Investigating the 19th Century Irish Potato Famine with Ancient DNA

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Brown patches on potato leaves are the first sign of Late Blight disease. Image by Fk

Brown patches on potato leaves are the first sign of Late Blight disease. Image by Fk

Late Blight quickly spreads from the leaves into the potato tubers. Image courtesy of the Agricultural Research Service

Late Blight quickly spreads from the leaves into the potato tubers. Image courtesy of the Agricultural Research Service

How can you investigate historical plant diseases? It’s a tough proposition when your samples turned to slimy mush 150 years ago.

However, an international team of Forensic Plant Pathologists have used ancient DNA to unlock some interesting information about the potato disease responsible for the Great Famine.

The Great Famine was caused by a mixture of catastrophic plant disease and politics – and was responsible for about a million deaths due to starvation and a million emigrations from Ireland.

The country relied heavily on potatoes for food as they grew plentifully in the cool moist conditions. The dominant potato variety, ‘Irish Lumper’, had little genetic variety because the tubers were saved from one season to the next. Each crop was effectively a clone of the last, leaving it wide open to disease.

For the first three centuries of potato growing in Europe, the plants grew beautifully. They provided the people with a cheap and nutritious (if bland) diet. This changed with the arrival of the Late Blight pathogen, Phytopthera infestans. This oomycete fungus infected the leaves of the plant and thrived in the moist conditions.

Phytopthera infestans killed the leaves of the plant and spread into the tubers, infecting all the plants due to the lack of genetic variation. There was no treatment for the disease and the crop simply rotted in the field.

Potatoes remain a staple food and the economic importance of Late Blight means it is a well studied disease. In the last twenty years researchers have used rapidly-developing genetic methods to reveal more information about the pathogen strain which caused the Great Famine. Is it still out there? Is another devastating epidemic just around the corner?

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