Submitted to: Archives of Phytopathology and Plant Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/29/2003
Publication Date: 2/1/2004
Citation: VANDEMARK, G.J., GRUNWALD, N.J. REACTION OF MEDICAGO TRUNCATULA TO APHANOMYCES EUTEICHES RACE 2. ARCHIVES OF PHYTOPATHOLOGY AND PLANT PROTECTION.37:59-67. Interpretive Summary: Alfalfa is the fourth most important crop in American agriculture in terms of both value and acreage under cultivation. Alfalfa is typically grown as a perennial crop, with fields remaining in production for three or more years. Many soilborne organisms cause disease in alfalfa, and losses due to disease are estimated to exceed $ 1 billion annually. Applying chemical fungicides to control soilborne diseases is not an economically viable practice. Consequently, the best approach to limit losses from disease is to cultivate alfalfa varieties that are disease resistant. Developing resistant varieties is facilitated by an understanding of the genetic basis of resistance. Unfortunately, alfalfa is a tetraploid organism, having four copies of each chromosome, as opposed to many other plants and animals, which are diploid organisms having two copies of each chromosome. It is very difficult at times to study genetics in an organism such as alfalfa that has four copies of each chromosome. We chose to study resistance to Aphanomyces euteiches, a soilborne microbe that causes root rot disease of alfalfa, by identifying sources of disease resistance in barrel medic (Medicago truncatula) , a close relative of alfalfa that only has two copies of each chromosome. We identified lines of M. truncatula that were as resistant as a highly resistant alfalfa variety, and also identified lines that were as susceptible as a very susceptible alfalfa variety. The resistant and susceptible lines of M. truncatula have been crossed to produce populations that can be used for determining the genetic basis of resistance to root rot.
Technical Abstract: Aphanomyces euteiches causes root rot disease of alfalfa, and two races of the pathogen have been identified based on the reaction of established check cultivars. A majority of alfalfa cultivars grown in regions where A. euteiches is prevalent have resistance to race 1 of the pathogen, but very few cultivars have resistance to race 2. The genetic basis of resistance in alfalfa to A. euteiches is unknown, in part due to characteristics of alfalfa that impede genetic analysis, including its tetraploid genome and susceptibility to inbreeding depression. The barrel medic, Medicago truncatula, is a model organism for the study of legume genetics. Our objective was to evaluate plant introduction (PI) accessions of M. truncatula for resistance to A. euteiches race 2. In repeated experiments, employing standardized testing conditions used for evaluating resistance in alfalfa to A. euteiches, we identified seven M. truncatula accessions with levels of resistance similar to the resistant alfalfa check population WAPH-5. Two M. truncatula accessions were identified that were as susceptible as the susceptible alfalfa check cultivar Saranac. The utility of M. truncatula as a model for studying the genetics of resistance in alfalfa to A. euteiches race 2 is discussed.