How to spot Potato Blight

Potato Blight

Blight is one of the worst diseases of potatoes. It was partly the cause of the great famine in Ireland (1845) in which tens of thousands of people died of starvation because their main food crop – the potato – was wiped out for several years in a row by blight. And now Russia is in the grip of a powerful new type of the same blight. Small Russian farmers can’t afford to spray with expensive fungus-killing chemicals to try and control the outbreak.

The first symptoms are a dark brown patch and yellowing of the leaves which then turn black, and a white bloom develops on the underside as the foliage dies. The spores produced by this bloom are washed down to the tubers causing dark spots on the skin and reddish-brown stains through the flesh. It survives the winter as mycelium and spores in tubers left behind at harvest. The fungus grows on shoots from these tubers the following year, and produces asexual spores that are carried by the wind to infect the new crop during warm moist conditions. Early crops are usually harvested before serious attacks.

Use only certified seed, which are produced in areas where infection is not prevalent. Try to harvest every tuber and destroy any infected ones. Avoid overhead watering which would wash spores down to the soil, and earth up well to protect the tubers. If infection occurs remove all the top-growth so that it does not spread to the tubers. Planting early or second early varieties may mean that spraying can be avoided as they will be mature enough to harvest before infection risk is high or if infection is detected they are mature enough to be harvested immediately. King Edwards are more resistant, but Maris Piper and Desiree are very susceptible.

Spraying should be carried out if the humidity is above 92% with a temperature over 10 C. Protective sprays every 10-14 days of Bio Dithane 945, Murphy Traditional Copper Fungicide Bordeaux MixtureBordeaux Mixture: –

• Copper Sulphate 1 lb. (250g)
• Hydrated Lime 11û4 lb. (315g)
• Water 10 gal (25 litres)

How to avoid potato blight

Potato blight can be avoided by useing disease-free seed potatoes and good garden hygiene will help to prevent blight but the spores of the disease are carried on the breeze, so few gardens will escape. To stave off the worst of blight outbreaks in drier areas, try using second early varieties and gather them up in August. 

Resistant varieties

‘Cara’, ‘Record’, ‘Valor’ and ‘Balmoral’ have partial resistance to tuber blight.

‘Stirling’* ‘Brodick’ and ‘Spunta’* have high levels of resistance to foliage and tuber blight. However, none is immune.