Evaluation location: Michigan, United States
The accessions were evaluated 2003 at the Michigan State University (MSU) Agronomy Farm in two 12.2 x 18.3 m polypropylene cages with a 0.49 mm mesh size that are aphid- and predator- proof. In each cage, a complete set of all accessions were planted as a randomized complete block. The accessions were sown on May 30 in one cage and on June 6 in the second cage. Five seeds/accession were planted in plots 0.3 m long with a row spacing of 0.3 m. At the V1 stage, 2 plants/accession were inoculated on the partially expanded trifoliate with 2 wingless aphids each, obtained from naturally infested fields on the MSU Agronomy Farm. The aphids were left to multiply and move among plants. Plants were visually rated 4 weeks after inoculation, using the following rating scale: 0 = No aphids, plant appears normal and healthy; 1 = Less than 100 aphids per plant, plant appears normal and healthy; 2 = 101 - 300 aphids per plant, mostly on the young leaves and the tender stem at top of plant, plant appears normal and healthy; 3 = 301 - 800 aphids per plant, leaves slightly curly and shiny, young leaves and stems covered with aphids; 4 = More than 800 aphids per plant, plants stunted, leaves severely curled, yellow, covered with sooty mold and cast skins. A damage index (DI) for each accession was calculated using the following formula: DI =sum of (Scale value x No. of plants in the category)/ (4 x Total no. of plants evaluated) ? 100. The DI ranges between 0% for no infestation and 100% for the most severe damage. The DIs obtained in both field cages for the same accession were averaged as the DI for the accession.