How did Phytophthora infestans reach Ireland in 1845?

Phytophthora infestans, the water mold responsible for the Great Famine, was introduced to Ireland from North America in 1845.1

While the exact mechanism of its arrival is often discussed in the context of global trade, the pathogen did not originate with the initial potato varieties brought to Europe in the 16th century. Once introduced in 1845, the mold found ideal conditions in Ireland’s unusually wet summer, allowing it to rapidly infect the country's genetically uniform potato crops. This lack of genetic diversity, combined with the population's heavy reliance on the crop, led to the catastrophic failure of the potato harvest and the subsequent humanitarian crisis.1