By Dennis T. Avery


Center for Global Food Issues

Hudson Institute

Churchville, VA

Ireland is still dotted with roofless, stone-walled "famine cottages," abandoned since the 1840s, a time when 1 million Irish starved and another 1.5 million fled overseas.

The people had to either flee or starve because the crucial potato crop had failed. They were victims of the world's most vicious crop disease--potato late blight.

The blight was stopped in the 19th century when farmers found and planted blight-resistant varieties of potatoes and gave them the additional protection of chemical fungicides.

In recent years, unfortunately, more aggressive strains of potato late blight have spread around the world.

The Irish blight did not reproduce sexually. Today's late blight strains do, so they spread faster. By comparison, the blight strain that struck Ireland in the 1840s was only a pale, half-hearted shadow of the current threat.

Potato growers on five continents have been reeling under the blights' onslaught, and many Third World countries are almost as heavily dependent on potatoes as the Irish in the 1840s.

No crop

produces as much food per acre as potatoes. That is a big reason why world potato production has soared to nearly 300 million tons a year, with about 40% grown in the densely populated countries of the Third World.

Today, Asia and Africa are potato cultures, which makes the new strains of late blight a serious threat to the food security of millions of people. It has already cut potato production in poor countries by 15% and threatens to inflict even worse damage.

Late blight sporangia can produce infections within three or four days. Two days later each lesion can be producing 300,000 spores, spreading the disease miles in all direction.

Because of this explosive growth, a field of blight-stricken potatoes can turn from flourishing health to horrifying rot seemingly overnight. The plant leaves turn black and wither. Many of the potatoes look healthy, but turn into an oozing mess within a day or two of harvest.

First World farmers' best chemical weapon against the blight has been a fungicide named metalaxyl, but some Mexican farmers have to spray every three days to save their crops. This means the chemicals cost more than the potatoes will bring at the market.

of commercially viable potatoes carrying deep-seated resistance to all the strains of potato late blight.

These potatoes were bred by a method called recurrent selection, in which researchers crossbreed dozens of different potato clones and then recross their most promising offspring. Repeated several times, this recurrent selection allows the potatoes to accumulate many "minor" resistance factors.

Center scientists believe that a major gene carried the original resistance to late blight. Its narrow, one-gene base was eventually surrounded and overwhelmed by hordes of constantly mutating blight fungi. Researchers believe that it will be much more difficult for the fungi to overcome the new, multiple-resistance varieties.

to late blight."

Three sets of blight-resistant potatoes are being bred at two separate center sites in Peru, producing European-style potatoes for Africa and the First World; yellow-fleshed potatoes for parts of South America; and tropical potatoes produced by crossing the white and yellow varieties.

Potato farmers will need to combine the resistant potatoes with integrated pest management, including the elimination of all possible places where the blight can survive between growing seasons. That means digging up any potentially infected tubers missed at harvest and destroying all vines.

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