Development of Two Alfalfa Populations with Resistance to Insect Pests, Nematodes, and Diseases. I. Aphid Resistance1

‘Turkistan’ alfalfa (Medicago saliva L.) is a good source of resistance to aphids, to organisms such as stem nematode associated with moist soil conditions, and diseases such as bacterial wilt and Phytophthora root rot. It is very susceptible to foliar pathogens. In 1959, two germplasm pools, MSE and MSF, were initiated at Beltsville, Md., by intercrossing 50 plants of Turkistan origin with 50 foliar disease‐resistant plants of MSA‐9, MSB‐9, and ‘DuPuits’ alfalfa. After four cycles of intercrossing and recombination within pools, the two pools were sent to Reno, Nev., and later to Tucson, Ariz., to undergo two cycles of penotypic selection for resistance to spotted alfalfa aphid [Therioaphis maculata (Buckton)] and pea aphid, [Acyrthosiphon pisum (Harris)]. From 65 to 300 plants were intercrossed within each cycle and pool to initiate a new cycle. Cycles 4, 5, and 6 were evaluated for seedling survival under severe aphid infestations. An average of 95 and 92% of the plants of MSE‐6 and MSF‐6, respectively, survived the spotted alfalfa aphid test, compared to 1% for ‘Caliverde’. An average of 60 and 42% of the plants of MSE‐6 and MSF‐6, respectively, survived the pea aphid test, compared to 0% for Caliverde. This form of mass selection was effective in developing resistance to spotted alfalfa aphids and pea aphids.