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WSU Computer Models Give Early Warnings On Late Blight

PULLMAN, Wash. — The disease that caused the Irish Potato Famine in 1845-1850 is stalking the world once again, and the fungus that causes it is meaner and nastier today than it was 150 years ago.

Dennis Johnson, a Washington State University plant pathologist who specializes in potato diseases, predicts severe late blight in this year’s Northwest potato crop.

The culprits are the weather and the presence of aggressive strains of Phytophthora infestans fungus.

Fortunately, scientists have given vigilant farmers the means to control late blight. Johnson has developed computer models that help predict when farmers should spray their fields with fungicides and take other precautions to avoid the disease, which can devastate a potato field in a matter of days.

Today [August 12] Johnson is in Rochester, New York, reporting to the American Phytopathological Society on his system for forecasting late blight outbreaks.

The plant pathologist’s computer models run on data from three automated weather stations in WSU’s Public Agriculture Weather System — PAWS for short.

Johnson runs the data through computer models and posts late blight warnings on an telephone answering machine in his Pullman office. Growers call 1 (800) 984-7400 to hear reports. Johnson updates reports as often as necessary, sometimes as frequently as twice a day.

In 1996 the machine received 1,512 calls.

Johnson developed his computer models after he and his WSU colleagues studied 25 years of weather and late blight data and found that the relative disease status of a crop in a given year can be predicted before 1 June. This is four-10 weeks after planting and 14 days before late blight has been observed in the Columbia Basin in any year.

This allows Johnson to alert growers in time for them to thoroughly monitor fields and spray fungicides.

Outbreaks correlate with cool, wet weather with humidity above 90 percent and temperatures of 45-70 degrees F. The number of rainy days in April and May, and in July and August, and total precipitation in May when daily minimum temperatures are above 41 degrees F also are good indicators of late blight risk.

Johnson says late blight in Washington usually begins near the Columbia River, either south or east of Prosser, and moves north as the growing season progresses.

Johnson says growers can reduce the risk of P. infestans infections by good management practices.

First, they need to buy seed potatoes that aren’t infected with the fungus. They need to eliminate volunteer potato plants that have survived in the field from the previous year and destroy potato tuber culls. Infected tubers are the main means of enabling the fungus to persist from season to season.

“Over-irrigation, or too many applications of water, can make a bad problem worse,” Johnson says. Potatoes are especially susceptible where sprinklers from adjacent fields overlap.

Fungus populations “explode” when leaf canopies close, creating an ideal micro-climate for the fungus. This occurs when leaves from plants in one row grow to the point they touch or overlap leaves from plants in adjacent rows.

Infections can be controlled by spraying fungicides, but only if applied before infection occurs.

Johnson says conditions this year favor the disease. So far late blight has appeared in several potato fields near Hermiston, Ore., and in about 30 in Washington’s Columbia Basin.

“Late blight is a community disease,” Johnson says. Farmers need to quickly report outbreaks to neighbors, and to WSU so other farmers can be protected. Unfortunately, some don’t want to admit that their fields are infected. This only delays treatment of adjacent fields.

Farmers also can reduce the risk of outbreaks by buying uninfected seed pieces and vigorously fighting volunteer plants.

Farmers are going to have to learn to manage for late blight because they’re going to live with it for a long time, Johnson says.

Total cost of managing late blight in the Columbia Basin in 1995 was estimated at $30 million. More than $18 million of that was spent to apply chemicals to control late blight.

In 1995, the last year for which data are available, Washington farmers harvested 80 million hundredweight of potatoes from 147,000 acres. They were Washington’s fourth most valuable agricultural commodity, behind apples, wheat and milk.

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Late Blight, The Disease

PULLMAN, Wash. — A hundred and fifty years ago a fungus dramatically changed history.

The fungus, Phytophthora infestans, caused the late blight that devastated world potato fields between 1845 and 1850. The most severe consequences were in Ireland where 1 million-1.5 million peasants starved to death and another 1.5 million – 2 million fled Ireland during the great famine. Emigrant included ancestors of John F. Kennedy, 35th president of the United States.

The fungus releases enzymes that dissolve plant tissue and fungus then absorbs it. Leaves and stems die and tubers rot in the ground or in storage.

