Virulent fungus threatens world’s wheat

U. MINNESOTA (US) — A fungus discovered in Uganda in 1999, now threatens up to 80 percent of the world’s wheat crop, according to researchers.

Discovered in Uganda in 1999, Ug99 is a strain of “stem rust” fungus that burrows into the tissues of wheat and barley and damages or kills the plants, even some of the most rust-resistant lines.

The fungus has spread into Iran, and variants have been found in southern Africa.

Scientists believe it also threatens wheat in India and China, where a billion people depend on the grain. If it finds its way to the Americas, the fungus could pose a threat to most of the U.S. wheat crop.

“Ug99 has mutated at least twice since its discovery,” says Martin Carson, research leader at the Cereal Disease Laboratory at the University of Minnesota. Each time it has produced a variant with added virulence.

Two of the variants were discovered by CDL researcher Yue Jin and colleagues.

First, they observed two wheat lines growing in Kenya, each carrying a gene for resistance to Ug99, but those lines became susceptible to stem rust in Kenyan fields.

Laboratory studies revealed that the rust strains attacking plants with the resistant genes were new variants of Ug99.

“[Ug99] has the potential to undo the Green Revolution led by Norman Borlaug in East Asia, because it’s capable of attacking the high-yielding wheats that he worked so assiduously to breed for resistance,” says rust researcher Brian Steffenson.

Steffenson, Jin, and colleagues are screening wheat in heavily affected areas like Ethiopia and Kenya in a race to find long-lasting resistance to the fungus and incorporate it into wheat and barley.

“The way to slow down the evolution of virulence is to have multiple genes for resistance to Ug99,” says Matt Rouse, who recently completed his doctoral research on resistance to Ug99 in the department of plant pathology.

Rouse has already found two new resistance genes and identified molecular markers that can be easily detected and serve as flags for their presence. The markers help scientists track which plants carry the resistance genes so that only they will be used for breeding.

Researchers are also looking for the precise agents—called effector proteins—that Ug99 uses to infect plants.

If these can be identified other plants, such as rice, can be tested to see if they have genes that can protect against effector proteins. Those genes could then provide new sources of resistance to help protect wheat and barley.

“If we understand enough, we may be able to design ‘de novo’ genes that block key points essential for this fungus to infect its host,” says Les Szabo of the Cereal Disease Lab.

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