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How the humble potato could feed the world
http://www.newscientist.com/article.ns?id=mg19926671.600&print=true
8.8.1
Debora MacKenzie

FOR some reason, it's always called the "humble" potato. But the
tasty tuber from the Andes is poised to take over the world. As the
food crisis bites, the land area planted with potatoes is increasing
faster than for any other staple crop. Developing countries now grow
and eat more of them than the traditional potato-eaters of the rich
countries: today, the world's biggest potato producer is China, and
India produces twice as much by weight each year as the US.

Yet behind this success story lies a problem. The blight that wiped
out Ireland's potato crop in the 1840s is becoming more virulent and
is increasingly resistant to the fungicides used to control it.
Without a new weapon against blight, we could be setting ourselves
up for a replay of the famine wherever the disease strikes. And this
time even more people could suffer.

There are good reasons why the world is turning to potatoes. Much of
the world's food comes either from grain or animals fed on grain,
but rising populations and increasing demand for meat, dairy
products and biofuel means that global demand for grain is
outstripping supply. Grain yields must ultimately increase to meet
this demand but cranking up the global food system will take time,
and yields won't increase overnight. In many places potatoes can
plug the gap, providing food and income for the people who need them
most. "Worldwide we see an overlap between where the poorest live
and where people grow potatoes," says Pamela Anderson, head of the
International Potato Center (CIP) in Lima, Peru, part of the
Consultative Group on International Agricultural Research, which
works on crop improvement for poor countries.

Potatoes can squeeze in between grain crops, which means a field
yields three harvests a year instead of two. Since there is little
international trade in potatoes, their prices tend to be more stable
than those of grain. All these things have led the UN to dub 2008
the International Year of the Potato and to hail it as the "food of
the future".

In fact, listen to a potato enthusiast, and you may wonder why
people bother with grain at all. Potatoes are more nutritious,
faster growing, need less land and water and can thrive in worse
growing conditions than any other major crop. They provide up to
four times as much complex carbohydrate per hectare as grain, better
quality protein and several vitamins - a medium-size potato boiled
in its skin has half an adult's daily dose of vitamin C, for
example. They also contain B vitamins, plus many of the trace
elements poor people, and grain, lack. And, unless you douse them
with it, potatoes have almost no fat (see table).

Potatoes do have their downsides, of course. They are more
perishable than grain and because they are heavier and bulkier, they
are more expensive to transport - one reason why there is little
international trade. Their main weakness, though, is disease.

Potatoes are rolling in genetic diversity - there are some 150
species in the potato family and countless varieties. The problem
is, almost all potatoes grown outside the Andes are of a single
subspecies, Solanum tuberosum tuberosum, first cultivated 8000 years
ago in the highlands around Lake Titicaca. Keeping all our potatoes
in one basket leaves the world's crop vulnerable to being wiped out.

The most likely candidate to do this is late blight, which is what
destroyed the potato crops in Ireland and other parts of Europe in
the mid-19th century. It is caused by Phytophthora infestans, a
fungus-like organism called an oomycete, which spreads by producing
spores. The disease originated in Mexico, where it infects wild
potatoes, and spread north as American agriculture expanded in the
19th century. In 1845, it arrived in Belgium on seed potatoes
imported from the US.

The blight quickly spread across Europe, wiping out crops and
causing catastrophe in Ireland, where the damp, cool soils and
climate, plus the fact that the colonial landowners took the best
land to grow grain for export to England, made the Irish poor more
reliant on potatoes than other Europeans. Breeders eventually found
potatoes that partially resisted the blight, but the crop's future
was only secured when fungicides were invented in the 1880s. Now
potatoes are more dependent on chemical treatment than any other
crop. The potato industry in the European Union is worth ¬6 billion
a year; farmers spend a sixth of that on fungicide.

Farmers in developing countries can rarely afford to buy fungicide,
a big reason, along with the pervasive lack of fertiliser and water,
why average potato yields in African countries are half those in
China or Peru, which are in turn half those of rich countries.

