Why did cool and moist weather in 1845 worsen potato blight in Ireland?

The potato blight that triggered the Great Famine was caused by Phytophthora infestans, a water mold that thrives in humid conditions with temperatures between 4 and 29 °C (40 and 80 °F).12

In 1845, the arrival of this pathogen coincided with an unusually cool and wet summer in Ireland. These specific environmental conditions were ideal for the mold's rapid development and spread:13

• Optimal Growth: The moisture and cool temperatures allowed the pathogen to thrive and reproduce efficiently, as hot, dry weather typically checks its spread.12
• Rapid Infection Cycle: Under these conditions, the mold could produce new spores and infect healthy foliage within four to six days, repeating the cycle as long as the cool, moist weather persisted.2
• Vulnerability: Because the Irish potato crop lacked genetic diversity, the mold was able to exploit the identically vulnerable plants, causing widespread rot in the fields.13

This combination of a susceptible, uniform crop and favorable weather allowed the blight to establish itself, leading to catastrophic agricultural failure.3