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Title:   Author  item
                                            Samac, Deborah

                                                         - *Debby*item
                                            Bucciarelli, Brunaitem
                                            Lamb, Joann
  Submitted to: Meeting Abstract
                                    Publication Type: Abstract Only
                                    Publication Acceptance Date: 3/17/2015
                                    Publication  Date: 3/17/2015
                                    Citation: Samac, D.A., Bucciarelli, B., Lamb, J.F. 2015. Mechanisms of qualitative and quantitative resistance to Aphanomyces root rot in alfalfa [abstract]. Oomycete Molecular Genetics Network. p. 34.Interpretive Summary:
                                Technical Abstract: Aphanomyces root rot (ARR), caused by Aphanomyces euteiches, is one of the most important diseases of alfalfa (Medicago sativa) in the United States. Two races of the pathogen are currently recognized. Most modern alfalfa cultivars have high levels of resistance to race 1 but few cultivars have resistance to both races. Surveys in the eastern and Midwestern US found that race 2 is predominant and new pathotypes are emerging that overcome resistance in current cultivars. Broad race-nonspecific resistance is needed to avoid the boom and bust cycles of disease resistance breeding. The aim of this project is to identify alfalfa plants with quantitative race-nonspecific resistance, understand the mechanisms of race-specific and race-nonspecific resistance, and identify DNA markers associated with resistance. Analysis of 98 alfalfa accessions from the USDA National Genetic Resources Program identified three with novel quantitative resistance to A. euteiches. Although the pathogen initially colonizes hypocotyl cortical cells, it does not cause decay, is excluded from the stele, and colonized cells are ablated as the vascular system enlarges and the seedling develops. In contrast, race-specific resistance is characterized by a hypersensitive response of individual epidermal cells upon pathogen attack. In both types of resistance, suberized cells surround the stele and strong autofluorescence occurs in cortical cells after inoculation, indicating the presence of phenolic compounds. In susceptible plants extensive cortical decay occurs, the stele is colonized, and little suberization develops. These accessions are promising source materials for developing cultivars with durable resistance to ARR.


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