Date of Award
Master of Science
Silvia R Cianzio
Leonor F Leandro
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 = 0.82, P<0.05) with field ratings at 36 days after planting (dap). It was also determined that the best time to evaluate genotypes was from 30 to 36 dap. This modified mixed method was capable of shifting the frequency of the SDS scores of the progeny from the RILs to approach a normal curve.
Using this modified method, the 12 QTL from five resistant parents (EF23, FH13, FH33, FH35, and PD98) obtained from the recombinant inbred line (RIL) populations `Essex' X `Forrest', `Flyer' x `Hartwig', and `Pyramid' x `Douglas' were evaluated. A total of 321 F2:3-derived lines were evaluated for four disease assessment criteria, eg. disease incidence (DI), disease severity (DS), area under the disease progress curve (AUDPC), and root rot severity. Five QTL (qRfs4, qRfs5, qRfs7, qRfs12, and Rfs12) were shown to be associated with at least one disease assessment criteria across multiple populations. Two of the QTL also showed an association with more than one disease assessment criteria across populations, eg. qRfs4 was associated with DI, AUDPC, and root rot severity and qRfs12 was associated with AUDPC and root rot severity. These findings provide breeders with new tools to evaluate germplasm for resistance to SDS and show the 12 QTL have differences in SDS resistance. This information is of importance to develop new and improved SDS resistant cultivars.
Alexander Stephen Luckew
Luckew, Alexander Stephen, "Evaluating usefulness of 10 genomic regions associated with Sudden Death Syndrome resistance in soybean" (2012). Graduate Theses and Dissertations. 12391.