So THIS is what caused the Irish potato famine: Scientists finally identify unique pathogen that caused the blight

  • A strain called US-1 was long thought to have been the cause
  • Plant pathogen dubbed HERB-1 now identified as cause of Irish potato blight
  • Specimen held at Kew Gardens allowed scientists to decode the genome
  • HERB-1 originated in Mexico during the Spanish conquest of Inca Empire
  • It has now died out because of the use of disease resistant potato plants

The unique strain of potato blight which triggered the Irish potato famine in the mid-19th century has been identified by scientists.

The plant pathogen dubbed HERB-1 has been identified from potato samples collected in 1847 at the height of the famine.

The specimen, held at Kew Gardens, allowed scientists to decode the genome of a plant pathogen and its plant host from dried plant samples.

The unique strain of potato blight which triggered the Irish potato famine in the mid-19th century has been identified by scientists and named HERB-1

The unique strain of potato blight which triggered the Irish potato famine in the mid-19th century has been identified by scientists and named HERB-1

Molecular biologists from Europe and the U.S. reconstructed the spread of the potato blight pathogen from dried plants which, although 120 to 170-years-old, had many intact pieces of DNA.

The researchers found that the strain of blight originated in Mexico during the Spanish conquest of the Inca Empire and evolved until it caused the massive crop failure in Ireland three hundred years later.

It has since died out after new disease resistant potato crops began to be planted at the turn of the last century.

Dr Kentaro Yoshida from The Sainsbury Laboratory in Norwich said: ‘Preserved plant samples represent a rich and untapped source from which we can learn a tremendous amount about the historical distribution of plants and their pests - and also about the history of the people who grew these plants.

The researchers examined the historical spread of the fungus-like oomycete Phytophthora infestans, known as the Irish potato famine pathogen.

A strain called US-1 was long thought to have been the cause of the fatal outbreak but the researchers concluded a strain new to science was actually responsible.

A strain called US-1 was long thought to have been the cause of the fatal outbreak but the plant pathogen dubbed HERB-1 has now been identified from potato samples collected in 1847 at the height of the famine

A strain called US-1 was long thought to have been the cause of the fatal outbreak but the plant pathogen dubbed HERB-1 has now been identified from potato samples collected in 1847 at the height of the famine

While more closely related to the US-1 strain than to other modern strains, HERB-1 is unique.

Hernan Burbano, from the Max Planck Institute for Developmental Biology in Germany, said: ‘We have finally discovered the identity of the exact strain that caused all this havoc.

‘Both strains seem to have separated from each other only years before the first major outbreak in Europe.’

The researchers compared the historic samples with modern strains from Europe, Africa and the Americas as well as two closely related Phytophthora species.

The scientists were able to estimate with confidence when the various Phytophthora strains diverged from each other during evolutionary time.

The strain of blight originated in Mexico during the Spanish conquest of the Inca Empire and evolved until it caused the massive crop failure in Ireland three hundred years later

The strain of blight originated in Mexico during the Spanish conquest of the Inca Empire and evolved until it caused the massive crop failure in Ireland three hundred years later

The HERB-1 strain of Phytophthora infestans likely emerged in the early 1800s and continued its global conquest throughout the 19th century.

Only in the 20th century, after new potato varieties were introduced, was HERB-1 replaced by another Phytophthora infestans strain, US-1.

The scientists found several connections with historic events.

The first contact between Europeans and Americans in Mexico in the sixteenth century coincides with a remarkable increase in the genetic diversity of Phytophthora.

The social upheaval during that time may have led to a spread of the pathogen from its centre of origin in Toluca Valley, Mexico. This in turn would have accelerated its evolution.

The potato blight specimen, held at Kew Gardens (pictured), allowed scientists to decode the genome of a plant pathogen and its plant host from dried plant samples

The potato blight specimen, held at Kew Gardens (pictured), allowed scientists to decode the genome of a plant pathogen and its plant host from dried plant samples

The international team came to these conclusions after deciphering the entire genomes of 11 historical samples of Phytophthora infestans from potato leaves collected over more than 50 years.

Because of the remarkable DNA quality and quantity in the samples, the research team could evaluate the entire genome of Phytophthora infestans and its host, the potato, within just a few weeks.

Dr Yoshida said: ‘Perhaps this strain became extinct when the first resistant potato varieties were bred at the beginning of the twentieth century.

‘What is for certain is that these findings will greatly help us to understand the dynamics of emerging pathogens. This type of work paves the way for the discovery of many more treasures of knowledge hidden in preserved plant samples.’

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