hear the older term "Smith periods").

In the UK, that usually means July and August, occasionally creeping into a wet June or a mild September. A cool, dry summer can pass with barely a case; a warm, wet one can see blight reported across the country within days. Allotment sites are especially vulnerable, because once one plot is infected the spores drift straight onto its neighbours.

It's worth knowing that potatoes and tomatoes get blight from the same organism, which is why the two crops spread it to each other. If you grow both, keep them apart, and read up on tomato blight too — the recognition and prevention are closely related. Never plant tomatoes downwind of, or right next to, your potato bed.

What to do immediately

If you find blight, don't panic — but do act the same day. The aim is to stop spores reaching the tubers below ground, where your actual crop is sitting.

1. Cut off and remove all the foliage. Use shears or a knife to take the stems right down to ground level. Remove every leaf and stem — this halts spore production and protects the potatoes underneath. With early varieties that are already a decent size, you may have a perfectly good crop waiting.

2. Never compost the infected material. Blight can survive on plant debris, and a home compost heap rarely gets hot enough to kill it. Bag it up for your council green-waste collection, burn it if you're allowed to, or bin it. Don't leave cut foliage lying on the bed.

3. Wait about two weeks before lifting. Leave the tubers in the ground for roughly a fortnight after removing the tops. This lets any spores already on the soil surface die off and gives the potato skins time to firm up ("set"), which helps them store. Lifting straight away risks dragging live spores onto freshly exposed tubers.

4. Lift on a dry day. Choose dry weather and gently fork up the crop, taking care not to spear the potatoes. Let them dry on the surface for an hour or two if conditions allow.

5. Store only healthy, unblemished tubers. Inspect every potato. Discard any with rusty patches, soft spots or a sour smell — one infected tuber can rot a whole sack. Store the sound ones somewhere cool, dark and frost-free in paper or hessian sacks, and check them every couple of weeks. As a rule, blighted maincrop won't keep well, so use them up sooner rather than later.

How to prevent it

You can't keep every spore out of your garden, but you can stack the odds heavily in your favour. These are the habits that make the biggest difference, and most of them cost nothing.

Grow blight-resistant varieties. This is the single best defence. The Hungarian-bred 'Sarpo' varieties — particularly 'Sarpo Mira' (a floury maincrop) and 'Sarpo Axona' — shrug off blight that would flatten older types, and they're widely sold by UK seed-potato suppliers. Other reliable choices include 'Setanta', 'Cara' and the salad potato 'Charlotte', which has reasonable resistance. If your plot or allotment gets blight most years, switch to resistant varieties and you may never lose a crop again.

Grow first earlies that finish before blight season. Early varieties such as 'Rocket', 'Swift' and 'Pentland Javelin' are ready to lift in June and early July — often before the main blight periods arrive. By choosing the right type and getting them in promptly, you can harvest a full crop before the disease is even a risk. The planting calendar will help you line up your sowing and lifting dates with the UK season.

Earth up well. Drawing soil up around the stems doesn't just boost your yield and stop tubers going green — it puts a protective barrier of soil between any spores washing off the leaves and the potatoes below. Good, deep ridges genuinely reduce tuber blight. Our guide to earthing up potatoes walks through the timing and technique.

Give plants room to breathe. Space your seed potatoes properly — around 30cm apart for earlies and 40cm for maincrop, with about 60–75cm between rows. Crowded plants stay damp longer after rain, and damp foliage is exactly what blight wants. Good airflow lets leaves dry quickly.

Rotate your crops. Don't grow potatoes (or tomatoes) in the same ground two years running. A three- or four-year rotation reduces the build-up of spores and the related problem of leftover "volunteer" potatoes carrying the disease over winter — always dig up any stray tubers you find the following spring.

Keep the foliage dry where you can. Water the soil, not the leaves, and do it in the morning so any splashes dry off during the day. Avoid overhead watering during a humid spell.

For the full picture on planting, feeding and harvesting a healthy crop from the start, see our main guide to growing potatoes — getting the basics right makes your plants far more able to shrug off a bad blight year. And if you've not yet decided what to put in, browse the wider range of vegetable growing guides to plan a plot that spreads your risk.

Blight can feel like a disaster the first time you see it, but it rarely is. Recognise it early, cut the tops off without hesitation, and you'll usually still have a meal's worth of potatoes — or more — waiting safely underground.