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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.

We 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 “Lr46.”

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

Varieties 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.

The 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
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