The worst outbreak of potato blight in Britain for five years is another sign of this summer’s atrocious weather. The wet, humid and warm conditions are ideal for Phytophthora infestans, the pathogen that causes the disease. Potato blight only spreads from one plant to another in great humidity, and the plants must be wet for the spores to infect leaves. In fact, the weather plays such a critical role that forecasts of blight can be made based on a minimum temperature of 10C (50F) and 90 per cent humidity.
Similar weather set off the disastrous potato blight of 1845 in Ireland. That summer began sunny, hot and dry across northern Europe, and the potato crop grew well. But the weather deteriorated and turned overcast, cool and rainy for several weeks. The blight first appeared in Belgium and by September had reached Ireland where the crop quickly rotted. The following summers in Ireland stayed exceptionally wet and the potato blight continued destroying the potato harvests.
The disease was particularly harsh in Ireland because the potato was the staple food, and the population was left starving in the worst famine in Europe of the 19th century. About a million people died and another million emigrated to the US, Canada, Australia and England. Altogether, about a quarter of the population died, devastating the country for generations.