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Phytophthora infestans, also: late blight

The pathogen that causes potato late blight, the most serious fungal disease affecting potatoes

Early symptoms of late blight on potato leaves

Advanced infestation

Phytophthora infestans affects around 20 per cent of the global potato harvest.

Early symptoms of potato late blight are visible on plant stems and leaves. Grey-green patches appear, which turn brown later. In very humid conditions, white mould forms on the underside of the leaves. Phytophthora spreads rapidly in warm, humid weather. It releases fungal spores that penetrate the plant tissue with the help of a germ tube.

The fungus-like pathogen also attacks the potato tubers. Grey-blue patches appear and the potato flesh underneath them turns brown. Phytophthora overwinters in the tubers, which means that some of the seed tubers already contain a latent infection when they are sown. A single infected tuber can be enough to destroy a whole crop.

Phytophthora entered Europe in the middle of the last century and gained notoriety as the cause of Ireland’s potato famine of 1845, as a result of which two million Irish citizens emigrated to America and Australia.

The small oomycete is extremely flexible and adaptable and always finds ways to circumvent resistance. For this reason, researchers and plant-breeders are increasingly focusing on developing a type of resistance that involves several different genes and that will slow the spread of the fungus for a long time.

Genetic engineering experts are also working on strategies to combat Phytophthora. Field trials have been running since 2006 in a number of European countries to test a potato containing two resistance genes from a Mexican wild potato. Authorisation for the Fortuna potato was applied for at the end of 2011.