Polygenes in Diploid Alfalfa That Affect Resistance to Downy Mildew
- D. Z. Skinner and
- D. L. Stuteville
The genetics of resistance in alfalfa (Medicago sativa L.) to downy mildew (caused by Peronospora trifoliorum d By.) is complex. Our previous studies with diploid alfalfa indicated that resistance (absence of conidial production) resulted from single dominant genes in some plants, and combinations of minor genes in other plants. The present sludy was conducted with diploid plants possessing only minor resistance genes to determine if different combinations of minor genes conditioned a single disease phenotype, and to determine the dominance relationships of the alleles involved. A seedling test was used in which downy mildew symptoms were described on a 0 (resistant, no conidial production) to 5 (very susceptible, copious conidiai production) infection type (IT) scale. Chi-square analysis of progeny IT data revealed that 30% of paired plants with the same IT were genetically different. Most F2 plants from resistant F1, plants were resistant, whereas susceptible F1, plants produced F2's of all disease scores, including resistance. This was consistent with dominant susceptibility at most loci, and additivity of effects among loci.
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