Inoculum Rate Influences Selection for Field Resistance to Soybean Sudden Death Syndrome in the Greenhouse

Effective selection of field resistance to soybean sudden death syndrome (SDS) caused by Fusarium solani (Mart.) Sacc. f. sp. glycines (Fsg) (Roy, 1997), measured by disease index (DX), requires multiple environments. Current greenhouse assays reduce genotype × environment interactions, but fail to predict field resistance. Our objective was to compare selection for field resistance to SDS in the greenhouse among recombinant inbred lines (RILs) inoculated with Fsg at three rates. Thirty soybean [Glycine max (L.) Merr.] RILs with characterized field resistance to SDS were evaluated in the greenhouse for scorch severity at three inoculum rates in four experiments. Ten cultivars with characterized field resistance were compared using disease severity (DS) readings from one experiment at one inoculum rate. The heritability of DS among RILs in the greenhouse was 46% at the low, 66% at the moderate, and 37% at the high inoculum rates. Reduced inoculum rates in the greenhouse (3500 to 5000 spores cm−3 plant growth medium) provided DS values that explained ≈65% of variation in the field Using a Fsg inoculum rate of 5000 spores cm−3 plant growth medium and greenhouse midparent DS as criterion for selection, the number of lines potentially resistant to SDS within a segregating population could be reduced by 53%. Errors caused ≈10% of field resistant lines to be eliminated. Among unrelated soybean cultivars, greenhouse DS values from an inoculum rate of 4000 spores cm−3 plant growth medium explained 81 and 73% of variations in field DS and DX, respectively. Therefore, the method is an effective tool for inheritance studies and cultivar evaluation for SDS.