This article will discuss the significance of Phytophthora infestans in relation to the Irish famine of the mid-nineteenth century. Phytophthora infestans is an organism that infects potatoes and tomatoes and is the cause of the potato famine that invaded Ireland in 1845. The invasiveness and complexity of Phytophthora infestans were responsible for the spread and severity of the epidemic which caused emigration of mass and death. Ultimately, Ireland was ill-prepared economically, socially and intellectually to control the Phytophthora infestans infestation of the mid-nineteenth century. Phytophthora infestans, also known as P. infestans, is an oomycete and host-specific parasite (Shumann and D'Arcy, 2000). P. infestans primarily infects plant tissue of potatoes and tomatoes, causing irregularly shaped lesions to develop on the leaves of the plant. Potatoes infected with P. infestans develop large black lesions on the skin of the potato that make it slimy, soft and completely inedible. The damage that P. infestans inflicts on the potato was a major problem in mid-19th century Ireland because half the country relied on the potato for its livelihood (Mintz and McNeil, 2014). Many Irish farmers could not afford to rent many acres of land, so the potato was the ideal crop because one acre of potatoes could support a family for a year (Mintz and McNeil, 2014). The arrival of P. infestans eliminated almost half of Ireland's food supply, causing nutritional deficiencies that led to other diseases such as scurvy and death (Mintz and McNeil, 2014). Ireland's dependence on the potato ultimately increased the severity of the Irish famine.P. infestans breeds and spreads easily in a moist, cool environment...... middle of paper......the list of victims and emigrants suggests that countries other than Ireland could never be prepared for the uncontrollability of P. infestans in the mid-19th century. The devastation caused by P. infestans in Ireland in the mid-19th century was too immense for Ireland to cope with. The invasiveness of P. infestans was responsible for mass emigration and death. The complexity and aggressiveness of P. infestans suggests that a potato disease of similar scale to the Irish one would cause as much devastation if it occurred in modern society as research on P. infestans is new and emerging slowly. The potato blight not only exemplifies the instability of Ireland in 1845, but also demonstrates that a close relationship between science and history is important because the two areas of study allow for a different understanding of the environmental, medical and social issues of the past.