Association mapping and genomic prediction for resistance to sudden death syndrome in early maturing soybean germplasm.

Sudden death syndrome (SDS), caused by Fusarium virguliforme, has spread to northern soybean growing regions in the US causing significant yield losses. The objectives of this study were to identify loci underlying variation in plant responses to SDS through association mapping (AM) and to assess prediction accuracy of genomic selection (GS) in a panel of early maturing soybean germplasm. A set of 282 soybean breeding lines was selected from the University of Minnesota soybean breeding program and then genotyped using a genome-wide panel of 1536 single-nucleotide polymorphism markers. Four resistance traits, root lesion severity (RLS), foliar symptom severity (FSS), root retention (RR), and dry matter reduction (DMR), were evaluated using soil inoculation in the greenhouse. AM identified significant peaks in genomic regions of known SDS resistance quantitative trait loci cqSDS001, cqRfs4, and SDS11-2. Additionally, two novel loci, one on chromosome 3 and another on chromosome 18, were tentatively identified. A ninefold cross-validation scheme was used to assess the prediction accuracy of GS for SDS resistance. The prediction accuracy of single-trait GS (ST-GS) was 0.64 for RLS, but less than 0.30 for RR, DMR, and FSS. Compared to ST-GS, none of multi-trait GS (MT-GS) models significantly improved the prediction accuracy due to weak correlations between the four traits. This study suggests both AM and GS hold promise for implementation in genetic improvement of SDS resistance in existing soybean breeding programs.

Predicting the risk of soybean rust in Minnesota based on an integrated atmospheric model.

To minimize crop loss by assisting in timely disease management and reducing fungicide use, an integrated atmospheric model was developed and tested for predicting the risk of occurrence of soybean rust in Minnesota. The model includes a long-range atmospheric spore transport and deposition module coupled to a leaf wetness module. The latter is required for spore germination and infection. Predictions are made on a daily basis for up to 7 days in advance using forecast data from the United States National Weather Service. Complementing the transport and leaf wetness modules, bulk (wet plus dry) atmospheric deposition samples from Minnesota were examined for soybean rust spores using a specific DNA test and sequence analysis. Overall, the risk prediction worked satisfactorily within the bounds of the uncertainty associated with the use of modeled 7-day weather forecasts, with more than 65% agreement between the model forecast and the DNA test results. The daily predictions are available as an advisory to the user community through the University of Minnesota Extension. However, users must take the actual decision to implement the disease management strategy.

Introduction of Asian Soybean Rust Urediniospores into the Midwestern United States-A Case Study.

In 2005, weekly rain samples collected at 124 National Atmospheric Deposition Program/National Trends Network (NADP/NTN) sites in the eastern and central United States were screened for Asian soybean rust (ASR; Phakopsora pachyrhizi) urediniospores. Application of a quantitative polymerase chain reaction method detected P. pachyrhizi DNA in the filter residue of rain samples collected during the week of 19 to 26 July 2005 in Minnesota, Missouri, and South Dakota. To determine the geographic origin of ASR urediniospores in those weekly composite samples, back air trajectories of the lifted condensation and mixed boundary layers were calculated for each rain event within the week, by sampling site. The calculations, based on the hybrid single-particle lagrangian integrated trajectory model, pointed to source areas in eastern and southern Texas. In a separate case, DNA of P. pachyrhizi was detected in a 28 June to 5 July 2005 rain sample from an eastern Texas site. Back trajectories pointed to southern Texas and the Yucatan Peninsula in Mexico as potential source areas of ASR urediniospores. Vertical motions of those back trajectories indicated a ventilation of the boundary layer in the upwind areas, suggesting the possible injection of urediniospores into the free troposphere where they can be transported for long distances before wet deposition.