Evaluation location: South Dakota, United States
A total of 334 soybean genotypes including resistant and susceptible checks were screened for resistance to the soybean aphid (SA) in three field tests in summer 2011. Genotypes included 33 from maturity group (MG) 00 and 293 from MG 0, and six resistant and two susceptible checks of later maturity. Each test was replicated three times. One of the field evaluations was conducted at the Eastern South Dakota Research Farm near Brookings, and the other two field tests were conducted at the Aurora Research Farm, South Dakota State University. At Brookings, a subset of 330 accessions was tested. SA infestations were scored on 9 Aug by using a 1-to-6 scale (1, =<25 aphids per plant, plant appears normal and healthy; 2 = 26 to 100 aphids per plant, plant appears normal and healthy; 3 = 101 to 200 aphids per plant, older leaves may be slightly yellowing; 4 = 201 to 500 aphids per plant, plant appears slightly stunted with yellowing of older leaves and minor curling of young leaves, and slight appearance of sooty mold on stem and leaves; 5, > 500 aphids per plant, often with obvious symptoms on foliage such as leaf curling, yellowing and sooty mold development; 6, > 500 aphids per plant, with extensive plant damage (e.g. curling, yellowing and sooty mold development). Natural infestations of SA in each replicate plot were supplemented by placing an aphid-infested stem across the plant canopy on 19 July; supplemental aphids were obtained from a colony housed at the USDA-ARS laboratory near Brookings. Most of the test plants were at R4 to R6 developmental stages when scored. A subset of 323 soybean genotypes was evaluated in one of two tests at Aurora when most of the genotypes were at R4 to R5 developmental stages. SA infestations were scored by using a 1-to-5 scale (similar as above). In the second test at Aurora, 64 selected genotypes plus 6 resistant and 2 susceptible checks. Test plants were inoculated by putting a leaflet or a part of a leaflet with 20 to 25 mixed-aged SA on the central plant of each plot at the V2 to V3 stage on 3 July. SA-infested leaves were obtained from colony plants at SDSU. Two weeks later, total number of aphids per plant was counted on five plants within each plot. Genotypes with <200 aphids per plant at the first count (and also resistant and susceptible checks) were counted again on the same five plants at four weeks after inoculation. Six weeks after inoculation, by which time most of the genotypes were at R5-R6 stage, all experimental plots were scored using the 1-to-6 scale PI 603712 was the only test accession rated as resistant, and PI 430491 was moderately resistant. Control lines PI 71506, PI 567541B, and PI 567543C were moderately resistant; however, control lines PI 243540, 543663, and 567597C were susceptible. All other lines were susceptible to soybean aphid.