IN a field outside Ayr in southwest Scotland, researchers are about to infect
a crop of potatoes with late-blight, a fungus that is the scourge of potatoes
worldwide and the cause of a famine that killed a million Irish people just 150
years ago. If the potato hybrids being tested can resist the disease here, and
in five other test plots around the world, it could mean that researchers have
cracked the late-blight plague once and for all, starting a potato revolution
that could feed a billion people in the 21st century. 鈥淚鈥檓 very optimistic,鈥�
says Jim Duncan of the Scottish Crop Research Institute, which is running the
experiment. 鈥淲e can beat this potato plague.鈥�
But they may not have much time. New versions of the deadly fungus, evolved
from types only recently escaped from their Mexican mountain fastnesses, are on
the march. One particularly aggressive strain has spread across the US in the
past five years and is growing resistant to fungicides. It has done hundreds of
millions of dollars worth of damage. British researchers believe it could reach
Europe this summer.
The potato is the best package of nutrition in the world, rich in minerals,
vitamins, calories and protein, and virtually fat-free. And potato fields yield
more tonnes鈥攁nd more calories鈥攑er hectare than any other form of
cultivation, averaging 13.5 tonnes per hectare worldwide, four times as much as
rice and five times as much as wheat. North American farmers can grow 37 tonnes
of potatoes per hectare. The potato grows quickly, too鈥攑roducing a crop
within two months in the tropics鈥攁nd will thrive almost anywhere, from
deserts to the soggy fields of a cold, damp Scottish summer. Not only that, it
also lends itself to improvement by modern techniques such as genetic
engineering.
Advertisement
The global potato harvest now stands at 270 million tonnes a year, making it
the world鈥檚 most valuable non-cereal food crop, with a cash value that is
surpassed only by the 鈥渂ig three鈥�: rice, wheat and maize. Potatoes fed the
Industrial Revolution in Europe 200 years ago, and may soon do the same for Asia
and Africa. But unless the late-blight fungus that dogs its every step can be
controlled, the shadow of the Irish potato famine could return to haunt
developing countries.
The potato, a native of the Americas, is a member of the Solanaceae family,
which includes tomatoes, peppers and aubergines. Altogether, there are 154
recorded wild species of potato, found from Chile to Mexico. Only seven species
are cultivated, but during 8000 years of cultivation farmers have developed
thousands of varieties. Today, the International Potato Centre (IPC) in Lima,
Peru, houses more than 6000 varieties in its gene banks.
The Spaniards introduced the potato to the Old World in 1570, and it spread
widely throughout Europe. Only one species, Solanum tuberosum, is grown
outside the Andes. But more than half the 600 potato varieties grown in Europe
contain some genes bred in from the large number of related wild species. These
genes confer some resistance to the plant鈥檚 many enemies, including nematodes,
potato viruses and the late-blight fungus. The enormous gene pool in wild
potatoes is a potentially invaluable tool kit for plant scientists trying to
engineer new potato varieties, especially those looking for disease resistance,
says Vincent Rubatzky of the University of California at Davis.
Although potatoes are traditionally associated with the dour climates of
Russia, Poland and Ireland, improved varieties have begun a long march into the
tropics. They have already infiltrated tropical highlands such as the Himalayas,
and are moving increasingly into tropical lowlands. Here they can be harvested
in just 60 days, a third of the time typical in temperate climates. Potato
production in developing countries is increasing faster than that of any other
food crop. By the end of this decade, a third of the world鈥檚 potatoes will be
grown in developing countries, compared to just 4 per cent in 1950.
Record harvests
Growth is fastest in Asia. Over the past decade, China鈥檚 potato output has
tripled, pushing it ahead of Russia as the world鈥檚 premier potato-growing
nation. India鈥檚 crop has quadrupled since 1980, making it the sixth largest
producer. In Vietnam, the potato is the biggest crop after rice. In China,
Vietnam and India, faster growing times for both potatoes and rice mean that
potatoes can be planted as a second crop in rice fields. Production is rising in
its home continent, too. Mexican output has increased sixfold since 1950, mostly
through a near quadrupling of yields to 16 tonnes per hectare.
