May
30, 2007 10:34 AM, By David Bennett
Farm Press Editorial Staff
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. 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/.
Vicki Morrone
Organic Vegetable and Crop Outreach Specialist
Michigan State University
C.S. Mott Sustainable Food Systems
303 Natural Resources Bldg.
East Lansing, MI 48824
517-353-3542
517-282-3557 (cell)
517-353-3834 (fax)
http://www.MichiganOrganic.msu.edu/
http://www.mottgroup.msu.edu/