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Virulent fungus threatens breadbaskets around the world

Story from world grain that would drive up wheat, especially, but should affect all food prices if the virus spreads.

The story from WorldGrain.com>>>

Tamar Kahn

Virulent fungus threatens breadbaskets around the world

New, resistant strains of wind-borne stem rust - the 'polio of wheat' - have spread from Uganda, writes

LOCAL scientists have found two new strains of a deadly wheat fungus, once thought to be confined to Uganda, here in SA, suggesting crops in many more parts of the world may be vulnerable to the devastating pathogen.

The spread of this fungus would have dramatic consequences for SA, which already does not grow enough wheat to meet demand. On a global level, if wheat crops failed, it would drive up the price of the grain and increase the price of staples, such as bread.

The fungus is stem rust, the polio of wheat. It leaves tell-tale red pustules on the wheat stems, and has destroyed crops since Roman times: a healthy crop close to harvest can be reduced to a shrivelled black mass within weeks. It damages the plant's vascular tissue, robbing it of its capacity to transport nutrients.

The last major rust epidemic occurred in the 1950s and spurred a global initiative led by Nobel laureate Norman Borlaug to develop varieties that could resist the wind-borne fungus Puccinia graminis.

Scientists have hunted for genes that defend wheat against rust and bred them into new varieties, a painstaking process that can take years to come to fruition. One of the key resistance gene clusters identified is called Sr31, which until very recently guaranteed protection against stem rust.

It offered an added bonus, as it increased yields, and seeds with Sr31 were rapidly adopted by farmers around the world.

But like all pathogens, stem rust mutates.

In 1999, a new and extremely virulent strain was discovered in Uganda that was immune to Sr31. It decimated local crops. Wheat rust, once regarded as under control, was back on the global agenda.

This strain, dubbed Ug99, quickly spread to Kenya, Ethiopia, Sudan, Yemen and Iran - and has now arrived in SA.

New and more virulent races are emerging all the time, as recent research by Prof Zak Pretorius, a plant pathologist from the University of the Free State, has shown. Prof Pretorius regularly surveys the rust susceptibility of commercial wheat varieties and the most advanced breeding lines under development by seed companies.

In 2007, he found a strain of rust that was impervious to the resistance gene Sr24. Last year he found a variant that knocked out both Sr24 and Sr31.

Both restrict the rust's growth by triggering the death of cells around the site where the pathogen has invaded the plant.

Prof Pretorius found that half the South African wheat cultivars he surveyed were susceptible to rust in the seedling stage, but cautions this is not likely to be the case with adult plants as many resistance genes only take effect once plants are mature. Work is under way to assess the vulnerability of adult wheat strains, and results are expected in November.

South African farmers do not grow wheat containing Sr31 so scientists are not particularly worried about harvests.

But Prof Pretorius's discovery that Ug99 is evolving with virulence for new combinations of genes is causing concern. "It shows the pathogen is mutating and adapting," he says.

Scientists around the world are racing to identify and isolate new resistance genes, hoping to develop new wheat varieties and get them to farmers faster than Ug99 can spread.

Ironically, it is the bread industry that is partly responsible for SA's relative protection from Ug99, says Dr Cobus le Roux, research institute manager for the Small Grains Institute in Bethlehem.

Wheat containing Sr31, a cluster of genes that originally come from rye, yields a dough that is too sticky for local millers and bakers, he says. Nevertheless, consumers could be affected by the potential spread of Ug99 to other wheat-producing regions, such as the Middle East and south Asia, where farmers grow varieties that will be open to attack unless scientists develop new cultivars impervious to attack before it blows there on the wind.

If their crops fail, global wheat prices will rise - with a consummate increase in the price of staples such as bread.

At present SA cannot grow enough wheat to meet its needs, and imports about 1-million tons of the 2,8-million tons it consumes each year, according to Dr le Roux.

Seed companies are keeping a close eye on developments too. Says Pannar's wheat breeder, Dr Willem Boshoff: "No one wants to release susceptible cultivars."

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July 13, 2010

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