St. Patrick’s Day, March 17, is a popular celebration in the United States, due to the number of Americans, 10.5%, with Irish heritage. One million Irish emigrated to North America, Australia, or other parts of Great Britain in the mid-1800s because of the potato disease now known as late blight. Late blight, caused by the water mold, Phytophthora infestans, destroyed the Irish potato crops in 1845 through 1849 and caused the Irish Potato Famine. Another one million people died from hunger or disease.
The irony of the famous story of the Irish Potato Famine is that the United States was the likely source of late blight. In a 2016 study, DNA fingerprints were generated for P. infestans samples preserved in 48 dried, diseased potato, tomato, or nightshade leaves collected between 1846 and 1939 in the United States (36 samples) and Europe (12 samples). Over three-quarters of the samples had the identical fingerprint, which suggests that an 1843 to 1844 outbreak of late blight in the eastern United States was the source of the Irish outbreak that started in 1845. It’s very likely that diseased potatoes shipped from the United States to Europe carried late blight, but it’s not known if diseased potatoes or airborne spores spread P. infestans from mainland Europe to Ireland.
The oldest known late blight sample was collected in Ireland but not from potato. Late blight also affected a shrub called red-striped yellow tailflower, Anthocercis ilicifolia, that, like potato, is a member of the nightshade family. Even more surprising is that the shrub is native to Australia, not Ireland.
The Irish Potato Famine is a good reminder of how trans-oceanic movement of plants can also move pests and diseases.
For more information on late blight, see HGIC 2217, Tomato Diseases & Disorders.