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DC17

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).

Blight damage Right: Undamaged blight resistant Sarpo variety
Left: Blight damaged potato crop
Right: Sarpo variety showing blight resistance

Blighted potato foliage
(P M Pears)

Blighted stem Sutton Bridge CSR
(Sutton Bridge CSR)

Blighted potato foliage
(Sutton Bridge CSR)

Blighted stem Sutton Bridge CSR
Early blight. Also known as Alternaria or Target
spot (Sutton Bridge CSR)

Typical symptoms

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

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