The disease called late blight killed most of Ireland’s potatoes during its infamous 1840 famine. Today, it still costs Florida tomato farmers far more than potato farmers, resulting in millions of dollars each year in lost yield, unmarketable crop and control.

A University of Florida scientist has pinpointed Mexico as the origin of the pathogen that caused the 1840s Irish Potato Famine, a finding that may help researchers solve the $6 billion-a-year disease that continues to torment potato and tomato growers around the world.

For more than a century, scientists thought the pathogen that caused late blight originated in Mexico. But a 2007 study contradicted earlier findings, concluding it came from the South American Andes.

After analyzing sequenced genes from four strains of the pathogen, UF plant pathology assistant professor Erica Goss found ancestral relationships among them that point to Mexico as the origin.

“The pathogen is very good at overcoming our management strategies,” said Goss. “To come up with better solutions to late blight, we need to understand the genetic changes that allow it to become more aggressive. By understanding past changes, we can design new strategies that are more likely to be robust to future genetic changes.”

Goss and eight colleagues analyzed the genes of potato late-blight pathogens from around the world. Potato late blight, which flourishes in cool, damp weather, is caused by the pathogen phytophthora infestans.

Scientists sequenced four genes from more than 100 phytophthora infestans samples, plus four closely related species, to tease out the pathogen’s origin. Knowing the origin provides insight into its genetic diversity and the ways it adapts to different environments, Goss said

The pathogen also moved from other related species to the potato late in the evolutionary history of potatoes, she said, perhaps one reason potatoes are so susceptible to the disease and why finding a breeding-based solution to the disease has been so difficult.

Florida farmers lose millions each year due to late blight, said Gene McAvoy, Hendry County Extension director, who has monitored late blight in Southwest Florida for years.

A late-blight pandemic in 2009 made the pathogen a household term in much of the eastern U.S. It made its way to the Northeast via tomatoes in big-box retailers. After planting the tomatoes, many home gardeners and organic producers lost most, if not all, of their crop, Goss said.

“Just when we think we’re on top of it, a new strain shows up,” she said. “New strains have repeatedly appeared in the U.S. that are more aggressive or resistant to fungicides. This pathogen just keeps coming.”

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