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Late
blight research zeroes
in on a moving target |
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Scientists
worldwide have made
notable advances in combating
late blight, the most damaging
potato disease known. But as
this pathogen travels around
the world and evolves,
researchers are realizing that
they must be as adaptable
as their elusive enemy
to keep it at bay |
People thought they knew late
blight. It has an infamous history
as the disease that caused the
Irish potato famine, and entire
books have been written about
it. But recent studies on the
biology and population dynamics
of Phytophthora infestans—the
fungus-like organism that causes
late blight disease—have
demonstrated that the pathogen
has far more genetic diversity
than previously realized.
“Over the past three or
four years we’ve been
finding new forms of P.
infestans—what scientists
call isolates—that have
never been seen before,”
says CIP late blight project
leader Greg Forbes.
“The pathogen is adapting
faster than the control measures
used to combat it, and new approaches
are urgently needed.”
The picture is complicated by
global warming, which is opening
up new opportunities for P.
infestans in areas where
it was previously not a problem,
because low temperatures kept
it under control. |
A
history lesson
A review of late blight history
helps to understand the evolving
problem. Part of this pathogen’s
adaptability results from the
fact that it can reproduce either
asexually or sexually. To date
science has recognized two main
“mating types,”
commonly distinguished as “A1”
and “A2.” An emerging
theory has it that these two
types co-evolved with potato’s
wild relatives among the Solanum
species—which include
wild potatoes, tree tomatoes,
pear melons, and numerous weedy
species and woody vines—in
the Andes. This contrasts with
the commonly held view that
P. infestans originated
in the central highlands of
Mexico, where it is thought
to have “jumped”
to cultivated potatoes. “There
are many hypotheses about the
origins of the pathogen,”
says Forbes, “and the
evidence is still coming in.
The fact is that to date, we’ve
generated a lot more questions
than answers.”
At any rate, pathologists believe
that the A1 mating type traveled
from Mexico to northeastern
USA sometime around 1840. It
went on to Europe where, in
the late 1840s, it caused one
of the greatest famines in human
history. The A1 type eventually
made its way to Africa, Asia,
and back to South America. The
vehicle: potatoes being traded
and sold to meet worldwide demand.
It was not until the 1970s that
the A2 mating type reached Europe,
probably carried in a shipment
of potatoes imported from Mexico
to offset the effects of a major
drought. From there, history
was repeated as this “new”
form of the pathogen spread
around the world.
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Blitecast, a late blight forecasting model, has been linked with geographic information system
technology to help researchers estimate potential global severity of the disease (expressed as
the number of fungicide sprays required for control).
|
When A1 and A2 are present in
the same environment they can
“recombine” through
sexual reproduction. The result
is an explosion of new types
of the pathogen, which makes
it even more difficult to manage.
Almost all potato-growing countries
are now affected by the problem.
Even so, in North America and
Europe farmers are willing to
grow highly susceptible potato
varieties that fetch good prices
on the market, resorting to
chemicals to control the disease.
But as late bight evolves, these
farmers are being forced to
use increasingly large amounts
of fungicides, and to use them
with greater frequency. What’s
more, a class of chemicals that
used to be considered invincible
is losing its effectiveness
in the face of the new disease
types.
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CIP
scientists are concerned
that Late Blight is “moving
up
the mountainside,”
becoming a
serious threat in places
where
it was rarely encountered
in the past. |
An expanding portfolio
The situation is even more complex
in the potato-growing areas
of the developing world, where
seasons, day-length regimes,
altitudes, and socio-economic
and agro-ecological conditions
are diverse, especially compared
with those found in industrialized
countries. “The solutions
used in the northern hemisphere
just don’t work here,”
says Pamela Anderson, CIP’s
Deputy Director General for
Research. “CIP has the
mandate for late blight research
in the tropics, and one of our
main goals is to reduce farmer
dependence on chemicals. This
makes the replacement of susceptible
varieties with more resistant
ones a pivotal point of our
late blight control programs.”
More than 20 developing countries—including
major potato producers such
as China, Peru, and Kenya—are
in the process of releasing
the latest lines of late-blight-resistant
potatoes produced by CIP plant
breeders in Lima (see Late
blight in China: A cause for
concern). Unlike early
late-blight-resistant populations,
these new potatoes carry multiple
resistance genes to help them
survive under high, and varied,
disease pressure. But breeding
is not a one-time fix, and there
is no miracle potato. Disease
resistance must not only be
matched with local require-ments
and preferences, it must also
be continually improved to keep
up with and withstand the evolving
forms of the disease (see Late
blight approaches a sexual frontier).
 |
| Late
Blight symptoms on tubers. |
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at
3,500 masl, Huanuco, Peru,
serves as a
good testing ground for
Late-Blight-
resistant varieties and
management
practices. |
In a similar manner, resistant
potatoes cannot do the job alone.
