The rise and fall of the Phytophthora infestans lineage that triggered the Irish potato famine

TL;DR

This study uses genomic analysis of historical and modern strains of Phytophthora infestans to trace the lineage responsible for the Irish potato famine, revealing a unique 19th-century genotype HERB-1 that was replaced by US-1 in the 20th century.

Contribution

It identifies and characterizes the HERB-1 lineage as the cause of the Irish famine and clarifies its relationship to later strains like US-1, providing historical and evolutionary insights.

Findings

HERB-1 caused the 19th-century epidemic

HERB-1 persisted for over 50 years

HERB-1 is closely related to but distinct from US-1

Abstract

Phytophthora infestans, the cause of potato late blight, is infamous for having triggered the Irish Great Famine in the 1840s. Until the late 1970s, P. infestans diversity outside of its Mexican center of origin was low, and one scenario held that a single strain, US-1, had dominated the global population for 150 years; this was later challenged based on DNA analysis of historical herbarium specimens. We have compared the genomes of 11 herbarium and 15 modern strains. We conclude that the nineteenth century epidemic was caused by a unique genotype, HERB-1, that persisted for over 50 years. HERB-1 is distinct from all examined modern strains, but it is a close relative of US-1, which replaced it outside of Mexico in the twentieth century. We propose that HERB-1 and US-1 emerged from a metapopulation that was established in the early 1800s outside of the species' center of diversity.