- 13 July 2011 by Debora MacKenzie
- Magazine issue 2821. Subscribe and save
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NEARLY 20 million people face starvation in east Africa as the region experiences its worst drought for 60 years. Hopes now focus on the return of the rains, perhaps as early as the autumn. But that could just bring another problem with it.
A new, aggressive strain of yellow rust, a fungal disease of wheat, is waiting in the wings, and east Africa isn’t the only region at risk.
The disease had already struck the US, Australia and Europe when, in 2010, a particularly virulent strain infested an area from Morocco to Pakistan, and spread faster than any known major crop disease (see map). Most wheat varieties in warm countries have no defence against it. Its march continued this year, with an outbreak in northern India.
Concern about yellow rust has languished as wheat scientists rushed to respond to a related disease, the Ug99 strain of stem rust against which little of the world’s wheat has any resistance. Yellow rust has historically been less deadly than stem rust, and thrived only in cool countries.
But in 2000, a new aggressive strain of the disease was detected in California, though it probably evolved in Africa or Asia, says Mogens Hovmøller of Aarhus University, Denmark. It generates more spores than its ancestors, so it spreads farther, faster.
It is also nastier. Yellow rust typically does not destroy entire crops, even in the absence of fungicide. But this lineage wiped out organic crops in Denmark, says Hovmøller. Based on the location of recent outbreaks, Colin Wellings of the University of Sydney in New South Wales, Australia, says some of the world’s major wheat producers are at risk from the new strain.
Concern is greatest in the tropics, where most farmers cannot afford fungicide and yellow rust has not typically been a problem: until now, the disease has not been adapted to warm temperatures. As a result, most local wheat varieties carry only one gene to resist it, called Yr27.
The new strain tolerates warmer temperatures and appears to have acquired genes to defeat Yr27, possibly from other yellow rusts. No one knows where or when this happened, but plant pathologists discovered last year that yellow rust can reproduce sexually, suggesting the new strain may have picked up genes from local strains by mating with them.
The result was an epidemic that in 2010 killed up to 40 per cent the wheat crop in Ethiopia, Syria, Turkey, Iran, Azerbaijan, Iraq, Uzbekistan, Morocco and Kenya. The spores like humidity, and struck irrigated fields in drought-stricken areas, says Mahmoud Solh, head of the International Center for Agricultural Research in the Dry Areas (ICARDA) in Aleppo, Syria. Drought is now holding them at bay in some places, but “when the rains return to Africa, the rust will be waiting”, he warns. Hovmøller agrees.
Wheat researchers meeting at ICARDA in April said they had developed wheat varieties that can resist the new strain of yellow rust. They also yield 15 per cent more grain and resist Ug99 stem rust. But it will take several years to breed enough seed for regions blighted by the new strain, cautions Solh. And the aggressive strains produce so many spores so fast, adds Hovmøller, that they mutate and adapt faster than yellow rust has in the past. “What works now may not last long,” he says.