Evaluation location: Illinois, United States
G. soja accessions were tested with P. sojae isolate race 3 using the plug-inoculation method. Seeds were planted in 10-cm-deep flats containing a 1:1 mix of sterilized sand and soil. The seedlings were allowed to grow for 10 days in a greenhouse with a 16-h photoperiod, a 26/21?C day/night temperature, and daily watering. Race 3 P. sojae isolate was grown for 14 days on V8 juice agar. A 3-mm cork-borer then was used to cut 3-mm plugs out of the colonies. A single plug, mycelial-side down, was placed on a cotyledon immediately adjacent to the stem of each seedling. Following inoculation, all seedlings were lightly misted with water using 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 buildup inside the domes. After 5 days, the domes were removed. Two days later the shade cloth was removed and the number of seedlings that survived or died was counted. If a plug fell from the cotyledon before infection, the seedling was counted as an escape. The soybean differentials Mack and Harosoy 63 were used as controls.