Evaluating usefulness of 10 genomic regions associated with Sudden Death Syndrome resistance in soybean

Soybean sudden death syndrome (SDS) is one of the most important soybean diseases in the Midwestern United States soybean producing region, leading to losses of $190 million per year from 1996 to 2007. The causal agent of SDS is the soilborne fungus, Fusarium virguliforme. The main management practice used to reduce the impact of SDS is planting resistant soybean cultivars. Resistance to SDS is known to be quantitative with 12 known quantitative trait loci (QTL). To identify the QTL with the largest effects on reducing disease severity recombinant inbred lines (RIL) were crossed among themselves and their progeny screened for resistance to the pathogen. The current greenhouse screening method, the layer method, proved to not be sensitive enough to evaluate the progeny of the RILs, which could possess up to 12 quantitative trait loci (QTL) resistant alleles, leading to a skewed distribution towards the highly resistant side. In order to increase the sensitivity of the current greenhouse screening method, three experiments were conducted using sorghum (Sorghum bicolor (L.) Moench) infested with F. virguliforme as inoculum to modify it. The first experiment was conducted to identify the optimum inoculum density to distinguish genotypic levels of resistance to SDS when mixed homogeneously throughout the pot. The second experiment was conducted to study the association between the modified greenhouse screening method and SDS ratings from the field using eight different soybean genotypes. In the third experiment, the modified mixed method was compared to a layer method with an equivalent level of inoculum to see potential negative effects. A 1:20 inoculum:soil ratio was found to be the most effective to differentiate among resistant soybean genotypes. The modified method correlated well (r =