There are three major wheat rusts — stripe, leaf and stem — and all three can cause serious problems for a crop. In terms of yield loss potential, though, stem rust is king.
At one time, stem rust regularly devastated wheat production around the world. Through use of resistant varieties, the disease has been held in check since the 1950s.
But in 1998, a new race of stem rust appeared in Uganda wheat: Ug99. This finding went rather unnoticed until 1999, when it showed up in Kenyan wheat fields.
“At that point, many international scientists said, ‘This is something we need to check because this new race can overcome many of the effective resistances,’” says David Marshall, research leader with the USDA-ARS in North Carolina.
“And that included the resistances that are in the international germ plasm out of CIMMYT (International Maize and Wheat Improvement Centre) in Mexico City. That's alarming and this rust has become a front-burner issue.”
The new race of rust is a “big concern and justifiably so,” says Rick Cartwright, Arkansas Extension plant pathologist. “Of all the wheat rusts, stem rust has historically been the most dangerous. And it's back to its bad, old ways. It's attacking all the wheat varieties in areas of Africa, there's no resistance and it's on the move.”
The spread of Ug99 through east Africa “raised a red flag and the USDA, in cooperation with CIMMYT and other international breeding centers, set up a program to identify germ plasm on a worldwide basis based on how it fares — resistant, intermediate, or susceptible — with the new rust race,” says Marshall.
Marshall and colleagues set up a testing program in Kenya. A few years ago, “it was the only place we could screen for this new rust. We didn't want to bring the pathogen into the United States. So, we've had a nursery over there for several years screening U.S. germ plasm for resistance.”
The good news is some resistance was found in U.S. wheat lines. But that resistance isn't widespread.
“There's work left to do to breed new resistances into U.S. varieties.”
If east African weather patterns remain constant, the disease should move north and then east. From Uganda and Kenya, Ug99 moved into Ethiopia. It has now jumped across the sea into Yemen on the Arabian Peninsula and there are unconfirmed reports that it has reached Egypt.
As it travels farther north, it will reach Syria and Turkey. From there it could move into Europe. It will likely also spread east — perhaps as far as China and Russia.
If its spread is natural, the new rust race will probably show up in South America and North America around 2009 or 2010 — about a decade after it was first in Uganda.
“So we have a little time to get things in order before it shows up here,” says Marshall. “We need to make sure we're not in a vulnerable state when it arrives.”
How frightening is this for the world's subsistence farmers?
“Very much. Many farmers in Kenya are subsistence growers. They have small plots of land where they grow enough for their family to survive. This rust can be devastating for them and the government knows it.”
Unfortunately, fungicides appear to be no guarantee against the stem rust. Kenyans have had mixed success with treatments.
“I've spoken with researchers who have been in Kenya where wheat is grown year-round,” says Gene Milus, University of Arkansas professor and wheat pathologist. “They report that wheat fields had been sprayed three times with Folicur — a good fungicide — and were still wiped out with this race of stem rust. The rust can develop quickly, and if comes in early, can wipe out a crop whether a fungicide is sprayed or not.”
To thwart stem rust, German wheat breeders in the 1930s were
able to incorporate a gene from rye — Sr31 — into wheat. The gene was widely adopted and until Ug99 showed up, it worked well.
After Ug99 developed and sources of resistance were being sought, breeders found a gene called Sr24. More than half the wheat varieties resistant to Ug99 rust could thank the Sr24 gene.
In 2006, though, a further mutation in Ug99 meant that in addition to being able to overcome Sr31, the rust race was also unbothered by Sr24.
“That means more than half the wheat varieties and breeding lines that were resistant to Ug99 are now susceptible to this new race,” says Milus. “There are only a small handful of resistance genes in U.S. wheat varieties that are effective against this new race of stem rust. And there are a few additional resistance genes that are in bad genetic backgrounds or are linked to bad traits.”
All hope isn't lost, says Milus. “But the outlook isn't rosy either. Several sources of resistance have been found but these are too few to protect the world's wheat supply and some aren't easy for breeders to utilize.”
Are we years from having a genetic answer to this new rust? “Yes. Because what we're talking about is replacing most of the current varieties with new ones that are yet to be developed. This is a long haul.”
It isn't a question of if the new stem rust will arrive in the United States, but when. Fortunately, USDA and its international counterparts appear to understand the urgent need for a solution.
“Money is being made available for research and there's an international effort to find answers to this,” says Marshall, who is involved with the Global Rust Initiative. “But there's a lot left to accomplish.”
For more, see http://www.globalrust.org/.