Giving farmers in the developing world access to fungicides would
certainly increase yields, but it may not be enough to protect them
from blight, as the disease is becoming ever more resistant to
fungicides. "Last year I had to spray 12 times, the most ever," says
Jim Godfrey, a potato farmer and former head of the Scottish Crop
Research Institute in Invergowrie. In the tropics, where both potato
and pathogen grow faster, farmers may need to spray every few days.

What's more, the blight is becoming more aggressive - P. infestans
has two "genders", only one of which came over in 1845, so it was
only able to reproduce asexually. Though it has spread in this way
through Europe and much of the world, the asexual spores can persist
only on susceptible plants. Then, in the drought of 1976, Europe's
crop failed and it imported tonnes of potatoes from Mexico. With
them came the other "gender" of the blight. Now it can breed
sexually, which means it can adapt more quickly to both fungicides
and resistant potatoes. Sexually produced spores can also survive in
soil, making the disease even more difficult to control.

Sexually reproducing blight and increasing fungicide resistance mean
more, and worse, outbreaks of the disease around the world. It may
not cause starvation on the same scale as the Irish famine - food
aid exists now, and few places are as exclusively reliant on
potatoes as the Irish were in 1845 - but even so, the potato's
potential for disaster is worrying.

That, says Anderson, is why we need to develop new varieties of
blight-resistant potatoes. This won't be easy. Potatoes are a
notoriously difficult crop to breed, thanks to their unusually
complex genetics. The common spud carries four copies of each of its
chromosomes where most organisms carry two. That means the potato
plant carries a possible four variations for each gene, so when two
plants are crossed, thousands of different combinations emerge. That
makes it an enormous task to select the best ones.

In other crops that have more than two pairs of chromosomes breeders
have found ways around the problem. Most wheat has six copies, but
wheat breeders start with plants that are already inbred so that for
most genes, all six copies are identical. That way they can predict
the outcome of crosses. Attempts to do this with potatoes, and also
to engineer potato plants with only two-copy genomes, have been
disappointing, says Shelley Jansky of the US Department of
Agriculture's potato lab in Madison, Wisconsin. The genetically
impoverished potatoes are spindly and weak. "Potatoes just need all
that internal genetic diversity to thrive," she says.

That means potato breeders are forced to take a broad approach when
looking for useful new varieties. First, they cross genetically
diverse parent plants to create up to 100,000 genetically different
progeny. Then, they "walk across the field and choose the potatoes
they think look promising, and get it down to a manageable number,
say a thousand", says Jansky, and examine those plants for the
qualities they want.

This kind of classical breeding has given us all the potato
varieties we have today, but it is very difficult to use this method
to breed a single desired trait into an existing commercial potato
variety. Recent efforts to cross commercial varieties with Solanum
bulbocastanum, a wild Mexican potato which has two genes for
resistance to all known strains of blight, did indeed result in
blight-resistant potatoes - but they had other, unwanted wild genes
as well, and lower yields.

Breeding these hybrids back with the original commercial potato will
produce tubers more similar to the original, but they will never be
quite the same. This is a problem for the potato industry, says
Jansky. Processing companies take a third of the crop in rich
countries, and the machines and processes are designed for potatoes
of particular shapes, sizes and chemical properties. They know their
King Edwards and their Russet Burbanks and they want nothing else -
and because potatoes are propagated vegetatively by tuber, they can
have exactly the same potato again and again, says Jansky.

Genetic engineering could be the answer to this problem, says Anton
Haverkort of Wageningen University in the Netherlands. He is running
a 10-year programme to find more genes for resistance to late blight
in several wild potato species - and then put them, and nothing
else, into three popular varieties of eating potato. Haverkort uses
a relatively new method of genetic engineering that doesn't require
an antibiotic resistance marker gene - a common tool in creating
engineered plants - to be introduced along with the desired genes.
So far he has isolated eight genes and the first of his genetically
modified plants are now in field trials.

"We call them cisgenic, instead of transgenic," he says. "They
contain no genes except what they could have acquired naturally by
breeding with other potatoes - except it hasn't taken decades." He
hopes EU law will take account of the development and lighten
restrictions on such plants, and that Europe's anti-GM public will
accept them. "The only genes in there are from potatoes," he says.

