The digging that got to the roots of potato blight

Have you dug your potatoes yet? As I write this I am packing for my holiday and wondering if blight will strike the potato plants in our absence. When I arrived in Ireland from urban New England, radio broadcasts warning of potato blight seemed a quaint throwback. Crop diseases are, of course, anything but quaint. As the world population skyrockets and the effects of climate change emerge, the looming food crisis is a global concern.

Famine has threatened in the recent past: the green revolution of the 1960s increased agricultural productivity (mostly through pesticides, fertilisers, plant breeding and irrigation) to avert an anticipated catastrophe. Scientists are now looking to genetic engineering to produce a similar increase over the next 40 years. Of key concern is developing resistance to disease, which, as every student of Irish history knows, can devastate crops and thus people.

One lesson not learned from the famine was that a monoculture (where vast areas cultivate a single species of plant) is particularly vulnerable to disease and pests. Instead, monoculture has become the cornerstone of industrial agriculture, the potential consequences of which have been averted only through the application of chemicals. Perhaps now is a good time to reconsider the history of the disease that changed Ireland: Phythophthora infestans.

A recent article in PNAS by Erica M Goss et al confirms the disease that caused the famine has its genetic origins in Mexico. Throughout the early 1840s the disease was prevalent in the US. It may have arrived in Europe when agriculturalists sought to improve European potato varieties by importing plants from South America.

There is some suggestion the disease was in Europe before the 1840s, but in 1845 it emerged in virulent form. In summer 1845 diseased potato plants were noted with alarm in Belgium and some scientists identified the cause as a fungus. Others believed the disease to be the result of poor weather conditions (cool and damp). The fungal theory was eventually dismissed in favour of environmental explanations or ideas about the declining vitality of common potato varieties. While scientists debated, potatoes rotted. The effects here can hardly be overstated: death from starvation and disease, and mass emigration, changed Ireland in ways historians are still exploring.

Contagion recognised 

Despite numerous scientific commissions across Europe, it was not until the experiments of Anton de Bary in the 1850s that the fungal theory gained ground again. De Bary was interested in the study of fungi as a contribution to the debate about the spontaneous generation of life, rather than as an application to improving agriculture. While the appeal of a contagious fungal disease to explain the spread of blight might seem obvious now, it was far from obvious in the 1840s.

Contagion was not the predominant explanation for the origin of human diseases. Robert Koch’s work in the century’s final decades eventually persuaded many scientists to embrace the idea of disease germs spreading between human bodies. The debate about the origins of the potato blight mirrored debates about the origins of human disease (explanations for cholera had included rotting matter producing “bad air” or the confluence of atmospheric and cosmological conditions). Likewise, potato blight was thought to arise from weak plants in poor weather and the decaying tubers sometimes seen as a cause and not a symptom.

Despite fungicides and blight-resistant potato varieties, P infestans is still with us. From October, Teagasc researchers will begin processing data accumulated over three years of field trials on a blight-resistant GM potato. Ewen Mullins, senior research officer at Teagasc, says one of the main scientific questions of interest was how blight would evolve in response to the potatoes' resistance.

As with all populations of pathogens, some P infestans may survive even in unfavourable conditions, and new varieties of blight may emerge that the GM potatoes cannot resist. Strains of P infestans have probably been conducting this dance between pathogen and host since long before the first potatoes were brought to Europe more than 400 years ago. Gaining the lead will be crucial to securing food supplies for future generations.

This article was inspired by Mary Mulvihill, who died in June and who once told me she would like to do a history of the potato blight.

• Juliana Adelman lectures in history at St Patrick's College, Drumcondra