Fungicides are still needed,
but they need to be used rationally
to protect the environment,
human health, and the investments
of resource-poor farmers. A
recent study in developing countries
revealed that the number of
fungicide sprays used to control
the disease is often far more
dependent on purchasing power
than it is on best practice
recommendations. At the same
time, farmers’ decisions
to use resistant varieties may
be overturned by local consumer
preferences or by market considerations
that affect the supply of high
quality seed.
CIP scientists have made headway
by developing and adapting integrated
control programs using the farmer
field school methodology. In
these programs, variety introduction
is balanced with discovery learning
to increase farmers’ understanding
of control options that will
enable them to use chemicals
sparingly while protecting profits
and productivity. Field schools
have not only helped speed up
location-specific selection
and introduction of new varieties,
farmer input has also contributed
to reorienting ongoing breeding
research.
Modeling research has also been
integrated into CIP’s
late blight portfolio. For instance,
tradeoff modeling is helping
farmers to visualize how they
can make better decisions about
optimum use of pesticides and
avoid unnecessary health risks
(see CIP’s Annual Report
2001). At the same time, disease
forecast models are increasing
researchers’ understanding
of relative late blight severity
in the diverse agro-ecological
areas of the developing world,
where information of this sort
is scarce. The data will serve
as a guide for allocation of
resources to the areas where
they can make the biggest difference
in production, food security,
and poverty alleviation.
Recombining research
“We are just beginning
to scratch the surface,”
says Forbes. ”We need
to move quickly because the
picture is changing rapidly
and there are new variables,
like climate change, that need
to be factored in. Basically,
we need to expect the unexpected.
Innovative kinds of research—like
modeling and pathogen studies—will
help. The question is, how can
we do it with the resources
now at our disposal?”
Partnership will help to achieve
some of these objectives. Simulations
of management tactics and scenarios
are being conducted, for example,
through strategic alliances
with researchers at Israel’s
Volcani Institute, the Brazilian
Agriculture Research Corporation
(EMBRAPA), Plant Research International
in the Netherlands and the USA’s
Cornell University. The models,
which allow scientists to process
huge amounts of information,
are helping researchers visualize
how variables such as climate,
socio-economic conditions, and
local preferences can make or
break a control strategy. The
Global Initiative on Late Blight
(GILB), a worldwide network
of researchers, technology developers,
and agricultural knowledge agents,
lends communication and information
support to these initiatives.
At the core of the problem,
nonetheless, is the understanding
of the elusive Phytophthora
infestans pathogen and
the way it reproduces and interacts
with its host, and the fact
that this information is still
incomplete. Continued studies
in population dynamics are fundamental
if scientists are to fill the
gap. Forbes notes, however,
that it will take at least three
years of additional research
and more than US$1 million to
fully understand the dynamics
of the late blight pathogen
in the Andes alone. Special
project funding for population
studies has thus far been provided
by the Netherlands Ministry
of Agriculture, Nature Management
and Fisheries (LNV), the Swiss
Agency for Development Cooperation
(SDC), and the United States
Agency for International Development
(USAID).
“Late
blight is CIP’s biggest
challenge,” says Pamela
Anderson. “It is also
our biggest opportunity. It
gives us a chance to show how
all that we are doing—in
conservation, characterization,
and breeding; integrated crop
management and systems analysis;
and partnerships for development—can
fit together TO MAKE A DIFFERENCE
IN PEOPLE’S LIVES AND
LIVELIHOODS.” |
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Late
blight approaches a sexual
frontier
Working from new data
sets and reports of increasing
late blight damage in
highland areas of the
Andes, CIP pathologists
believe that the emergence
of previously unknown
forms of the late blight
disease—and its
appearance in areas previously
unaffected—could
have significant consequences
for ancient potato varieties
and the farmers who grow
them. Their concern is
focused on the Lake Titicaca
region and surrounding
areas. This region is
thought to be the potato’s
genetic center of origin,
a theory borne out by
the significant diversity
of potatoes found there.
“What we are
seeing is the convergence
of two mating types, one
moving south from Colombia
and Ecuador, and the other
coming up through Bolivia
from Brazil,” says
CIP pathologist and late
blight project leader
Greg Forbes. “Our
fear is that farmers in
the high Andes—the
people who have served
as the traditional custodians
of potato biodiversity—may
lose native varieties
that have been grown for
many centuries, and thereby
their means of survival.”
Local consumers hold these
potatoes in high esteem.
