Why did late blight spread so quickly through Ireland’s potato crops?

The rapid spread of late blight through Ireland’s potato crops was driven by a combination of environmental conditions and a lack of agricultural diversity.

• Lack of Genetic Diversity: By the 1840s, the Irish population relied heavily on only one or two high-yielding varieties of potatoes. Because these crops were propagated from tubers rather than seeds, the plants were genetically identical. This lack of diversity meant there were no resistant strains to stop the disease from decimating the entire crop.123
• Favorable Weather: The pathogen responsible for the disease, the water mold Phytophthora infestans, thrives in cool, moist conditions. The summer of 1845 was unusually wet and cool, providing the ideal environment for the mold to flourish and spread rapidly across the fields.123

Once the blight was established, the identical vulnerability of the potato plants allowed the disease to exploit the crops ruthlessly, leading to catastrophic failures between 1845 and 1849.23