Potatoes, Once Again, Under Fungal
Plant physiologist John Helgeson examines potatoes growing at the University of
Wisconsin's research station at Hancock. Resistant to late blight, this hybrid
is also being used in breeding trials at the ARS Vegetable Laboratory, in
In ancient times, farmers danced, chanted, and even invoked spells to
protect their crops from the ravages of pests and disease.
Fortunately, today's growers are armed with more knowledge about crop
diseases and how to control them. Integrated pest management and pesticides
greatly increase the odds of winning the war against plant diseases. But as
time has brought better weapons, it has also brought more devastating diseases.
"Late blight caused by the fungus Phytophthora infestans is an
excellent example," says plant pathologist/microbiologist
Kenneth L. Deahl, who is with the
ARS Vegetable Laboratory
in Beltsville, Maryland. "It can destroy a potato crop in a matter of
"And new strains of the fungus that are now attacking potato crops
throughout the world are far more difficult to control than the strain that
struck in the 1800s," says Deahl. When this fungal plant disease attacked
Ireland's potato crop, half a million people starved, and another million
emigrated to North America.
Despite modified cultural practices and fungicides designed to slow the
blight down, the new, more severe strains of late blight have spread throughout
the world in just 6 years. These strains have sexually produced spores that can
live in infected stems, tubers, and soil over winter and be infective the next
According to Neil Anderson, University of Minnesota plant pathologist, the
more virulent strains of late blight produce spores on plant stems, while the
original strains sporulated only on plant leaves. "In the 25 years that
I've worked with late blight, I've never seen blight attack tubers like these
new strains do," he says.
The new strains of late blight that have appeared in the United States are
called US-6, US-7, and US-8, while US-1 is an A1, original strain that can be
controlled with the chemical metalaxyl.
Plant pathologist Ken Deahl examines a potato damaged by late blight
Unlike A1, the new A2 strains aren't deterred by this chemical or
imperfectly applied cultural practices, such as immediately culling and
destroying infected potatoes.
Since the A2 strains are resistant to metalaxyl, the
Environmental Protection Agency allowed
emergency use of three other chemicals against the disease in 1995. Although
that helped, isolated epidemics of new, aggressive strains of late blight
occurred in 1995 and 1996.
In November 1996, plant pathologist Robert W. Goth and plant geneticist
Kathleen G. Haynes, who are also based at the ARS Vegetable Laboratory,
released two potato breeding selections that resist the most virulent strains
of late blight.
In addition, Goth, along with colleague Judith Keane, has developed a way
to test potato leaves for resistance to late blight. Up till now, the only way
to find out how well a plant would do was to infect it in field plots and watch
for symptoms. This procedure risked spreading the highly contagious blight
throughout an entire test area.
"Because late blight had been controlled with chemicals since the
middle of the 20th century, breeding for resistance to the disease was not a
top priority in the United States," says Haynes. "And all major
potato-producing areas of the United States had a blight forecasting and
chemical spray program based on weather-oriented models. The fungus thrives in
cool, damp weather."
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U.S. Department of Agriculture efforts
to breed potatoes for race-specific resistance to the disease began in the
1920s. And former ARS plant breeder Ray Webb began the ARS Vegetable
Laboratory's breeding program for field resistance to late blight in 1976.
"The two resistant breeding selections that we released were the result
of three generations of plant crosses," Haynes says.
"Initially, this germplasm had shown resistance to the severe strains
of late blight found in the Toluca Valley, Mexico, but it did not have other
characteristics needed to be commercially acceptable."
When grown in the field, the plants produced irregularly shaped potatoes
that wouldn't process well into chips or fries. Since Haynes and Goth weren't
increasing the level of resistance to the disease with successive breeding, the
scientists decided to release the germplasm to other breeders, who they hoped
would combine desirable processing and fresh-market characteristics with the
late blight resistance in the two ARS selections.
Potatoes infected with late blight are purplish and shrunken on the outside,
corky and rotted inside.
Last year, Haynes gathered 17 potato clones that were reported to have some
resistance to late blight. From them, she produced virus-tested plantlets in
tissue culture so that minitubers could be distributed to State scientists at
eight U.S. locations. The clones came from research programs by ARS, university
(Cornell, Minnesota, and Colorado State), and European sponsors.