Not only are their varied
tastes, textures, and
colors a source of culinary
diversity, native potatoes
are also important in
traditional culture and
are often used in ceremonies
or as gifts.
Maria Scurrah, a CIP adjunct
scientist who has spent
years working with farmers
in the high Andes, can
testify that this is no
longer just a theoretical
problem. “Late blight
is encroaching on areas
that were rarely affected
by it in the past. Essentially,
the pathogen is moving
up the mountainside, showing
up in places where farmers
have hardly ever encountered
it.”
Biodiversity fights back
“Traditional
varieties are not going
to disappear because of
late blight,” says
CIP potato breeder Juan
Landeo, “but it’s
likely that they will
be under greater pressure
than in the past.”
Landeo bred one of Peru’s
most popular and widely
grown potatoes, known
as Canchan. Depended on
for years as a late-blight-resistant
variety, Canchan’s
ability to withstand the
disease has, in recent
years, broken down. To
help farmers cope, Landeo
has developed a new series
of blight-resistant potatoes,
suitable for production
under extreme highland
conditions.
The new “populations”,
now ready for selection
and release, were derived
from materials of the
andigena subspecies
collection held in CIP’s
genetic resources complex
in Lima. The genebank
safeguards about 85 percent
of all known native potato
varieties, including 15,000
farmer-selected andigena
potatoes collected in
nine countries during
the 1970s and 1980s. The
CIP genebank collections,
which also include sweetpotato
and other Andean roots
and tubers, are protected
under an agreement with
the UN Food and Agriculture
Organization that charges
the Center with conserving
genetic resources so as
to make them available
equitably and without
restriction. CIP uses
these materials, for instance,
to help preserve the diversity
of native varieties in
the Andes through restoration
programs (see Next
steps for Chayabamba).
Bred over a 12-year period
using conventional plant
breeding techniques, the
new andigena
plant types carry multiple
late blight resistance
genes, which should help
them compete against many
forms of the disease.
Most native Andean varieties
belong to the subspecies
andigena, but generally
lack such resistance.
For this reason, the search
for the resistance traits
incorporated in the new
varieties involved a long
and careful process of
screening and selection.
The new materials have
some added advantages:
they produce higher yields
than conventional varieties
in less time, a characteristic
that should reduce their
exposure to the disease
in farmers’ fields,
while offering most of
the eating and market
characteristics valued
by highland farmers.
“What we’ve
tried to do is breed a
highland-type potato that
has most of the qualities
that will make it acceptable
to processors and allow
it to compete in urban
markets,” Landeo
says. It is Landeo’s
hope that these new andigena
potatoes, now being distributed
in the Andes through farmer
field schools, will eventually
enable people in Africa
and Asia to enjoy the
special taste and texture
of native Andean potatoes.
Because of their unique
features, they are much
better prepared to adapt
to areas outside of their
Andean home than their
native relatives. |
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Next
steps for Chayabamba
The potato
farmers of Chayabamba,
an Andean community
some 300 kilometers
east of the old
Inca capital of
Cusco and more
than 4,000 meters
above sea level,
are facing a period
of consider-able
uncertainty. Late
blight not only
cost them last
year’s potato
harvest, it also
devastated the
stores of planting
materials needed
to sow next year’s
crop.
As a coping strategy,
Chayabamba farmers
would normally
try to borrow
seed from nearby
communities, but
neighboring farmers
may have also
suffered losses
and have little
seed to spare.
A second possibility
would be to purchase
seed on the open
market. Commercial
seed suppliers,
however, are unlikely
to have the varieties
that farmers need
and want, and
there are few
guarantees that
purchased seed
will meet adequate
quality standards.
The CIP genebank
is poised to help
by providing local
communities with
“starter
seed” for
their rebuilding
programs. “One
of the principal
functions of a
genebank is to
guarantee that
traditional varieties
survive. When
disaster strikes,
as it did last
year in Chayabamba,
we are there to
help,” says
Willy Roca, head
of CIP’s
genetic conservation
project.
Seed return programs
are not just the
right thing to
do, Roca adds,
they are also
the smart thing
to do. “We
not only provide
planting material,
we also work with
local NGOs and
community groups
to multiply seed
at locations close
to where it will
be needed.”
Future restoration
efforts will be
aided by the development
of community field
genebanks, closely
linked to the
CIP collections,
in strategic microcenters
of genetic diversity.
According to Roca,
“Traditional
brick and mortar
genebanks with
their cold storage
rooms are a last
line of defense.
If you want to
maintain genetic
diversity and
encourage evolution,
your best option
is help farmers
grow traditional
varieties in the
fields where they
first evolved,
rather than in
test plots at
an experiment
station.” |
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