Evaluation location: Illinois, United States
Seeds of the perennial Glycine species were scarified by scoring seed coats on the opposite side of the hilum with a razor blade. Seeds were then germinated on moist filter paper inside petri dishes under light at 25C for 4 to 6 d. Germinated seedlings were planted in a 1:1 steam-sterilized soil:sand mix (1:1 v/v) at pH 7. The soil was a Drummer silt loam. All plants were grown in greenhouses under a photoperiod of 16 h of daylight at 24 +/- 3C/18 +/- 3C day/night temperature. For all evaluations, transplanted seedlings were grown for 2 to 3 wk before being inoculated. Seeds of the soybean checks were sown when the perennial accessions were transplanted. The seedlings were grown in a greenhouse and watered daily for 10 to 14 d before inoculating. Five seedlings of each accession were screened once in a series of eight nonreplicated runs that averaged 98 accessions per run. Cultivar Williams 82 was used as a susceptible check in each run. An isolate of S. sclerotiorum originating from infected plants from a field near Dekalb, IL, in 1994 was maintained the dark at 48C on potato dextrose agar. Mycelial plugs were transferred to acidified potato dextrose agar (APDA) and after 1 d, this culture was used as starter inoculum by transferring 3-mm-diam plugs from the margin of the colony to new APDA plates. After 1 to 2 d, 3-mm-diam plugs from the colony edges were used to inoculate plants. To inoculate the perennial plants, a single plug, mycelial-side down, was placed on the newest leaf petiole touching the main stem or in the plant whorl. For soybean plants, a single mycelial plug was placed mycelial-side down on a cotyledon approximately 2 mm from the stem of each seedling. Following inoculation with the plug method, all seedlings were lightly misted with water from a hand atomizer to increase humidity and then covered with a plastic dome that fit over individual flats. The dome-covered flats were placed about 1 m under black mesh shade cloth (80% light reduction) to prevent heat build up inside the domes. After 2 d, the domes and shade cloth were removed. The number of total seedlings that survived was counted daily usually beginning with the day the dome was removed until plants stopped dying, which was 4 to 5 d after the first rating. A selected set of 53 accessions, which were based on the results of the non-replicated runs with some accession representing most species, was further screened in two replicated trials with five plants in each of four replications. Soybean cultivars Asgrow A2242 (susceptible) and NK S19-90 (partially resistant) were used as checks. A final selected set of 12 accessions, which were based on those 53 accessions that preformed well within some of the species, was screened in two trials with three replications each.