How did late blight damage potato plants in Ireland?

During the Great Famine, the potato crop in Ireland was decimated by late blight, a disease caused by the water mold Phytophthora infestans. Because Irish farmers relied on a limited number of genetically identical potato varieties, the pathogen was able to spread rapidly through the vulnerable crops.123

The disease damaged plants in several ways:

• Foliage Damage: Lesions ranging from dark green to purplish black appeared on the leaves, stems, and petioles, often accompanied by a whitish growth of spore-producing structures.2
• Tuber Rot: The mold caused potato tubers to rot up to 15 mm deep.2
• Secondary Infection: Once the tubers were compromised, they were frequently invaded by secondary bacteria and fungi, leading to widespread decay during storage and transit.2

Thriving in the cool, moist weather conditions of the mid-1840s, the blight effectively destroyed the primary food source for the Irish population, leading to catastrophic starvation and mass emigration.123