Two Leaf Rusts Found Where One
Until recently, plant pathologists could identify only one species of rust
fungus attacking the leaves of both wheat and rye plants.
Now, ARS scientists studying leaf rust resistance in wheat have determined
there are two distinct species: Puccinia recondita, which infects rye,
and Puccinia triticina, which infects wheat.
"There had long been disagreement about whether both rye and wheat are
infected by the same rust species," says Kurt Leonard, a plant pathologist
who leads research in the ARS Cereal Rust Research Unit at St. Paul, Minnesota.
"We were surprised by how big the genetic differences were between the two
This distinction between the two types of rust is important. It means plant
breeders must take the finding into account when crossing rye and wheat or
transferring genes to wheat from rye or a wild wheat relative. This is because
the crosses may result in a hybrid that is susceptible to infection by both
leaf rust species.
Working with researchers at Tel Aviv University in Israel, ARS scientists
found characteristics that distinguish between rye leaf rust and wheat leaf
"They also tried crossing the rye and wheat leaf rust fungi and found
they couldn't interbreed," says Bill Bushnell, who is a plant
physiologist. "This is strong evidence the two are not the same
ARS is working with scientists in the Middle East because wheat and related
cereals originated in that part of the world--and so did the leaf rusts that
Using sophisticated laboratory tests in cooperation with the scientists at
Tel Aviv and the University of Minnesota, ARS scientists were able to measure
the DNA content of the two rust species. It turns out the leaf rust that
affects rye and several wild relatives of wheat has significantly larger
amounts of DNA than the leaf rusts of bread wheat.
"Plant breeders have been crossing wheat with rye and wild relatives of
both for decades in an effort to bring in new resistance genes to thwart leaf
rust," says Leonard.
"Now we find that in Morocco, durum wheat, which is used primarily in
pasta, is infected with a leaf rust similar to rye leaf rust, while in the rest
of the world it is infected by the wheat leaf rust fungus. Future hybrids
between wheat and rye should be tested for resistance to both types, to be
safe." -- By Dawn Lyons-Johnson, ARS.