Evaluation location: South Dakota, United States
Expression of soybean aphid (SA) resistance was characterized among 746 soybean accessions in 56 growth chamber tests at the North Central Agricultural Research Laboratory, Brookings, SD, from 2012 through 2015. Test plants were prepared by placing two seeds of a particular soybean accession into an 8.5-cm square plastic pot filled with a 2:1:1 mixture of soil, vermiculite, and peat moss. Ten to 12 days after planting, pots were thinned to one plant by selecting for similarly sized seedlings among test accessions, and the soil surface was then covered with a thin (ca. 2 cm) layer of sand. Two wks after planting, test accessions were placed into a plastic tray that held 18 square pots. Fifty-four of the 56 tests used a RCBD in which each tray comprised a replicate consisting of 14 test accessions, a susceptible check, a resistant check, and two SA-source plants. The other two tests used a CRD: one in which two paired trays comprised a replicate of 30 test accessions, and the other with two replicates of 6 test accessions per tray. Tests had from six to eight replications. Each source plant was a 4-wk-old PI 667735 soybean plant with about 250 SA that were derived from a polyclonal stock colony established in 2009, were reared on PI 667735 soybean, and had shown no virulence to known SA-resistant lines. Source plants were placed at one of two foci in each tray equidistant from immediately surrounding test accessions for even dispersion of SA. The source plants were clipped at soil level at the start of each test to induce wilting and subsequent dispersal of SA to test plants, and clipped plants were supported by attaching them via paper clip to a small wooden stake in the soil. Tests were run in environmental chambers at 16:8 L:D photoregime and temperatures from about 18oC (nighttime) to 22oC (daytime). Two weeks later when test plants were in the second trifoliolate leaf stage (V2 stage), infestations were rated by two researchers on a 1-to-6 scale, with 1 = 0 to 50, 2 = 51 to 100, 3 = 101 to 150, 4 = 151 to 200, 5 = 201 to 250, and 6 with >250 SA per test plant, respectively. For a small number of accessions that were included in more than one test, individual ratings were averaged across tests. Medians of infestation ratings are reported for each accession. Accessions were pooled across tests and ranked by median infestation rating. An accession was classified as resistant if its median rating was less than 3.0; moderately resistant if median rating was from 3.0 to 4.9; and susceptible if rating exceeded 4.9. Results show that the vast majority of accessions were susceptible to SA. However, 10 accessions were resistant to SA, and 23 were moderately resistant.