Plant pathologists studied the genomes of about 140 pathogen samples – historic and modern – from 37 countries on six continents to track the evolution of differing strains of Phytophthora infestans, a major cause of late-blight disease on potato and tomato plants. The study, published in Scientific Reports, shows that the historic lineage called FAM-1 was found in nearly three-fourths of the samples (73%) and on all six continents.
“FAM-1 was much more widespread than previously assumed, spreading from Europe to Asia and Africa along British colony trade routes,” says corresponding author Jean Ristaino, professor of plant pathology at North Carolina State University. “The lineage was also found over a span of more than 140 years.”
FAM-1 caused outbreaks of potato late blight in the United States in 1843 and then two years later in Great Britain and Ireland. It was also found in historic samples from Colombia—suggesting a South American origin.
FAM-1 caused massive and debilitating late-blight disease outbreaks in Europe, leaving starvation and migration in its wake. Ristaino theorizes that the pathogen arrived in Europe via infected potatoes on South American ships or directly from infected potatoes from the United States. FAM-1 survived for about 100 years in the United States but then a different strain of the pathogen called US-1 displaced it, Ristaino says.
“US-1 is not a direct descendant of FAM-1, but rather a sister lineage,” Ristaino says. “We found US-1 in 27% of samples in the study and they were found much later.”
Even more aggressive strains of the pathogen that originated in Mexico US-1 have since elbowed out US-1. Winter tomato crops—grown in Mexico and imported into the US – harbor the pathogen, Ristaino says.
The study also suggests that the pathogen spread first in potatoes and then later jumped into tomatoes. Spread of the pathogen in ripe tomatoes in ships’ holds would have been unlikely, Ristaino says.
The pathogen’s effects aren’t limited to the decimation of Ireland’s potato crop some 175 years ago. Billions are spent worldwide each year in attempts to control the pathogen, Ristaino says. Potatoes in the developing world are particularly vulnerable as fungicides are less available and often unaffordable.