Haynes worked with scientists from the Universities of Florida, Maine,
Minnesota, and Wisconsin, and from Michigan State, North Dakota State, Penn
State, and Cornell Universities. They evaluated the level of resistance at each
location and ranked the clones from 1 to 17, with 1 being the most resistant to
late blight and 17, the least.
"Of the top four clones that showed the most blight resistance, three
came out of Beltsville and one from the
ARS potato breeding
program at Aberdeen, Idaho," Haynes reports. "We only released
two of the Beltsville clones and plan more work on the third."
Neil Anderson and his University of Minnesota colleague, Vergel Concibido,
field-tested the clones.
"Since we've only had the A2 strain in Minnesota for the last 4 or 5
years, we tested the new clones at our Rosemount Agricultural Experiment
Station, which is 80 miles from our potato-growing region," Anderson
reports. "The new material showed good resistance to the disease and
stayed green when other clones were dead from blight. We also had good results
from the clones developed at Aberdeen."
Anderson and Concibido planted the experimental clones close to Norchip, a
commercial chipping potato variety, and inoculated Norchip with the severe
strains of late blight so the fungus could spread naturally. Anderson reports
several important findings from the research.
To ensure continuing worldwide availability of this valuable
food staple, potato plant breeders must unite desirable processing and
fresh-market characteristics with late blight resistance.
"Even though some of the clones we evaluated are somewhat susceptible
to the disease, they would still require less chemical sprays than commercial
varieties," he says.
"This would save growers money and also help cut down on the amount of
chemicals released into the environment."
Roger Jones, extension plant pathologist at the University of Minnesota,
says that the major problem in trying to control new strains of P. infestans is
that preventive spraying of pesticides is necessary about every 5 days.
Historically, he says, growers sprayed for blight an average of once or twice a
season, and that spray regimen worked. But for the new strains, even increasing
the number to 8 or 10 applications doesn't always work. So not only are growers
incurring more costs, they're putting more chemicals into the environment and
Late Blight Economics
Just how devastating are these new strains of late blight?
The International Potato Center in Lima, Peru, estimates late blight losses
at about $3 billion annually worldwide. In the United States, losses over the
past several years are estimated in the hundreds of millions. Potato growers in
Washington and Oregon alone lost $30 million in 1995, says Cornell University
plant pathologist William E. Fry.
"A good example is a single potato grower in New York who lost $1
million to the new strains of the disease in 1994," says Fry.
"Despite a doubling of pesticide expenditures, the disease cut that
grower's marketable yields by 80 percent. In addition to defaulting on three
supply contracts, the grower had to dispose of 4,090 metric tons of rotting
potatoes in an environmentally acceptable way. Needless to say, he is no longer
"The rate at which these exotic strains of P. infestans spread and the
severity of the epidemics they produce are astounding," Fry says.
"The new strains appeared in the eastern United States beginning in 1992.
By 1996, immigrant strains had become established in most of the United States
ARS plant geneticist Kathleen Haynes and plant pathologist Robert Goth have
released two potato breeding selections that resist the most virulent strains
of late blight.
Test Quickly Measures Resistance
A test developed by Goth and colleague Judith Keane could help slow the
onslaught. It can determine in just 6 days if a plant can resist the
originalas well as exotic--strains of late blight. In addition to potato
plants, it works on tomato plants, which are also a victim of late blight.
By simply detaching leaves from plants and subjecting them to the pathogenic
fungus, the scientists can tell if a plant has resistance.
"Testing late blight resistance of plants in a field requires
introducing the pathogen and chancing the risk of infecting an entire growing
area," Goth says. "Our test can be conducted on individual leaves in
a greenhouse or in a lab. The detached-leaf technique is not plant destructive,
and leaves from the same plant can be used to test for other pathogens."
Unlike field testing, this method is not weather-dependent and can be done
at the convenience of the plant breeder or researcher.
Potato growers worldwide anxiously seek some relief from this
seemingly invincible foe. "Having resistant breeding selections and a way
to test disease susceptibility are a start," Goth says.
By Doris Stanley, Agricultural Research Service
G. Haynes are at the USDA-ARS Vegetable Laboratory, Building 010A BARC -
West, 10300 Baltimore Ave., Beltsville, MD 20705; phone 301-504-7380.