Yet this may just be the start. 鈥淭he potential of potato cultivation has
barely been touched in developing countries,鈥� the IPC says.
One driving force is the growing demand of urban markets, as town-dwelling
Asians in particular switch from a staple diet of rice to fast-food 鈥渇ries鈥�.
Farmers are being helped by the development at the IPC of hybrid varieties that
can produce a uniform crop from a few grams of seed sown directly into the soil.
The conventional method is to plant 鈥渟eed potatoes鈥濃�攖ubers kept from one
season for planting the next. Seeds can cut growing time, and end the problem of
handling and storing bulky tubers. They also reduce the risk of transmitting
diseases through the tuber.
But probably the biggest reason for plant breeders鈥� optimism about the
potato鈥檚 prospects, says Rubatzky, is the potential for improvement by
introducing genes from the large pool of wild potato species and cultivated
varieties. This offers the prospect, for instance, of protection against
bacterial disease such as wilt, which is a particular problem in hot and humid
tropical lowlands.
Genetic engineering can also help a plant to attack pests directly. In 1995,
the potato became the first commercial crop to be genetically engineered to
release its own pesticide. The transgenic variety of Russet Burbank, America鈥檚
favourite potato, has been specially equipped to fight off the Colorado beetle.
Developed by Monsanto, the world鈥檚 largest pesticides manufacturer, and the
University of Wisconsin at Madison, the potato carries a gene from the soil
bacterium Bacillus thuringiensis (Bt) that produces a protein poisonous
to Colorado beetles. A beetle eating the potato will also eat the toxin, which
breaks down the cells in its gut. The same team has also experimentally
introduced into potatoes genes that provide resistance to a pair of potato
viruses known as viruses X and Y, which are transmitted by aphids.
Poised to strike
Other developments with a 鈥渘atural pesticide鈥� extracted from potato roots
could head off the potato cyst nematode, which almost destroyed the British
potato harvest in the 1950s (Technology, 10 December 1994, p 21). But the big
prize is beating late-blight, which could be on the verge of another pandemic,
fuelled by an input of genetic diversity from its Mexican homeland.
Before 1842, potato late-blight, Phytophthora infestans, was known
only in Mexico, where it began as a local disease among the potato and tomato
fields of the Toluca Valley in the high centre of the country west of Mexico
City. But that year it turned up in New England. Three years later it appeared
in Belgium, from where it spread through Europe and beyond.
鈥淕enetically, that late-blight was very uniform,鈥� says Duncan. 鈥淎 very small
number of clones reached the US in the 1840s, and we think it was a single clone
that made it to Europe.鈥� Nonetheless, the fungus hit European potatoes hard. It
found the most favourable conditions in the Irish monoculture of the lumper
potato, a variety that dominated huge tracts of the wet boggy country and
provided the bulk of the nourishment for millions of its inhabitants. The lumper
had no resistance to late-blight. In the ensuing famine a million people died
and two million more emigrated.
In the years since the Irish potato famine, plant breeders have waged a
continuing war against late-blight. Until recently, they had one great
advantage: the fungus鈥檚 very limited ability to evolve, which arose because,
outside Mexico, the fungus could not breed sexually. The escaped clone, or
clones, of the fungus all belonged to the same 鈥渕ating type鈥�, called A1. Fungi
belonging to type A1 can only reproduce sexually with fungi belonging to a
second mating type, called A2. Without an A2 partner, the A1 fungus could only
reproduce asexually, producing endless clones of itself. This limited its
capacity to evolve by exchanging and rearranging genetic material, making it
harder to adapt to changed environments.
Added to this, spores produced by asexual reproduction cannot survive for
long outside their host plants. The spore bodies emerge from stomata鈥攑ores
in the surface of the potato leaves鈥攆rom where even the slightest breeze
dislodges them. If they land on a neighbouring plant they start a new infection.
If it rains, some spores are washed into the soil, where they will infect any
tubers they come into contact with. Spores that fail to find a host soon die,
and so cannot infect a later crop.
