Blight

Potato blight, caused by the microscopic fungus Phytophthora infestans, is by far the most important disease of potatoes, and it now occurs in every potato growing area of the world. In about 1840, the potatoes on a ship travelling from Mexico to New York mysteriously rotted. This ship was inadvertently carrying the blight fungus from its centre of origin in Mexico to the temperate Northern Hemisphere. Shortly after this, the cook on another ship, travelling from New York to Rotterdam, also found that all his potatoes were rotten. This ship carried the blight from the New World to Europe. The fungus then spread rapidly and, by 1845, it had reached Ireland.

The damage it caused was incredible, and potato crops all over Europe were reduced to a black, stinking mush. This was the first devastating plant disease known to history, and it was devastating because it was a 'new-encounter' disease. The potato had evolved and been domesticated in South America, while the fungus had evolved on botanical relatives in Mexico, and they had been brought together by people. The potato had very little resistance to the fungus.

In Ireland alone, about one million people died of starvation, and another one and a half million emigrated, mainly to North America. This loss of people reduced the population of Ireland by one third. It is thought that another million people died in the rest of Europe. The decade became known as "The Hungry Forties". A more detailed account is given in Return to Resistance (available as a free download at www.sharebooks.ca).

Then, after two or three years, the blight epidemics declined. For the next forty years, potatoes were cultivated all over Europe and North America. Although blight limited the yields, potatoes remained one of the most important food crops in the Northern Hemisphere.

The reason for this decline in the epidemics was the rapid and complete elimination of all the most susceptible clones of potato. Only relatively resistant clones remained. Furthermore, all new potato breeding automatically produced resistant clones, because every susceptible seedling was killed by blight. This was horizontal resistance, and it enabled massive potato cultivation to continue without any spraying with fungicides. Indeed, no fungicides were known at that time. Equally, no vertical resistances were known at that time.

In 1882, Millardet, in France, discovered the fungicide that he called bouillie bordellaise, or Bordeaux mixture, which consisted of a mixture of copper sulphate solution and newly slaked lime. He used it to control downy mildew of grapes, which had also been accidentally imported from the New World, and was threatening the wine industry with ruin. And it was quickly discovered that this fungicide would also control potato blight. This made potato cultivation easier and more profitable. But it ruined potato breeding. This was because the breeders could now spray their screening populations, and this made the breeding very much easier. All the most famous potato varieties were produced during this period which lasted another forty years.

Unfortunately, during this breeding, the accumulation of blight resistance not only stopped. It went into reverse, and horizontal resistance was gradually lost because of the 'vertifolia effect'. This loss of resistance was not recognised at the time and, even if it had been, it would have been considered unimportant, because the crops could be sprayed with Bordeaux mixture. The slow loss of quantitative resistance was not even noticed, and potato varieties bred after about 1885 became increasingly susceptible.

During World War I, Germany was critically short of many commodities but, in particular, copper was scarce. Copper was needed for making Bordeaux mixture. But it was also needed for making brass shell cases for the rifles and field guns. Because of the war, the military had priority, and the potato crops went unsprayed. The winter of 1917 was known as the 'turnip winter'. Germany lost the war mainly because of food shortages, and several countries decided that potato blight had military significance.

When the war was over, they began to breed potatoes for resistance to blight. And they used the most modern techniques available. They used single-gene resistances that obeyed the recently recognised Mendel's laws of inheritance. In a word, they used vertical resistance.

For the next forty years, there was great optimism, as scientists believed they would eliminate blight forever. But these unstable vertical resistances failed again and again. And, during the 1960s, the breeders decided that it was a waste of time breeding for vertical resistance to blight. They should have tried horizontal resistance but they did not, probably because they were so badly misled by various sources of error.

A notable exception to this rule was John S. Niederhauser, who was working in Mexico. He was the first scientist who deliberately avoided the use of vertical resistance and worked with horizontal resistance. He was very successful and, in 1991, he was awarded the World Food Prize, which is the agricultural equivalent of the Nobel Prize.

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