Selection for Resistance to Anthracnose (Colletotrichum trifolii) in Four Alfalfa Populations1

Three cultivars and one experimental population of alfalfa (Medicago saliva L.) were subjected to three cycles of recurrent phenotypic selection in the laboratory and greenhouse for resistance to anthracnose, caused by Colletotrichum trifolii Bain and Essary. The frequency of highly resistant plants in the cultivars ‘Glacier,‘ ‘Saranac,’ and ‘Vernal’ increased from 5, 5, and 1% to 82, 72, and 59%, respectively. The frequency of highly resistant plants in the experimental population MSHp6F, which had a higher initial level of resistance, increased from 20 to 88%. Highly resistant plants were characterized by the absence of lesions and acervuli. Rapid progress in selection and the bimodal distribution of the MSHp6F population series suggested that resistance in this population may be controlled by a single dominant gene. The distribution and selection progress in Vernal suggested a more complex mechanism for conferring resistance in this population.