Set against these frailties is the help that farmers have given the fungus in
starting fresh infections. The practice of storing tubers to plant the following
year meant that if any were infected, there was a good chance of setting off a
new infection. And the inability to reproduce sexually does not prevent the
clones responding in a limited way to their environment, says Duncan. Some
genetic variation can arise among the clones through accidental mutation. For a
century, the late-blight spread round the world in this form. It was
periodically tamed by the development of new resistant potato varieties, but
each time its limited powers for alteration succeeded in reasserting the
fungus鈥檚 power to infect potatoes.
That was the story until 1976, when drought caused another failure of the
potato crop in Europe. Pressurised by traders, European governments approved
imports of potatoes from Mexico. 鈥淭housands of tonnes arrived,鈥� says Duncan.
Somewhere the hygiene rules failed, and hidden among the imported potatoes were
new strains of late-blight. Among them were more A1 types, but critically there
was also a generous selection of A2 types. The newly imported strains, A1 or A2,
are generally regarded as more aggressive than the old clone. 鈥淭here are still
only a fraction of the number of strains outside Mexico as there are within it.
But now we have a genetically much more diverse late-blight epidemic going round
the world,鈥� says Duncan. In the past 20 years, this new population of A1s and
A2s has gained ascendancy over the old clone everywhere from Ireland to Japan.
Only Australia has escaped the invasion.
The presence of both A1 and A2 types creates new possibilities. 鈥淭hey can now
reproduce sexually whenever they are present in the same location. And,
certainly in Europe, we know they are now doing just that,鈥� says Duncan.
Sexual reproduction dramatically increases the potential for late-blight to
evolve to fit in with its environment, to seek out new habitats, to find new
avenues of attack against supposedly resistant potato varieties, and to develop
resistance to fungicides. Moreover, the offspring of sexual reproduction are
thick-walled spores called oospores which, unlike their asexual counterparts,
can survive in the soil for years, awaiting a potato crop to attack. 鈥淲e have
already seen in the Netherlands how this allows crops to be attacked much
earlier in the season,鈥� says Duncan.
Alarm bells
This combination of sexual reproduction and the ability to lie dormant in
soils has alarmed plant breeders fighting late-blight, and moved the disease to
the top of the research agenda at the IPC in Lima and in potato laboratories
around the world.
Worldwide, the various forms of late-blight today destroy 14 million tonnes
of potatoes a year, with a value of $3 billion, says Peter Gregory,
deputy director-general in charge of research at the IPC. 鈥淎nd it is poised to
strike hardest at millions of poor people in developing countries who rely on
potatoes but can least afford to buy expensive chemicals to keep the fungus in
肠丑别肠办.鈥�
Farmers who can afford it are fighting back in the only way they
know鈥攂y spraying ever more fungicide onto their crops. In Central America
25 sprayings a season have been reported as desperate farmers try to tame the
fungus. In the highland potato fields of Java and Sumatra in Indonesia, says
Duncan, they sometimes spray fungicide on potato crops more than twice a
week.
And there is worse on the way. In the early 1990s, a clutch of aggressive new
A2 strains were spotted in the US. No one is clear how they got there. European
researchers insist that they came direct from Mexico in contaminated shipments
of potatoes or tomatoes. Alternatively, they may have taken a more roundabout
route, arriving in potato seed grown in Europe from contaminated Mexican
potatoes, as Hubert Zandstra, the Canadian director-general of the IPC,
claims.
This puzzle will not be resolved until someone compares strains from both
sides of the Atlantic. The reason that has not yet happened, says Duncan, is
that national crop hygiene laws are so tight that researchers have not yet been
able to get US and European strains of late-blight together in the same
laboratory to look at them side by side. 鈥淣ot surprisingly, governments are not
keen to let us move the fungus around,鈥� he says. But he has recently obtained
permission to bring some American A2 strains to Scotland for genetic
fingerprinting later this year. 鈥淲e will be allowed to import the US strains
provided we do it in winter, when there is no chance of the spores escaping to
infect potato crops,鈥� Duncan says.
