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.
Here are organic and chemical ways of dealing with the problem.
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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.
Since most copper fungicides have gradually been withdrawn from sale, there are currently no chemical treatments available for use on blighted potatoes.