Evaluation location: Illinois, United States
Ancestral soybean lines representing approximately 99% of the genes in modern North American cultivars and lines identified in the pedigrees of previously identified mostly resistant (MR) lines were screened for resistance to SDS. Four liters of a steam-treated soil mix (1:1 sand/soil) was placed in 50 x 35 x 10 cm flats. A template was used to make seven furrows 2 cm deep and 35 cm long, and 10 cm3 of sorghum seed infested with F. solani f. sp. glycines isolate Mont-1 was evenly distributed in each furrow. The inoculum was then covered with 2 cm of soil, and the template was reapplied to make a 0.5-cm-deep furrow directly over the inoculum. Soybean seed was added to each furrow and covered with another 2 cm of soil. Plants were then placed on a greenhouse bench and grown under a 16-h photoperiod. Plants were watered to saturation after planting and maintained at near field capacity throughout the study. 24 days after inoculation, plants were rated for SDS foliar symptoms. Plots were rated if two or more seeds emerged. Five seeds per entry were planted in flats containing 20 entries and one check. The susceptible check was Great Lakes 3302, and the moderately resistant checks were PI 520733 and PI 567374. The ancestral set was evaluated 3 times and the pedigree set was evaluated twice using a randomized complete block design with 3 replications.