2007 U of M Research Special Report

Mapping New Genes for Leaf Rust Resistance in Wheat Cultivars

Leaf rust occurs annually on wheat in Minnesota, causing significant yield losses in susceptible cultivars such as Oxen and Reeder. New races that can attack leaf rust resistant wheat cultivars are constantly emerging.  In fact, many different leaf rust races are found in the U.S., over 50 each year. As a result, it is critical that new genes for leaf rust resistance be characterized and added to wheat breeding germplasm.

Jim Kolmer, a plant pathologist at the USDA-ARS Cereal Disease Laboratory, St. Paul, has been working with U of M wheat breeder Jim Anderson to characterize new genes for leaf rust resistance in wheat, and to improve the leaf rust resistance in advanced breeding lines. The goal is having wheat cultivars that have very good leaf rust resistance, high yield, good quality and agronomic traits combined with FHB resistance.

Wheat lines with new genes for leaf rust resistance that were originally derived from durum wheat, einkhorn wheat, and an accession of common bread wheat, were crossed with the leaf rust susceptible wheat Thatcher, and the segregating progenies were tested for leaf rust resistance. 

The wheat lines with the resistance genes and Thatcher were tested to determine which chromosome-specific DNA markers showed differences between the resistant lines and Thatcher.  More than 25 markers have been found that showed differences between the resistant and susceptible wheat lines. These markers will be used to determine which of the 42 chromosomes carry the new genes for leaf rust resistance.  The gene from einkhorn wheat has been mapped to the long arm of chromosome 3A.

Kolmer and Anderson will soon be working on mapping new genes for leaf rust resistance from wheat landraces that were grown in the early 20th century in South America.  The landraces are highly resistant to leaf rust and also carry new leaf rust resistance genes. Once the chromosome locations of all these new genes are determined, tightly linked DNA markers for the genes can be developed to assist in selecting wheat breeding lines with these genes.

The wheat improvement project at the University of Minnesota also tests all preliminary and advanced breeding lines for leaf rust resistance.  Preliminary lines are tested in field plots in St. Paul that have been inoculated with the most common leaf rust races in the spring wheat area of Minnesota, North Dakota and South Dakota.

Breeding lines that show good resistance in this test should also have high levels of resistance in plots and farm fields. Advanced breeding lines are also tested with a number of leaf rust races individually in greenhouse tests to characterize their leaf rust resistance genes.

New genes for leaf rust resistance are also being added to the wheat project. The leaf rust resistance genes Lr21, Lr22a, Lr46, Lr47, and Lr52, have been crossed into the wheat germplasm by testing for rust resistance and by using DNA markers. Wheat lines that have one of these genes combined with the resistance genes already present in the breeding lines and cultivars are highly resistant to leaf rust. Recent U of M released cultivars Oklee (2003), Ulen (2005), Ada (2006), and RB07 (2007) have better leaf rust leaf resistance compared to previously grown cultivars such as HJ98, Ingot, and Oxen.

– Dr James Kolmer, research plant pathologist, USDA-ARS Cereal Disease Laboratory, St. Paul