GENETICS OF RESISTANCE AND SUSCEPTIBILITY IN ALFALFA TO ALFALFA SICK SOIL

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Abstract
Alfalfa sickness occurs in central Alberta. It is characterized by yellowed, stunted, and poorly nodulated seedlings of alfalfa (Medicago media Pers.). Alfalfa seedlings were grown on "sick soil" in a growth room and evaluated for height, disease rating, and yield of dry matter, during three cycles of selection for resistance and susceptibility. Results showed that recurrent phenotypic selection was an effective breeding method to increase resistance to alfalfa sickness. High broad sense heritabilities for height and disease rating suggested that genotypic variability exists within locally adapted cultivars. Low narrow sense heritabilities for height and disease rating in resistant genotypes indicated that additive genetic variation was small and successive cycles of recurrent selection would be necessary to develop resistance. High specific combining ability variance in the resistant and susceptible diallel crosses demonstrated large nonadditive genetic variation.
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