Potato blight, also known as late blight, attacks the foliage and tubers of potatoes, eventually causing the crop to rot. Caused by the same fungus responsible for tomato blight, it is a particular problem in wet summers. Potato blight was responsible for the Irish potato famine in the 19th century, devastating crops in the ground, and those in storage too.

Blight attacks usually occur in July or August and can devastate potato crops in just a few days. As long as action is taken to remove the foliage as soon as the problem is spotted, any tubers that have developed should still be edible. Blight can occur as early as June, however, and this is more serious as many tubers have not fully developed by this time and the crop will be lost.

Blight is becoming increasingly common in the UK, thanks to warmer winters and wetter summers. There are now several strains circulating at any one time, and they are liable to merge and mutate.

What is potato blight?

Potato blight
Potato blight

Potato blight is a serious disease caused by a fungus called Phytopthera infestans. Its spores break away easily from an infected plant and are carried on the wind (as far as 30 miles) until they land on a susceptible plant. In dry weather, the spores remain dormant, but once the right conditions occur – warm, wet or humid weather – they develop and spread. This type of weather is known as a 'Hutton Period' – conditions that include a minimum air temperature of 10°C over a two-day period, plus at least six hours of 90 per cent humidity during that time.

Most infections in gardens and allotments are blown in from nearby plots, although it can also arise from infected material that has been left in the ground. Blight can occur as early as June but is more common in July and August. It usually starts in the South West of the UK.

Identifying potato blight symptoms

Blighted potato crop
Blighted potato crop

The first sign of potato blight is dark blotches on the leaves, starting at the leaf tips and edges. They then spread, causing the foliage to shrivel and collapse. Blotches also appear on the stems, which turn black and rot. At the same time, white fungus spores appear around the dark blotches and on the undersides of the leaves. If the problem is left unchecked, it will travel down to the tubers. The fungal spores can also be washed into the soil by rain, also infecting the tubers. Once one tuber is infected, the problem spreads rapidly, turning them red-brown. White, smelly slime then appears on the crop and it rapidly rots.

Treating potato blight

Binning blighted potato foliage
Binning blighted potato foliage

Chemical treatments

The traditional approach to preventing potato blight used to involve spraying plants with copper-based fungicides such as Bordeaux mixture before blight arrived. However copper fungicides have gradually been withdrawn from sale, and there are currently no chemical treatments to prevent potato blight.

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Organic treatments

As soon as you spot blight, you may be able to save your crop by cutting the foliage and stems down to ground level immediately. This obviously means that the tubers won't get any larger, but it should prevent the fungus travelling down to them. Leave the tubers in the soil undisturbed for three weeks. This should kill off any lingering spores, preventing them from infecting the crop when it is lifted, and will also allow the skins of the potatoes to harden. The potatoes will be edible but use them up as soon as possible – tubers from blighted plants do not store well. Discard any that show any signs of blight.

Add all infected plant material to your council green waste bin – municipal composting systems heat garden waste to higher temperatures, killing the spores. Never compost it in your own garden, as this may carry the problem over to next year.

If tubers have not yet developed, remove and destroy all affected plants as soon as you spot them.

Preventing potato blight

Grow early potatoes

Harvesting new potatoes
Harvesting new potatoes

The best way to avoid blight is to grow new or early potatoes, such as first earlies and second earlies. You can usually harvest these before blight strikes. Read about the best new potatoes to grow.

Grow blight-resistant varieties

Potato 'Sarpo Mira'
Potato 'Sarpo Mira'

Another option is to grow blight-resistant potatoes. The genetic make-up of potato blight is constantly changing, and so are blight-resistant potatoes. 'Cara', 'Kondor', 'Valor' and 'Remarka' previously showed good resistance to blight but this is no longer the case. The best blight-resistant potatoes are currently the 'Sarpo' cultivars, such as 'Sarpo Mira', 'Sarpo Blue Danube' and 'Sarpo Axona'; many seed potato suppliers sell collections of blight-resistant seed potatoes. Older varieties such as 'King Edward' and 'Arran Pilot' are particularly susceptible to blight.

Practise good hygiene

How to grow potatoes - earthing up potatoes
How to grow potatoes - earthing up potatoes

Be scrupulous about the hygiene on your veg plot, by ensuring you do the following:

Buy from a reputable supplier

Be sure to order your potatoes from a reputable supplier that supplies certified disease-free seed potatoes.

Space plants further apart
This increases air flow, which means that foliage dries more quickly after rain, minimising the conditions that blight thrives in.

Water in the morning
Take care not to splash the foliage. If you do, it will dry off quickly in the sun.

Earth up
Earthing up potatoes is a common practice when growing potatoes. While it won't prevent blight affecting the foliage, it may help to prevent the tubers from being infected.

Remove any infected material
Blight overwinters in on foliage or tubers left in the ground, so it is important to remove any affected material from your garden or allotment. Be sure not to leave any potatoes, even baby ones, in the ground. Dispose of them in your council green waste bin – do not add to the compost heap as this may help the problem persist the following year. Do not store blighted tubers for eating.

Practice crop rotation
Heavy rain can wash the spores of potato blight into the soil, where it overwinters. Growing potatoes in a fresh piece of ground each year can help avoid any disease build up in the soil.

Get blight alerts
While aimed at farmers, the Potato Growers' Association's Blightwatch service can alert you to blight outbreaks in your area, allowing you to look out for signs of blight and take prompt action if necessary.