AgronomyThe Potato Murrain - The Birth of Modern Plant Pathology

The Potato Murrain – The Birth of Modern Plant Pathology

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A short history of late blight, and the beginnings of the agri-chemical market.

Because of both the economic and climatic conditions in Ireland in the eighteenth century, the potato quickly became the main food for the large Irish farm families who each had only a small amount of land from which to extract a living. The dependence of the Irish population on the potato became so great that when the late blight disease attacked the crop for several successive years in the 1840s, the result was the catastrophic Irish Potato Famine, or An Gorta Mór, “The Great Hunger,” as it is known in Ireland. The famine resulted in the deaths of more than one million people from starvation and associated diseases such as cholera and typhus. At least another million Irish refugees emigrated, mostly to North America.

The Birth of Modern Plant Pathology

The first alarm bell of the calamity was sounded by Dr. John Lindley, editor of the prestigious The Gardeners’ Chronicle and Agricultural Gazette in his August 23, 1845  editorial. Three weeks later, he made another dramatic announcement. Lindley was obviously well aware of the impending disaster when he asked, “Where will Ireland be, in the event of a universal potato rot?” The subsequent pages of the Gardener’s Chronicle contain much discussion about the perceived cause of, and remedy for, the “Potato Murrain.”

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If there is a silver lining in this disastrous famine, it was the debate among scientists about the cause of this plant disease, which eventually led to the birth of modern plant pathology. At that time it was thought that diseases were caused by spontaneous generation rather than by microorganisms. Regarding late blight, there were several different views about its cause. Some opined that the disease was caused by the devil; others that it was a punishment from God. Yet others thought that the disease was caused by the “puffing, hooting locomotives that thundered up and down the countryside at the unholy speeds of 20 miles per hour,” which were “discharging electricity into the air.”

A major debate ensued but Lindley, also a professor of botany, maintained that the “Potato Murrain” was caused by environmental factors such as rain. This position was quite common at the time; the germ theory of disease had not yet been proposed. It was not until the early 1860s that the French scientist Louis Pasteur proved that bacteria are the cause, rather than the consequence, of disease.

Lindley was opposed by Reverend Miles Berkeley, an amateur biologist who nevertheless made a monumental contribution to the understanding of plant disease. In his 1846 classic, Observations, Botanical and Physiological, on the Potato Murrain, Berkeley described how he had observed the fungus microscopically on potato leaves, and named it Botrytis infestans. Berkeley made the (at the time) amazing suggestion that this fungus might be the cause of the disease. This was revolutionary thinking. Both Lindley and Berkeley published their heated arguments in the Gardener’s Chronicle, but Berkeley was unable to provide further proof of his ideas.

The matter was eventually settled in 1861 by the German scientist Anton de Bary, the “father of modern plant pathology.” De Bary grew two groups of potato plants. One of these groups he infected with spores of diseased plants while keeping the other group healthy. The study yielded clear results: only the infected group became diseased. De Bary renamed the fungus Phytophthora infestans, the terrible plant destroyer.

We now know that professor Lindley was wrong in his views that the late blight fungus was a consequence rather than the cause of the disease. On the other hand, we now also recognize that the spread of late blight is greatly enhanced by environmental factors such as humidity and temperature. The cultivar “Lumper,” which was widely grown at the time, was devastated by the disease. Along with many other heritage cultivars, Lumper is now maintained by the Potato Gene Resources Repository in Fredericton.

Copper as a Fungicide

In the middle of the nineteenth century, the town of Swansea, Wales, had several copper smelters. The Gardener’s Chronicle of September 26, 1846 contained the following note from Matthew Moggridge, of Swansea:

“On the 31st Aug., I examined many pieces of Potatoes within the immediate influence of the copper smoke from the smelting works in this neighbourhood. There is no occasion perhaps to note the individual cases, but the general result is that the leaves, haulm, and tubers improve as you approach the works, and that the nearest gardens, little more than 200 yards from them, are entirely free from blight, and the crop good in quality, quantity, and flavor. The Potatoes are of different sorts. These last named gardens, as I am informed by the proprietor, entirely escaped the disease in 1845, and have borne Potatoes for 40 years. The Potatoes are also said to have escaped in the vicinity of the chemical works at Newcastle.”

Lindley accepted that the copper smoke effectively protected the potato crop, but he interpreted it as simply one environmental factor suppressing another, and/or “atmospheric influence.” Nevertheless, it is amazing that it would take another 40 years until it was finally realized that copper can control plant diseases such as late blight. This happened in 1882 in the Bordeaux region of France, where Professor Pierre-Marie-Alexis Millardet discovered that a mixture of copper sulfate and lime could be used to control downy mildew on grapes. Millardet soon realized that his “Bouillie bordelaise” could also be used to control late blight in potatoes.

However, several more years went by until the Bordeaux mixture was used to control late blight in the British Isles. The now famous Bordeaux mixture was the first agricultural chemical application on a global scale, used on potatoes and several other crops including grapevines, and thus served as a catalyst in the development of the agro-chemical industry. It is still in use today, sometimes marketed as “Bordo,” especially in organic potato production and for use in home gardens.

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Hielke De Jong is a retired Agriculture and Agri-Food Canada potato breeder, consultant and co-author of The Complete Book of Potatoes: What Every Grower and Gardener Needs to Know.