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Leaf rust is the most common disease of wheat in Minnesota and in the U.S. Jim Kolmer, a plant
pathologist at the USDA-ARS Cereal Disease Laboratory has been working with Jim Anderson, wheat breeder at the University of Minnesota, to improve the leaf rust resistance in advanced breeding lines,
with the goal of having wheat cultivars that have very good leaf rust resistance, high yield, good quality and agronomic traits combined with FHB resistance.
From 2005-2007, in test plots throughout Minnesota, the commonly grown spring wheats Oklee, Freyr, and Ada that have
intermediate leaf rust resistance had an average yield loss of 9%, 8.5%, and 8%, respectively due to leaf rust. The cultivars Marshall and Oxen that are highly susceptible to leaf rust had average losses
of 30% and 19% respectively in the same years. New races of leaf rust with higher virulence to some of the resistance genes in the highly resistant cultivars Knudson and Briggs have also increased in
frequency, placing greater leaf rust pressure on these cultivars.
The development of rust resistant cultivars depends on leaf rust resistance genes that are highly effective against the
current population of leaf rust races. In recent years, University of Minnesota wheat breeding lines with leaf rust resistance genes Lr21, Lr22a, Lr32, Lr47, Lr52,
and Lr60 have been highly resistant in research plot tests. These genes have not been widely used in wheat in the U.S. or Canada and will provide good
levels of leaf rust resistance in the Minnesota spring wheats.
Over 50 different races of leaf rust are found in the U.S. each year. New races that can attack resistant wheat cultivars
are constantly increasing. As a result, it is critical that new genes for leaf rust resistance be characterized and added to wheat breeding programs. A new gene for leaf rust resistance derived
originally from einkorn wheat has been mapped to chromosome 3AS using molecular markers. A new gene originally from durum wheat was located on chromosome 6AL, and an additional gene from common wheat has
been mapped to chromosome 6BL. Wheat lines with new genes derived from wheat landraces that were grown in the early 20th century in South America, and from soft red winter wheat germplasm, have been
tested for leaf rust resistance. Currently in development is the process of segregating populations with these genes for chromosome mapping with molecular markers. Wheat lines with the new leaf rust
resistance genes have been used in crosses with Minnesota breeding lines and cultivars in order to develop new cultivars with new and highly effective genes for leaf rust resistance.
The cultivars Oklee (2003) with genes Lr34; Ulen (2005) with Lr23 and an additional gene that has not been characterized; Ada with Lr23 and Lr34 (2006), RB07 with Lr21 (2007), Tom (2008) and Sabin (2009) with Lr34, released by the University of Minnesota AES have improved leaf rust resistance
compared to previously grown cultivars such as Oxen, Ingot, and HJ98. Genes Lr23 and Lr34 have provided
long lasting resistance in spring wheats in Minnesota and Lr21 is currently effective to all races of leaf rust in the U.S.
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