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 in Kansas over the years – a variety starts out being resistant to leaf rust then becomes susceptible after a few years or so.
Figure 1. Leaf rust in wheat. Photo by K-State Research and Extension.
For the past several years, we have been working on 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.”
Most of our new crosses in the wheat breeding program in Manhattan now include minor gene resistance to leaf rust. When several of these genes are present in a line, we prefer to call this “non-race-specific resistance” because if conveys a moderate level of resistance to a wide range of known races of leaf rust, rather than strong resistance to just one or a few specific races of leaf rust.
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 known 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 susceptibility.
None of the newest varieties released by K-State has this kind of leaf rust resistance yet, with the possible exception of KanMark. We expect to have lines with non-race-specific, durable leaf rust resistance in advanced stages of development, or possibly ready for release, within about five years or so.
At least some of the minor genes for leaf rust resistance are also effective against stripe rust. We initially thought that varieties with minor gene, non-race-specific leaf rust resistance would also provide the same level of resistance for stripe rust. This has not turned out to be the case as these genes are somewhat less effective against stripe rust. These genes do keep stripe rust from developing as rapidly after the initial infection, but stripe rust eventually is able to become moderate to severe on these varieties. While there is some benefit to having a variety on which stripe rust develops more slowly, we believe we will need to either increase the number of minor genes used to combat stripe rust or combine them with traditional major-gene, strong resistance to specific races of stripe rust in our varieties.
Allan Fritz, Wheat Breeder