The pathogen Phytophtera infestans causes a disease of potatoes called “late blight”. The pathogen grows in potato plants, breaking down cell walls so that it can use the nutrients found within them. Severely infected plants have an acrid odor which is the result of dying plant tissue. In the 1800s, Irish peasants subsisted almost entirely on potatoes. The late blight fungus arrived in 1845 and destroyed 40% of the Irish crop. In 1846, 100% of the crop was destroyed. Over 1.5 million Irish died of famine and a comparable number emigrated to America and other countries. Today, the fungus is still present in Ireland and would destroy the crop again if not for fungicides.

“Without the routine use of fungicides, large-scale commercial potato production in Ireland would be impossible. The cool, damp climate, which favours the cultivation of the potato and limits problems with virus diseases, is also ideal for the spread of blight. … In warm, wet weather when the humidity is high, P. infestans will lay waste an unprotected crop. … To prevent such devastating losses, the potato industry in Ireland has long been reliant on a substantial annual usage of fungicides.”

Author: L.R. Cooke
Affiliation: Plant Pathology Research Division, Department of Agriculture for Northern Ireland
Title: Potato blight control in Ireland: a challenging problem.
Publication: Pesticide Outlook. 1992. 13(4):28-31.