Final answer:
The Irish Potato Famine was caused by the late blight pathogen Phytophthora infestans in genetically uniform potato crops during favorable cool and moist environmental conditions, leading to mass starvation and emigration.
Step-by-step explanation:
The historic epidemics of late blight of potato in Ireland, which led to the Irish Potato Famine, can be understood in terms of the critical aspects of the host, the pathogen, and environmental conditions. The host, the potato (Solanum tuberosum), was grown in monocultures, meaning a single, genetically uniform variety susceptible to the blight was planted extensively. This lack of genetic diversity greatly facilitated the spread of the disease, which was caused by the pathogen Phytophthora infestans, a type of oomycete. Its severe impact was compounded by the environmental conditions of Ireland, which were cool and moist, ideal for the pathogen to thrive. The catastrophe resulted in over a million deaths due to starvation and forced another million to emigrate, mostly to the United States.