A Method to Increase Precision In Screening to Identify Differences In Resistance to Sudden Death Syndrome of Soybeans From Resistant by Resistant Crosses.

In order to stack quantitative trait loci (QTL) for resistance to soybean sudden death syndrome (SDS), crosses between resistant parents are becoming more common. Our goal was to modify a screening method to accurately and precisely identify differences in resistance to SDS in soybeans from resistant by resistant crosses. Severity of SDS foliar and root symptoms were assessed on a susceptible and a resistant genotype grown in soil mixed with sorghum infested with Fusarium virguliforme at four different inoculum ratios (1:5, 1:10, 1:15, 1:20 sorghum:soil). In a second experiment, SDS severity was compared over time on soybeans of eight genotypes (two resistant, two moderately resistant, two moderately susceptible, and two susceptible) grown in pots with soil at the 1:20 inoculum ratio. There was a significant effect (P<0.05) of genotype on disease severity at the 1:10, 1:15 and 1:20 ratios, with the resistant genotype showing lower foliar severity than the susceptible. Root rot severity was only significantly different at the 1:20 ratio, with the resistant having more rot (P<0.05). However, the susceptible genotype showed a greater loss in root dry weight (P<0.05). In the second experiment, the assessment time that best discriminated SDS resistance among varieties was between 30 and 33 days after planting. The two resistant genotypes displayed significantly (P<0.01) lower foliar symptoms than the moderately susceptible and susceptible genotypes. A susceptible and a moderately susceptible genotype showed higher (P<0.01) foliar severity than all the other genotypes, but the other moderate genotypes were not significantly different from each other. In this study, we were able to identify an optimum assessment time and distinguish between resistant, moderate and susceptible genotypes using a 1:20 mixture of infested sorghum and soil. Work will continue to improve the differentiation between moderate genotypes.