Appearance, Impact and Biology

Late blight is a serious disease of potatoes and tomatoes and was the causal agent of the Irish potato famine. It remains the single most important potato disease, costing the industry an estimated £50M annually in crop protection chemicals in a typical blight pressure season. Blight spores are produced on infected leaves and are dispersed by the wind. They can travel long distances. Spread of blight is favoured by warm, humid weather. Irrigation increases humidity within the crop canopy and prolongs leaf surface wetness. Such conditions encourage blight to sporulate, spread and infect potato plants.

The first sign of infection is likely to be brown freckles on the surface of the leaf, often at the margins, which spread to form larger dark brown patches. These lesions will quickly spread in humid conditions to cover the whole leaf which will soon die off. The disease spreads through the plant ultimately infecting the tubers. Infected potato tubers have a brown discolouration on the skin and a brown marbled appearance to the flesh when cut. The tubers may remain firm but are highly likely to develop soft rot in storage.

New strains of the disease infect potato varieties which were previously resistant. Over the last few years two dominant strains, 13_A2 and 6_A1, have been found. These strains can infect a plant more rapidly and therefore limit the curative effects of some fungicides.

Management recommendations

Maintaining crop hygiene is an important way to prevent spread of late blight. When planting avoid potential sources of infection such as outgrade piles or volunteer potatoes. Avoid short rotations, especially if it is known that the previous potato crop was blighted, or blighted volunteers were observed in the intervening years. Consider distance to watercourses as this may affect blight fungicide spraying options later in the season. Avoid discarding unplanted tubers in headlands as they could become sources of infection. Control volunteer potatoes.

AHDB mapping of late blight outbreaks has shown that outgrade piles are a significant source of blight for crops. Minimise the number of tubers being put in the outgrade pile and adopt a zero tolerance approach to sprouting and production of green foliage on outgrade piles by taking action early; preferably covering with black plastic sheeting. If sheeting is not used early and the outgrade pile has been allowed to sprout, you should spray the pile as soon as possible with Reglone (Diquat) or glyphosate (please note diquat is about to be banned - the EU has proposed a market withdrawal date for diquat of 4 May 2019, with a use-up period for growers until 4 February 2020 and Defra will comply with this even after Brexit). Check outgrade piles regularly. Outgrade piles should be accessible but well away from your own, and your neighbour’s, potato crops and farm buildings. Try to locate piles on land not intended for any crop, especially any potatoes in the future.

There is a risk of blight from infected seed. A single seed tuber with blight symptoms in every 100 tubers can produce at least two primary infection sources per hectare. In the right conditions this could start a blight outbreak in your crop. Check with your seed supplier where the seed was grown and what the blight risk was. Treat farm-saved seed with particular caution and don’t save seed from crops known to have had a blight infection.

Independent information on the relative effectiveness of different blight fungicides is available in the Euroblight fungicide comparison table. The aim should be to use blight fungicides protectively, that is, apply them before blight infects the crop. Starting the spray programme early enough and maintaining the correct spray intervals for the prevailing risk during the season are just as important as the choice of fungicide. Further information is available from AHDB on late blight. Decision support tools Fight against Blight and BlightCast are available for the UK.

Current resistance ratings for varieties are available on the British Potato Variety Database. With more resistant varieties, the timing of fungicide application is not quite so critical. During periods in the growing season when it is difficult to apply fungicide to all crops at the planned time, then consider treating the more susceptible varieties first. Blight resistant varieties are being developed such as Sarpo Mira which has good blight resistance but not the highest quality flavour. Transgenic potatoes that are blight resistant have been developed by The Sainsbury Laboratory but are not commercially available yet.

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