Whether consumers accept cisgenic potatoes remains to be seen.
Meanwhile, genetically engineered blight-resistant potatoes created
by the German chemical giant BASF are already in their third year of
field trials. The company has put the two resistance genes from
Solanum bulbocastanum into commercial potato varieties along with an
antibiotic resistance marker. BASF says the plants seem to have
durable resistance to blight strains circulating in Europe, and it
is hoping to start selling them by the middle of next decade.

The antibiotic resistance gene could be a problem, however. Its
presence is central to objections to GM food; opponents say the gene
could be taken up by bacteria in the environment, creating
superbugs. BASF has another genetically engineered potato that
yields more uniform starch for the paper and fabrics industries,
which the European Commission declared safe last year, but as
countries such as Austria harden their resistance to GM crops, it is
holding back on the go-ahead for release. The same fate may await
the company's GM food potatoes.

Developing countries, having had the potato for less time, seem to
be more open to non-traditional varieties, and in some places GM
food is less unpopular. China, for example, is rumoured to have
developed varieties similar to BASF's.

In Peru, CIP plans to keep studying how potatoes resist blight, and
using its potato gene bank - the world's largest - to find genes
that confer resistance. CIP is using GM to develop
late-blight-resistant strains for Asia and is also breeding potatoes
conventionally. This is partly because CIP has imposed a moratorium
on releasing GM potatoes in South America, where most governments
are opposed to GM and where most of the potato's wild relatives
exist, until more is known about whether introduced genes might
escape into wild potatoes. But it is also, she says, because "GM is
one tool, it doesn't do everything." Resistance to blight, for
instance, might be achievable by implanting one or two genes at a
time, but eventually, the blight will adapt to those few genes. And
other, more complex traits like nutritional quality and yield depend
on many genes, few of which are known, and can only be bred into
farmed varieties the old-fashioned way, says Anderson.

However we come by new varieties, as the humble potato spreads
around the world, and more and more people depend on it for
sustenance, the need to win the battle against disease becomes more
urgent. Blight is a disaster waiting to happen, and this time we
have no alternative but to fight back.

The plant that changed the world

The Spanish brought the potato from South America to Europe in 1536.
Most histories say it was then ignored for 200 years, but according
to University of Chicago historian William McNeil, peasants knew all
about it, and quietly took to growing potatoes as insurance against
the frequent loss of their grain stores to marauding armies.

When Prussia then other European governments realised in the
mid-1700s that potatoes could slash the cost of warfare, they made
peasants grow them. Potato-pushers such as nutritionist Antoine
Parmentier, whose name still graces French potato dishes, and
Austrian empress Maria Theresa, were in fact pushing civil defence.
The empress's daughter, French queen Marie Antoinette, is best known
for suggesting peasants eat cake, but she wore potato flowers to
promote another alternative to bread.

In the 1800s potatoes moved into the mainstream. Before then, half
of Europe's farmland lay fallow between grain crops. As the
population rose, people planted potatoes on this ground instead. The
crop needed weeding, but produced more than enough to feed the
labour-force.

Then, in 1845, late blight hit Europe. It is remembered as the Irish
potato famine, but hundreds of thousands died in the rest of Europe
too. In the early 1850s, yields recovered, and Europe's potato
fields continued to feed the population explosion and booming cities
of the 19th century. By fuelling the industrial revolution and the
economic and military rise of Europeans, says McNeil, potatoes
changed the world.

Now history is repeating itself. Asian farmers are feeding a growing
population with scarce land and abundant labour by squeezing
potatoes between crops of grain. If suitable varieties allow
Africans to do the same, the potato may once again be a lifeline for
growing, urbanising and war-torn populations. But only if - this
time - we can keep blight at bay.

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Weblinks

International Year of the Potato
http://www.potato2008.org/en/index.html
International Potato Center
http://www.cipotato.org/
The potato song
http://www.youtube.com/watch?v=q7uyKYeGPdE