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Combining Ability Effects for Resistance to Four Insects in Selected Alfalfa Clones¹

Diallel cross progenies, including reciprocals, of six alfalfa (Medicago sativa L.) clones that differed in miridinduced leaf malformation and aphid resistance were studied to obtain estimates of combining ability effect for characters associated with resistance to the spotted alfalfa aphid (SAA), Therioaphis maculata Buckton; the peaaphid (PA), Acyrthosiphon pisum Harris; and two mirids: the alfalfa plant bug (APB), Adelphocoris lineolatus Goeze, and the tarnished plant bug (TPB), Lygus lineolaris Beauv. The responses to mirid feeding were measured in the field with controlled APB and TPB infestations under cages. Aphid tests were conducted on seedlings in the greenhouse. The hypothesis that the same genetic system controlled feeding by APB and TPB was supported by the absence of significant genotype x mirid species interactions. Forage yield and stem length were reduced 5 and 20%, respectively, whereas percentages of dry matter were unchanged and protein percentages increased 4% due to mirid infestation. Resistance to mirid-induced forage yield reduction and stunting was not found. Leaf malformation, a symptom of mirid feeding, appeared to be a highly heritable trait, but was not correlated with forage yield reduction, stunting, forage quality, or aphid resistance. General combining ability effects were far more important than specific combining ability effects among single crosses for forage yield, stem length, and leaf malformation under mirid infestation, and for PA and SAA resistance.

Key Words: Breeding • Genetics • Pest resistance

² Formerly graduate student, presently research agronomist, Waterman-Loomis Co., 2000 Century Plaza, Suite 110, Columbia, MD 21044; research agronomist, ARS-USDA, and professor of agronomy; and assistant professor, Laboratory of Agricultural Biochemistry, Univ. of Nebraska, Lincoln, NE 68583.

Received for publication September 24, 1976.