Potato blight

Potato late blight is caused by the fungus-like organism Phytophthora infestans. This infects leaves, stems and tubers and can cause devastating crop losses. Tomato blight is also caused by the same organism (see factsheet DC20 Tomato blight).

Typical symptoms

• Leaves and stems: Dark brown/ blackish round patches, often surrounded by a pale halo. In warm, damp weather the patches quickly spread to rot the whole leaf. Stems can also be infected. The underside of the infected leaf develops a downy white coating of spores in moist conditions, particularly noticeable early in the morning. If weather remains warm and damp, the disease spreads rapidly, reducing the foliage to a rotting mass within a few days.
• Tubers: Dark, sunken areas on the surface, which may extend to cover the whole potato, giving a dry firm rot. Cutting the potato in half will show patches of chestnut-coloured rot starting just under the skin. Other fungi and bacteria may follow, invading the tuber to produce a wet, foul smelling soft-rot. Seemingly healthy tubers may rot later when in store.

Another blight
Potato early blight (Target spot) symptoms may be mistaken for late blight. Early blight, Alternaria solani, however generally occurs earlier in the season (July) and spreads under warmer and drier conditions than late blight. The distinctive smaller dark brown spots, somewhat angular with concentric rings, are bounded by the leaf veins. Early blight rarely causes significant loss of yield and no treatment is necessary.

Life cycle

Potato late blight survives the winter in infected potato tubers. Infected ‘volunteers’ left in the soil, or discarded, will sprout in the spring and develop blight, acting as a source of infection for new crops. Home-saved tubers from an infected crop may also develop the disease when planted.

The initial infection may come from a local source, but the spores may be blown in from many miles away.

Spores can only infect the plant if they land on wet foliage. Spread is very rapid throughout the crop when conditions are warm and moist for two days or more at a time. Rain washes spores from the leaves down into the soil where they infect the tubers.

There are two types of blight in the UK. When both of these are present, winter spores can form with the potential for new blight strains to develop.

Prevention and control

• A healthy start: Plant good quality, certified, seed tubers. Don’t bring seed tubers from other countries, or save your own.
• Variety choice: Choose a variety with highly blight resistant foliage and tubers if blight is a regular problem in your area. Most early varieties are very susceptible, but can often be harvested before blight arrives. Similarly, early maincrop varieties are more likely to produce a reasonable crop before blight appears.
• Resistant varieties: A recent change in the blight fungus has meant that almost all the varieties that had high or moderate resistance to blight now only show moderate or low resistance. To be sure that the varieties you choose still have good resistance, consult a database (such as http://varieties.potato.org.uk/) that is kept up-to-date. Fortunately, potatoes in the Sarpo range, such as Sarpo Mira and Sarpo Gwyn, have still shown good foliage and tuber resistance in recent tests.
• Spacing and watering: Avoid planting in sheltered sites. If watering is required, apply water to the base of plants, preferably early in the morning, to reduce long periods of moisture on foliage. Digging in, or mulching with, organic matter will reduce the need for watering.
• Good hygiene: Try to harvest all tubers, even the tiniest, so that there are none to regrow. Remove all volunteer plants that come up. Never abandon old tubers around the garden or allotment.
• Ridging and mulching: Blight spores on foliage are washed down through the soil to infect tubers. Earthing-up potatoes, or mulching the soil with a thick layer of hay or straw or other organic material, can reduce the levels of tuber infection.
• Blight forecasting: When on at least two consecutive days the minimum temperature is 10ºC or above and on each day relative humidity is greater than 90% for at least 11 hours, the blight organism can infect plants, if it is in your locality. This is known as a ‘Smith’ period. Go to blightwatch.co.uk to sign up for a blight alert in your area.
• If blight strikes: If you notice what you think might be blight on a few leaves or stems, cut these out immediately. The disease may not spread if the weather turns dry.
  If the crop is badly infected (over 10% foliage killed), cut off and remove all potato foliage to help prevent spread. This obviously stops any further growth.
  Wait two to three weeks before digging the crop. By then, tubers will have thicker skins and blight spores in the soil, that could infect tubers when lifted, will have died. Dry off tubers carefully before storing in cool, dark conditions. Do not wash tubers that are to be stored.
• Composting tops and tubers: Blight infected potato haulms (foliage) can be composted – well buried in a good active heap. Alternatively bury them in trench in the soil.The likelihood of resistant spores being present is very slim. Never try to compost tubers; they are best put in your green waste bin. Potato peelings tend to break down quickly and rarely re-sprout and so are considered fine to compost, but not if they are particularly thickly cut.
• Storing potatoes: Do not wash potatoes that are to be stored. Remove blighted tubers (see symptoms overleaf) before putting potatoes in store. Check stored potatoes every few weeks and remove any rotting tubers.

	For more information about Sarpo varieties and the work of the Sarvari Research Trust click on the logo.
	For an even more detailed factsheet on potato blight, click on the Potato council logo.

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