Final answer:
The Irish Potato Famine of 1845 was caused by the pathogen Phytophthora infestans, which thrived in Ireland's climate and devastated the genetically uniform potato crop, leading to over a million deaths and a similar number of emigrations.
Step-by-step explanation:
The Cause of Late Blight in Ireland in 1845
The cause of late blight in Ireland in 1845, which led to the catastrophic Irish Potato Famine, was the oomycete Phytophthora infestans. This pathogen decimated the potato crops, which were a staple diet for the Irish population. The conditions in Ireland were well-suited to the growth and reproduction of P. infestans, and the lack of genetic diversity in the potato plants made them highly susceptible to the disease. The result was a devastating loss of life due to starvation, and a massive wave of emigration primarily to the United States. Over one million Irish died, and another million left the country. This period is marked as one of the worst tragedies in Irish history and was further exacerbated by ineffective and prejudiced British government aid at the time.