What caused the Great Famine in Ireland between 1845 and 1849?

The Great Famine in Ireland (1845–1849) was primarily caused by the arrival of Phytophthora infestans, a water mold that causes late blight in potato plants.12

The disaster was exacerbated by several critical factors:

• Genetic Vulnerability: By the 1840s, nearly half of the Irish population depended almost exclusively on the potato for their diet. Because the crop was propagated from tubers rather than seeds, the plants lacked genetic diversity, making them uniformly susceptible to the pathogen.12
• Environmental Conditions: The blight thrives in cool, moist weather. The unusually wet summer of 1845 allowed the mold to spread rapidly, rotting much of the crop in the fields.12
• Socio-Economic Factors: Many Irish tenant farmers lived at a subsistence level. When the crop failed, they could not pay rent, leading to mass evictions.12
• Inadequate Relief: The British government’s laissez-faire approach and ineffective relief policies failed to prevent widespread starvation and disease, resulting in approximately one million deaths and the emigration of up to two million people.12