Today 
Caused by the same fungus responsible for tomato blight, potato blight causes the leaves to develop dark patches, starting at the edges. In humid conditions a white fungus appears around the marks. The infection then travels down the plant to the tubers, which turn a reddish-brown and, if a secondary infection strikes, the crop turns slimy and rots.
Solution
Organic
Grow cultivars that have some blight resistance, such as 'Cara', 'Kondor', 'Valor' and 'Remarka'. Immediately destroy any leaves or stems showing the symptoms (never compost them), and any infected tubers.
Chemical
As a preventative measure, spray the crop with mancozeb or copper fungicide every 10 days or so. They don't provide 100 per cent protection, but an attack will be slowed down and you should get a ripe crop.