Dennis Johnson, a Washington State University plant pathologist who specializes in potato diseases, says late blight can be remarkably rapid and destructive. Fields that appear healthy, but containing low levels of disease can be wiped out within days. Importantly, low levels of disease are very difficult to detect. This gives the fungus a head start.

Each lesion in a leaf can produce as many as 300,000 spores per day. Soft-rot diseases often invade potato tubers infected with P. infestans, resulting in “meltdown” of stored tubers. Entire storages must be discarded.

Scientists believe the fungus originated in Mexico, coming to the United States around 1840. The disease was first reported in the Northeastern United States in 1843.

Late blight first appeared in Europe in June 1845 when it was discovered in Belgium. By August it was reported throughout northwestern Europe and southern England. Soon after it was introduced to Asia, Africa and South America.

The disease resulted in the Irish famine for several reasons:

Potatoes (introduced to the British Isles from America in 1586, by Sir Walter Raleigh) had become the basic subsistence food of Irish peasants.

Crops were swiftly and utterly devastated by the disease.

Relief measures were inadequate.

Late blight in potatoes has been difficult at times to control since the Irish Potato Famine, but genetic changes in the population of the late blight fungus recently has made control many times more difficult, Johnson says.

A dramatic increase in late blight in potatoes was observed in the early 1980s in Europe, and subsequently in the Middle and Far East. By March 1996, the problem was so bad that the International Potato Center in Lima, Peru, called for a global initiative to combat late blight.

Johnson says two reasons apparently explain the late blight comeback. Strains of the fungus are more aggressive than previous ones that have migrated into Washington. They also have developed resistence to metalaxyl, a fungicide that potato growers have relied on to stop the fungus in its tracks. At the same time, the fungus has “discovered” sexual reproduction. A second type of P. infestans known as A-2 also has recently migrated to Washington. It is compatible for sexual reproduction with the older type, A-1.

Resistance to drugs is a growing problem throughout the biological world. Until recently late blight in potatoes has been caused by a mode of asexual reproduction. But, beginning in 1995, European scientists confirmed P. infestans is reproducing sexually in the Netherlands and in Poland.

Johnson is convinced that the fungus is reproducing sexually in Northwest potato fields as well.

“We haven’t seen sexual spores yet,” Johnson says, “but we have DNA fingerprinting data that says P. infestans is reproducing sexually here.”

That’s bad news for potato growers because sexual reproduction introduces new gene combinations from sexual spores. This introduces more genetic variety in the fungus and that makes it more difficult to control late blight.

Developing resistant potato cultivars becomes more difficult and more aggressive strains of the fungus may result from sexual reproduction.

Spores created by sexual reproduction also help the organism survive adverse conditions such as freezing and drying, and to survive without a host of living plant material.

New strains are colonizing rapidly. One new strain, called US-8, was detected only in New York in 1992 and in Maine in 1993. In 1994 and 1995 it was found in 23 states, including Washington, Johnson says.

Even with the tools of modern science at hand, late blight can be devastating. In 1993 a farmer in the Northeastern United States saw his production fall from 79 tons per acre in 1993 to 14.8 tons in 1994. The value of his crop dropped from $1.2 million to $212,000 while his production costs went from $762,954 to $835,940.

Late blight was first reported in south central Washington during the 1947 growing season, which was unusually cool, cloudy and wet. The next report was 27 years later, in 1974. It has been observed in fields seven of the 16 years between 1974 and 1989.

From 1947 to 1990 the population of P. infestans in this area was composed of old clonal US-1 lineage. Resistance to metalazyl appeared in 1991 and A2 compatibility was detected in 1993.

Johnson believes metalaxyl-resistant strains of the fungus likely were introduced via infected potato seed brought to the Columbia Basin at the beginning of each growing season.

The scientist said 163,000 acres of potatoes in Washington’s Columbia Basin and north-central Oregon were affected by late blight in 1995. That was virtually every potato field in the region. Johnson estimated increased costs associated with coping with the problem at $30 million. Costs could run in the $20 million neighborhood this year.

Computer simulations predict 15 percent to 25 percent increased use of prophylactic fungicides may be necessary to control late blight in the U.S. and Canada.

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