American rampage
Duncan expects this will reveal that the US strains are different from those
in Europe. But whatever the immediate source of the strains now rampant in the
US, the damage they have inflicted on potato crops has been huge. 鈥淥ne strain,
called US8, has spread right across the US in the past five years,鈥� says Bill
Fry, professor of plant pathology at Cornell University in New York, who is a
board member of the IPC. US8 is particularly aggressive and is resistant to some
fungicides. 鈥淥nce it arrives, it stays,鈥� says Fry. 鈥淚n 1994, it did hundreds of
millions of dollars worth of damage. And 1995 was as bad. I know of individual
growers who have lost up to a million dollars.鈥�
Even before the new 鈥減otato plague鈥� arrived in the US, North American farmers
sprayed more pesticides on their potato fields than on any other crop.
鈥淔antastic sums are being spent on pesticides,鈥� says Fry. 鈥淚t鈥檚 been especially
bad for organic growers, many of whom have given up and started spraying. I
don鈥檛 think this is sustainable either economically or environmentally.鈥� The
only defence, he says, will be to develop potato varieties that are resistant to
the new strains of late-blight, and to keep a vigilant watch for the first signs
of the fungus, so that it can be rooted out as soon as it appears.
American potato breeders are partly to blame for the state of the national
crop, says Duncan. They did little to develop varieties with resistance to the
fungus. This is a charge Fry accepts. The priority was always to meet the needs
of potato processors. Now European farmers and plant breeders wait in
trepidation for the accidental arrival of US8 or one of its cousins in potatoes
or tomatoes imported from North America. 鈥淭he possibility of US strains getting
to Britain this year can鈥檛 be ruled out,鈥� says Duncan. 鈥淓specially if we have a
drought and more imports are allowed.鈥�
The IPC has just launched a 10-year, $25 million Global Initiative on
Late Blight. Its priority is to protect Third World farmers, but 鈥渇or once the
interests of everyone are much the same鈥�, Fry says. The initiative, drawn up at
meetings in Lima in March 1996, will focus on the genetics of the potato, with
the aim of developing varieties that are resistant to the fungus. In the first
phase, researchers will identify the genes in wild species and traditional
cultivated varieties from the Andes and Mexico that can resist the blight. They
may also look for similar genes that confer resistance to late-blight in other
plants of the same family, such as tomatoes.
The researchers will build on past potato-breeding programmes using wild
species. This work was pioneered by John Niederhauser, professor of plant
pathology at the University of Arizona at Tucson, who spent 35 years in Mexico.
So far, Mexican researchers have identified more than 25 resistant native
varieties. Many of them have been grown for several years, without the aid of
fungicides, in the mountains of central Mexico. One is a hybrid, known as Greta,
that carries genes from Solanum demissum, a Mexican wild potato.
These resistant Mexican varieties 鈥渉ave the potential to become the most
promising plant materials for expanding world food production since the Green
Revolution鈥�, says Niederhauser. Last year, trials began in the US, Ecuador,
Canada, Britain, France and the Netherlands to test them in different
environments. In Britain, that work is being conducted by Duncan and his
colleagues in Scotland. The research sites will test 15 varieties from around
the world, infecting them with local races of late-blight. Duncan is preparing
to plant more varieties to continue the experiments this month. 鈥淟ast year we
found several of them performed well everywhere, which suggests they have quite
broad resistance.鈥�
But such optimism must be tempered with caution. As Niederhauser points out,
a century鈥檚 search for a lasting resistance to blight has not stopped the fungus
menacing crops around the world. Every time supposedly resistant cultivars were
developed, new races of the fungus quickly got the upper hand. Fry agrees. 鈥淚n
the past, we looked for a silver bullet, used it and then forgot about the
problem until a new race of the fungus evolved and late-blight reappeared.鈥� He
doesn鈥檛 believe the battle can ever finally be won.
Perhaps the biggest danger lies in partial success. A few years of respite
would be followed by a rapid spread of the potato into poor countries without
ready access to fungicides, such as the IPC鈥檚 current target areas in Kenya,
Rwanda and Burundi. Then bang and the blight is back. It could be Ireland 1845
all over again.