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Resistance to Bean Rust
April 15, 2003

Jack Rasmussen
Department of Plant Pathology
and Ken Grafton
Department of Plant Sciences
North Dakota State University

Bean rust is a continuous threat to dry bean production in the Northarvest region. Dry bean varieties released from breeding programs at NDSU possess excellent rust resistance, and widespread use of these varieties has reduced the incidence and severity of rust. However, the resistance in these varieties is conditioned by a single gene, Ur-3. The natural population of the bean rust pathogen can overcome genetic resistance, especially single gene resistance, over time. New races of the fungus may develop in the future that could cause disease on varieties protected by only Ur-3. Once defeated by the pathogen, a gene such as Ur-3 is no value and must be discarded. The best strategy from preventing this from happening is to combine other resistance genes with Ur-3 in future cultivar releases. The combining of multiple rust resistance genes into a single cultivar, known as gene pyrimiding, has extended the life of rust resistance genes in other crops. A Nicaraguan dry bean, designated CNC, contains genetic resistance effective against all races of the bean rust fungus found in the Northarvest region. Thus, the resistance found in this line could be extremely valuable to the NDSU breeding program, especially if the resistance could be combined with Ur-3 in future releases.

As a first step toward using the rust resistance, CNC was crossed with Othello, a rust-susceptible pinto bean cultivar adapted to the Northarvest production area. Progeny of this cross were used to establish a population of over 100 lines that segregated for rust resistance and susceptibility. To our surprise, experiments with these lines and appropriate statistical analyses clearly demonstrated that CNC possesses at least two rust resistance genes located on two different chromosomes. During the last year, we have advanced the population to the F5 generation with the ultimate goal of advancing the population later this summer or fall to the F6 generation. F6 lines are genetically stable and may be suitable for use in the breeding program. We have also conducted inoculation tests with multiple races of the fungus on the population to determine which of the two genes in CNC is most useful for combination with Ur-3. It is possible that both genes are required for widespread control of the many races of bean rust, but it would be more difficult to combine two genes with Ur-3 than it would be to combine one. Currently, we are approximately half way through the races we want to test. Preliminary tests indicate that one of the genes in particular would be effective with Ur-3 against many races that we wish to control. We are actively conducting additional experiments to confirm and extend this observation.


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Northarvest Bean Growers Association | 50072 East Lake Seven Road | Frazee, MN 56544
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