Evaluation location: Illinois, United States
An isolate of F. solani, Monticello, was increased on 200 g of sorghum seeds soaked overnight and autoclaved twice in a 1-liter flask. Each flask was inoculated with 10 plugs (4 mm diameter) of fungal mycelium and incubated for 2 weeks. Two liters of a steam-treated soil mix (1:1 sand/soil) was placed in 8 x 12-cm trays. A template was used to make three furrows that were 12 cm long, 2 cm deep, and 3 cm apart, and 5 cm3 of infested sorghum seeds was evenly distributed in each furrow. The infested seeds were covered to a depth of 2 cm. The template was reapplied to make a 2-cm-deep furrow directly over the inoculum. 12 seeds per entry were added to each furrow and covered with another 2 cm soil. After seedling emergence, the soil was maintained at near water-holding capacity by flooding the flats twice daily as needed. Three weeks after planting, each plant was evaluated for SDS leaf symptoms: 1 = no symptoms, 2 = slight symptom development, with mottling and mosaic on leaves (1 to 20% foliage affected), 3 = moderate symptom development, with interveinal chlorosis and necrosis on foliage (21 to 50% foliage affected), 4 = heavy symptom development, with interveinal chlorosis and necrosis (51 to 80%), and 5 = severe interveinal chlorosis and necrosis (81 to 100% foliage affected). In the first stage, all genotypes plus 3 standards (PI 520.733 and cvs. Ripley and Great Lakes 3202) were evaluated in the greenhouse. The entries were arranged in 8 sets. Entries within sets that had a mean disease severity of 1 standard deviation (SD) below the mean of the set were advanced. In the second stage, 133 PIs from stage one and the standards plus cvs. Jack (intermediate) and Spencer (susceptible) were inoculated in two sets. Entries with mean disease severity ratings less than 1 SD from the mean disease severity of the set were tested in a randomized complete block design with two reps. The experiment was repeated in a growth chamber at 25C and 12 hours of light per day.