Samac, DeborahBucciarelli, BrunaDornbusch, MelindaMiller, SusanYu, Long-Xi2018-02-272018-02-272017https://hdl.handle.net/11299/194186Aphanomyces root rot, caused by Aphanomyces euteiches, is one of the most important diseases of alfalfa in the United States. Two races of the pathogen are recognized and although most cultivars are resistant to race 1, fewer have resistance to race 2, the predominant race in North America. Molecular markers are needed to facilitate breeding for resistance and to clarify race/resistance gene structure. Resistant and susceptible seedlings were identified from three resistant cultivars, WAPH1, WAPH5 and 53V52, and used as parents to produce F1 populations. Severity of symptoms corresponded with amount of pathogen DNA and oospore density in roots. Race-specific resistance involves a hypersensitive response of individual epidermal or cortical cells upon pathogen attack followed by suberization of cells surrounding the stele and strong autofluorescence in cortical cells, indicating the presence of phenolic compounds. Segregation ratios of F1 populations suggested that resistance to race 1 in WAPH1 is conditioned by a single gene but resistance to race 1 is multigenic in WAPH5 and 53V52, and resistance to race 2 is multigenic in all three cultivars. Segregation for resistance to seven strains of A. euteiches in 70 F1 full-sib plants derived from 53V52 suggested the presence of clustered resistance genes and multiple race types. Identification of resistance gene loci is in progress using genotyping by sequencing and genetic mapping of F1 populations.enalfalfaAphanomyces euteichesdisease resistanceOther