Which organism caused the potato late blight outbreak that began in Europe in 1845?

The potato late blight outbreak that began in Europe in 1845 was caused by the water mold Phytophthora infestans.12

Although often described as funguslike, Phytophthora infestans is an oomycete, a member of the phylum Oomycota, and is distinct from true fungi. The pathogen was introduced to Ireland from North America in 1845, where it thrived in the unusually wet summer conditions of that year. Because the Irish potato crop lacked genetic diversity, the organism was able to spread rapidly, leading to the Great Famine, which resulted in mass starvation, death, and significant emigration.23