Tracking A Plant Killer: Historical and Scientific Reflections on the Irish Famine Pathogen and Its Control

In 1845 a hitherto unknown plant pathogen attacked the Irish potato crop, leading to one of the most significant famines in modern history. The first reported outbreaks of the pathogen were recorded in the United States in 1843. Two years later, in late summer of 1845, outbreaks were reported in the Flanders region of Belgium. By September that same year, the disease had spread to Ireland. Immediately, experiments were conducted to understand the cause of the disease: was it spontaneous generation, lightening, worn out potatoes, harsh weather or the wrath of God that caused the murrain? The presence of a minute ‘fungus’, which was eventually named Phytophthora infestans , was confirmed. Forensic DNA from historic outbreak samples found in mycological herbaria were used to identify and genotype the strain that caused the Irish famine. Historical agricultural reports provide evidence of shipping routes and the movement of potatoes from the Andes to the US and then to Europe. Modern forensic DNA studies have helped identify the strain that caused potato blight and track the pathogen’s Andean origins and evolution since the famine in Ireland. Modern tools including agricultural fungicides, disease prediction systems and resistant varieties have helped growers manage this plant disease, which still aggressively attacks potatoes in Ireland 180 years after its introduction.