Devastating Irish potato famine caused by deadly pathogen with South American origins

The pathogen that cause the terrible potato famine that began in Ireland in 1845 may have arrived in Europe via infected potatoes on South American ships or from the United States, scientists have discovered. This knowledge is crucial as the devastating effects of this Phytophthora infestans pathogen still have the potential to cause famines in the developing world today.

The potato famine led to the death of about one million people due to starvation and epidemic disease, between 1846 and 1851. It is considered to be one of the deadliest famines in modern times.

Previous studies have identified Phytophthora infestans as the plant pest that caused potato blight at the root of the famine. However, it wasn't clear where the pathogen had originated and how it had spread to Ireland.

In the study published in PLOS One, a team from North Carolina State University studied historic and modern samples of the pathogen to track the evolution of the different strains of Phytophthora infestans.

Signs of a Colombian origin

A total 183 historic and modern pathogen samples with different geographic origins was used in this study. Genetic analysis suggests that a lineage called FAM-1 caused outbreaks of potato late blight in the United States in 1843. The disease destroys both the leaves and the edible roots of the potato plant.

It is this same FAM-1 lineage which started to devastate Irish potato crops two years later. Next, the team found that older, historic samples from Colombia also belonged to this lineage, indicating that FAM-1 potentially had South American origins. They hypothesise that the strain ended up in Ireland after being carried by potatoes from ships coming from the US or South America.

This theory is consistent with previous studies which indicated a South American origin for the famine - in 2013 scientists suggested that a strain known as Herb-1 which originated in Mexico, was the ancestor of the Phytophthora Infestans strain which caused the famine.

The new analysis also revealed that the FAM-1 lineage subsisted in the US for 100 years after the epidemic started, before it was displaced by a sister lineage of the pathogen known as US-1 in the 20th century.

While the pathogen has evolved and other lineages have now taken over, the threat it can pose on agricultural crops remains. The scientists point out that billions are spent each year globally to control the pathogen but in some developing countries countries - where fungicides are less available - potato blight may still emerge and bring disease and starvation in its wake.