Strains of the disease that caused the Great Famine in Ireland have been traced to tomato crops in the Salinas Valley, researchers have found.

“It’s still a problem today,” said Frank Martin, a Salinas native and plant pathologist at the USDA, according to “It hasn’t gone away.”

Although the Irish strain has since died out, both potato or late blight and tomato blight are caused by a fungus-like, single-celled microbe called Phytophthera infestans, which thrives in wet environments and produces long-lived spores that travel in the wind.

Wayne Gularte, a tomato farmer for Rincon Farms in Gonzales, said that he lost about half of his crop this year to tomato blight. Just a little rain or fog can create the perfect environment for an outbreak. Gularte says there are preventative sprays that protect the fruit before it rains, but once the blight sets in, there is nothing a farmer can do to save his crop.

The original organism showed up suddenly in Belgium in 1845 and spread rapidly throughout the rest of Europe. Ireland's heavy dependence on potatoes meant the resulting famine was especially devastating. One million Irish people died and another million emigrated.

Martin worked with researchers at the Sainsbury Laboratory in the United Kingdom and the Max Planck Institute for Developmental Biology in Tübingen, Germany to find out where this devastating strain of potato blight originated.

By comparing entire DNA sequences from 11 historical strains and 15 modern strains of Phytophthera infestans and by looking at similarities and differences in their DNA sequences, the research group created a family tree of different blight strains.

The historical strains, which are like fossils in that they represent older forms of the organism, came from European herbariums that collected and preserved infected potato and tomato leaves from 1845 to 1896.  

Scientists believe the disease originated in Toluca Valley, Mexico. It traveled through the US in the 1800s and then jumped across the Atlantic Ocean to Europe in 1845. The organism that caused the Great Famine was a pandemic strain that spread throughout Europe and went extinct after potato breeding programs developed a tuber resistant to the organism.

The origin of modern strains is still unclear to researchers, although they do believe they also came from the US and then spread to Africa, Asia and South America.

About 120 different species of Phytophthera exist. Farmers lose over $6 billion a year on damaged crops and fungicide costs, and tomato growers in the Salinas Valley feel the effects of the blight keenly.

Martin is currently working on developing tests that will rapidly detect Phytophthera infestans as well as other related organisms that cause plant diseases worldwide.