Tracing Trouble

The Irish Potato Famine almost two centuries ago might be history, but the late blight that caused it isn’t.

Scientists now say they may have traced the origin of the pathogen and gotten one step closer to stopping it.

A new PLOS ONE study suggests that the potato blight – a microorganism known as Phytophthora infestans – likely originated in the Andes Mountains in South America and later spread to North America and Europe.

P. infestans is a type of single-celled eukaryote – also known as oomycete or water mold – that destroys the leaves and edible parts of nightshade plants, like potatoes and tomatoes. In the 1840s in Ireland, the late blight attacked crops and led to the starvation of more than one million people in what is known as the Great Hunger.

Resistance develops in the same place where the host and pathogen evolved together, so tracing the origin of the potato blight is important to discover ways in which native plants can resist the infection. However, this task is becoming more challenging due to global trade, which is mixing different pathogen populations.

To conduct the study, researchers analyzed the genome sequence of the P. infestans and compared it with close relative pathogens only found in South America.

“By sequencing these genomes and accounting for evolutionary relationships and migration patterns, we show that the whole Andean region is a hot spot for speciation,” explained study corresponding author Jean Ristaino, defining speciation as the process in which a species splits into two or more distinct species.

Scientists have been previously split among those who believed the pathogen originated in Mexico and those who believed it came from South America, but this new study shows clear differences between Mexican and Andean late blight.

While there is gene flow from the Andes to Mexico and vice versa, the Mexican pathogens are the most recent, which supports historical accounts that, at the time of the Great Hunger, the potato blight originated in the Andean region.

These findings encourage the study of wild Andean potato species to learn more about resistance to the pathogen, not excluding methods like breeding and gene-editing, which have previously been effective.

However, Ristaino explains that as climate change increases drought in the higher Andean regions, there is a growing risk of losing some potato species before being able to determine if they can resist the late blight.