Late Blight Potato Disease

Late blight is a common potato disease, caused by Phytophthora infestans, which affects potato and tomato plants with potentially devastating effects, even destroying entire crops in as little as 10 days.  It was late blight which resulted in the Irish Potato Famine back in the 1840’s and 1950’s.

Common Signs and Symptoms of Late Blight

Symptoms of late blight include:

Dark blotches appearing on leaf tips and plant stems.

White mould will appear on the underside of the leaves during humid conditions and the whole plant may quickly collapse.

Grey or dark patches that are reddish brown beneath the skin will appear on tuubers infected with late blight, and the potato’s flesh will quickly decay to a horrible smelling, putrid mush caused by soft rotting bacteria.

Seemingly healthy tubers afftected with late blight may well rot at a later stage during storage.

Common Causes and Conditions of Late Blight

The fungus of blight infected plants is spread via millions of spores through the air travelling on the wind for miles. Heavy rainfall washes these spores down into the soil before reaching the potatoes planted in the ground.

Late blight flourishes during periods of warm, humid weather, referred to as ‘Smith Periods’ (two or more consecutive days where min temperature is 10ºC or above and on each day at least 11 hours when the relative humidity is greater than 90%).

Affected / Vulnerable Varieties

Potato Varities which tend to be more susceptible to late blight include Brodick, Cara, Cosmos, Remarka, Sante, Stirling, Teena, Torridon and Romano.

Control of Late Blight

Cultural and Methods:

Blight can be prevented by avoiding already diseased seed, earthing up well and keeping a good cover of soil over your tubers.

Allow at least two weeks between haulm killing and lifting.

Destroy any blight affected tubers at lifting and grading stages.

Ensuring that stored potatoes are keep in cool, dry conditions will also help.

Chemicals and Pesticides:
Late blight cannot be cured, only prevented and controlled using specialist commercial fungicides.