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ARS scientists have discovered that the fungus
that causes stripe rust in wheat may use sexual recombination to overcome
resistant wheat varieties as fast as they do. Photo courtesy of Mary
Burrows, Montana State University, Bugwood.org
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Uncovering the Mystery of a Major Threat to Wheat
By Dennis
O'Brien
June 1, 2010 Agricultural Research Service (ARS)
scientists have solved a longstanding mystery as to why a pathogen that
threatens the worlds wheat supply can be so adaptable, diverse and
virulent. It is because the fungus that causes the wheat disease called stripe
rust may use sexual recombination to adapt to resistant varieties of wheat.
ARS plant pathologist
Yue
Jin and his colleagues
Les
Szabo and
Marty
Carson at the agencys
Cereal
Disease Laboratory at St. Paul, Minn., have shown for the first time that
stripe rust, caused by Puccinia striiformis, is capable of sexually
reproducing on the leaves of an alternate host called barberry, a common
ornamental. The fungus also goes through asexual mutation. But sexual
recombination offers an advantage because it promotes rapid reshuffling of
virulence gene combinations and produces a genetic mix more likely to pass
along traits that improve the chances for survival.
Barberry (Berberis spp) is already controlled in areas where wheat is
threatened by stem rust, caused by another fungal pathogen. But the work by the
ARS team is expected to lead to better control of barberry in areas like the
Pacific Northwest, where cool temperatures during most of the wheat growing
season make stripe rust a particular threat.
The researchers suspended wheat straw infected with the stripe rust pathogen
over barberry plants and found that fungal spores from the wheat infected the
barberry. They also took infected barberry leaves, treated them to promote the
release of spores, and exposed them to wheat. Tests confirmed that the wheat
plants were infected within about 10 days.
The researchers began the study last year after finding infected leaves on
barberry plants at two sites on the
University of Minnesota
campus. They initially thought the symptoms were a sign that the stem rust
pathogen had overcome the resistance commonly found in U.S. varieties of
barberry.
Instead, they found barberry serving as a sexual or alternate
host for stripe rust. When the overwintering spores of the stripe rust fungus
germinate in the spring, they produce spores that reach barberry leaves,
forming structures on the top of the leaves that allow mating between races or
strains of the fungus. Spores resulting from this mating can, in turn, infect
wheat.
The results were recently published in Phytopathology.
ARS is the principal intramural scientific research agency of the
U.S. Department of Agriculture (USDA). The
research supports the USDA priorities of promoting international food security.