New Approach to Breeding for Leaf Rust Resistance in Wheat
Traditionally, wheat breeders have been selecting for leaf rust
resistance by finding lines with strong resistance to one or more races
of leaf rust. The problem has been that other races of leaf rust
inevitably come along that can overcome that strong, but narrow source
of resistance. If that different race of leaf rust becomes widespread
in the Southern Plains, the resistant variety rapidly becomes
susceptible to leaf rust.
That has happened to most wheat varieties farmers are familiar with – such as Overley, Jagalene, Jagger, and many more.
are now taking a different approach, similar to the methods used for 20
years or more by wheat breeders at CIMMYT (International Maize and
Wheat Improvement Center in Mexico) to develop leaf rust resistance.
This approach involves combining 3 or more minor genes for leaf rust
resistance into a durable, effective, non-race-specific, slow-rusting
form of resistance.
There are several of these so-called minor
genes for leaf rust resistance. Two of the better known to plant
pathologists and wheat breeders are “Lr34” and
We have been crossing a genotype called
“Amadina” onto Overley. Amadina has four minor genes for
leaf rust resistance. We currently have 11 such crosses in the Kansas
Intrastate Nursery tests. If all goes well with these experimental
lines, we may have a new variety with durable leaf rust resistance
ready for release in 2011.
Within the next year or two, we plan
for all of our new crosses in the wheat breeding program in Manhattan
to include minor gene resistance to leaf rust. This is a departure from
the past. Previously, all of our leaf rust resistance breeding was
focused on major gene resistance to specific races of leaf rust.
Minor gene, slow-rusting resistance is characterized by:
* Smaller leaf rust pustules
* Fewer leaf rust pustules
* A longer latent period of time between the time of infection and rust pustule development
with minor gene resistance will be susceptible to leaf rust in the
seedling stage, but have durable, non-race-specific resistance in the
adult stage. Varieties with a combination of at least three minor genes
for leaf rust resistance will usually not be entirely free of leaf rust
pustules if leaf rust is present in the area. But the leaf rust will
typically occur late in the season and with only light to moderate
severity – causing relatively little, if any, yield loss. The
best thing is that this type of resistance is effective against all
races of leaf rust, making it durable from year to year. Varieties with
minor gene resistance should maintain a good level of leaf rust
resistance year after year, without becoming fully susceptible.
minor genes for leaf rust resistance also carry minor gene resistance
for stripe rust (yellow rust). Lr34 is linked with Yr18, and Lr46 is
linked with Yr29. As a result, the new varieties developed for minor
gene, durable, slow-rusting leaf rust resistance will also have the
same type of durable resistance for stripe rust.
If this type of
durable leaf rust and stripe rust resistance can be incorporated into
most or all of our new wheat varieties, that will give us more
resources to focus on other traits in our breeding problem, such as
scab resistance, Hessian fly resistance, quality, and others.
Source: K-State Research and Extension Agronomy