Organisms known as oomycetes or water moulds sound innocuous enough. There are 700 or so species of them, and although they are referred to as fungi (as the 'mycetes' in their name implies), they are more closely related to kelps and diatoms than mushrooms. But these microorganisms can have global effects. Earlier this year, for instance, one species — Saprolegnia ferax — was implicated in the widespread decline in amphibian populations (J. M. Kiesecker et al., Nature 410, 681–684; 2001).
The most infamous member of the group is Phytophthora infestans, the agent of late blight of potato. In 1845 and 1846 it devastated the potato crop in Ireland, and one million people starved. The historical image here portrays the struggle for existence in the aftermath of the famine. The micrograph shows the infectious Phytophthora spores being released from structures known as sporangia.
By identifying a fungus as the cause of the epidemic, the Reverend Miles Berkeley broke ranks with his fellow clergy who blamed the devil. But where had the disease come from? In this issue, Jean Ristaino and colleagues describe how they have used modern methods to tackle the question (Nature 411, 695–697; 2001).
In the late 1980s and early 1990s, fungicide-resistant strains of P. infestans emerged, and potato blight is now recognized as a significant present-day menace to agriculture. Much of the genetic diversity in the species is found in the Toluca Valley of Mexico, where a variety of strains reproduce sexually. Elsewhere, until recently, most plant infections were caused by a single strain called US-1, which reproduces asexually and has spawned a single, extended clonal lineage. Given its contemporary distribution, plant pathologists believed that US-1 was a direct descendant of the fungus responsible for the Irish potato famine.
In a remarkable piece of molecular detective work, Ristaino and colleagues have isolated and analysed Phytophthora DNA from herbarium specimens of infected leaves collected during and after the famine. Mutations in the mitochondrial DNA of the pathogen serve as markers that distinguish between four of the most common clones. Amplification of these sequences from fragments of the dried leaves confirms Berkeley's identification, but shows that the Irish famine was not caused by US-1. This study also casts doubt on the idea that the fungus spread from Mexico, and shows that further genetic analysis will be needed to work out its recent evolutionary history. Locating the centre of origin of P. infestans should help in identifying resistant varieties of host plants and genes that might confer protection against it. As the ancestral home of the potato, South America may be a fruitful hunting ground.
The new findings mean that some ideas about the origin of historical plant disease epidemics will need to be re-evaluated. But this application of molecular archaeology should hearten herbarium directors who are fighting to preserve their treasured archives.
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Journal of Risk and Uncertainty (2015)
Plant Biotechnology (2007)
Mycological Research (2001)