Evaluation location: Illinois, United States
Fifty-two soybean accessions were selected to represent the major ancestral lines of NA cultivars. Williams 82 was used as the susceptible check. Bean pod mottle virus isolate 98 was collected from infected soybean at the CSREC, Urbana, IL, and was maintained by continuous transfer and stored long term in desiccated refrigerated leaves. It belongs to subgroup I. Screening was done in 2002 and 2003. In the greenhouse, eight seeds of each line tested were planted in a pasteurized soil mixture (1:1:1 soil-sand-peat) in plastic flats (52- by 37-cm) or in 10-cm plastic pots with a soilless mix (Sunshine Mix LC1, Sun GroHorticulture, Inc., Bellevue, WA) and covered with coarse vermiculite. All entries without symptoms were retested using five to six plants. Inoculum consisted of extracts from infected leaves of Williams 82 plants maintained in the greenhouse that were prepared by homogenizing infected leaves in chilled 0.025 M KPO4 buffer, pH 7.1, plus 0.01 M sodium sulfite with sterilized pestles and mortars. Pestles were used to apply inoculum to carborundum-dusted leaf surfaces. Plants were inoculated with BPMV 7 to 10 d after planting at growth stage Vi. Two to three weeks after inoculation, trifoliolate leaves were examined for systemic virus symptoms. Resistance reactions were recorded when plants were symptomless. In cases where it was difficult to distinguish between mild and no symptoms, ELISA results were used to determine the reaction. At least three plants were tested by ELISA to verify phenotypic observations. Lines with a negative ELISA reaction and no systemic symptoms were considered virus resistant.