What is blight?

Blight is the most devastating disease of potatoes worldwide, and can result in complete destruction of crops.  It was responsible for the Irish potato famine of the 1840's that led to the deaths of one million people.  Caused by the fungus-like organism Phytophthora infestans, late blight spreads rapidly in warm, wet or very humid weather, destroying plants quickly if undetected.  Infected tubers can decay to a foul smelling mush, caused by secondary bacterial rots.

What does it look like?

Brown patches at the tips and margins of leaflets.  In high humidity and temperatures of 10oC or above, the patches enlarge and the leaflets curl and wither.  In moist weather, the fungus is often visible as a delicate white growth on the underside of the leaf, at the margins of the brown patches.  Brown patches may also develop on stems, and the entire plant may be killed in a matter of days if the weather is favourable.  Blighted tubers have a reddish-brown rot under a discoloured skin.

How is it spread?

Spores are spread by wind and rain.  Infected tubers from the previous year may be discarded at the edge of the vegetable plot, left in the ground or accidentally replanted.  A small percentage of them will produce ‘primary infector’ plants, which under favourable conditions soon develop severe blight and usually act as the source of the following year’s outbreak.