Evaluations of Soybean Accessions for Resistance to Soybean Rust in the United States David R. Walker 1, Randall L. Nelson 1, Glen L. Hartman 1, Monte R. Miles 1, Raymond W. Schneider 2, Steve Moore 2, Blair Buckley 2, David Weaver 3, Emerson Shipe 4, John D. Mueller 4, and H. Roger Boerma 5 1 USDA-ARS Soybean-Maize Germplasm, Pathology & Genetics Research Unit, Urbana, IL; 2 Louisiana State University, Baton Rouge (RWS), Alexandria (SM), and Bossier City (BB), LA; 3 Auburn University, Auburn, AL; 4 Clemson University, Clemson (ES) and Blackville (JDM), SC; and 5 University of Georgia, Athens, GA. Abstract Since the arrival of soybean rust in North America in November 2004, field evaluations have been conducted at multiple locations in the southern USA. Plant Introductions (PIs) from maturity groups 000 to X have been tested, so late summer planting dates and artificial lighting have been used to compress maturity dates among entries. Although this has been effective, accessions from early maturity groups still flower sooner after the extended photoperiod is discontinued, so subdivision of the entries into classes composed of PIs from similar maturity groups may reduce bias in interpretation of rating data. Some accessions that appeared relatively resistant in one location/test were more susceptible in others, but several dozen PIs, including representatives from most of the maturity groups, have exhibited high to moderate resistance in all of the field tests as well as in greenhouse assays. Resistance evaluation methods continue to evolve as were learn more about soybean rust and how to assess resistance. Evaluations are being conducted at seven locations in five states between Louisiana and South Carolina in Altogether these data will help to guide plant breeders in making decisions about which sources of resistance appear most appropriate for their breeding programs to develop rust-resistant cultivars adapted to North America Locations with Supplementary Lighting -452 germplasm accessions and susceptible cultivars from MGs 000 through X were planted in Blackville (SC), Attapulgus (GA), Quincy (FL), Alexandria (LA) and Baton Rouge (LA) ( Fig. 1). -Accessions were divided into four tests on the basis of maturity group: (1) MGs 000-I; (2) MGs II-IV; (3) MGs V-VII; and (4) MGs VIII-X. Grouping accessions with similar maturities minimized differences in supplementary light intensity among accessions with similar maturities and reduced evaluation bias. -The tests were planted between July and September, and the natural photoperiod was extended for one month using mobile lighting towers (Fig. 2) -Two replications were planted and plots were rated in October and November Introduction -Soybean rust (SBR), caused by Phakopsora pachyrhizi, can cause severe yield losses when temperatures of 16-26°C coincide with abundant moisture. -Resistance can be conditioned by single genes (e.g., Rpp1) or through quantitative action of genes that slow development of the disease and/or production of urediniospores. -Resistance can be manifested as immunity (no lesion development), reddish-brown (RB) instead of tan (TAN) lesions, reduced lesion development (also referred to as “slow rusting”), and/or reduced sporulation from uredinia. -Greenhouse evaluations of resistance are limited by available space and inoculum, and by an unclear correspondence between resistance at the seedling and adult stages of the host. -Evaluation of resistance in field tests is complicated by (i) the effects of environmental factors, (ii) temporal and geographical variation in pathogen isolates, (iii) interactions between plant growth stage and disease development, (iv) disease and damage caused by other pests and pathogens, (v) differences in maturity among entries, and (vi) the difficulty of synchronizing plant maturity with the peak of a rust epidemic at a specific location. -A field evaluation strategy used by the University of Georgia in 2005 at Attapulgus, GA involved planting soybean germplasm accessions from MGs 000 to X between mid-August and early September, and extending the natural photoperiod through the use of mobile lighting towers. This method, which succeeded in compressing maturity dates and synchronizing plant maturity with the fall SBR epidemic in the area, has served as a model for subsequent germplasm evaluations for SBR resistance. -Researchers from five public institutions collaborated in 2007 to evaluate USDA germplasm accessions for SBR resistance in seven locations (Fig. 1). -The objective of these evaluations is to identify or confirm plant introductions (PIs) with resistance to rust isolates in multiple locations to provide North American plant breeders with guidance in choosing parents to develop rust-resistant soybean cultivars Locations with No Supplementary Lighting -293 germplasm accessions and check cultivars from MGs IV through X were planted in Bossier City, LA and Fairhope, AL (Fig. 1). -Two replications were planted and plots were rated in October and November Acknowledgements We thank the United Soybean Board for funding this research including the cost of supplementary lighting and colleagues at the University of Florida’s North Florida Research & Education Center, the Attapulgus (GA) REC, the Dean Lee (Alexandria), Red River (Bossier City), Central (Baton Rouge), and Edisto (Blackville) Research Stations, and Auburn’s Gulf Coast Substation in Fairhope. Special thanks to Katey Pauls (USDA-ARS), Tristan Mueller (Univ. of Florida), and Nick Dufault (Pennsylvania State Univ.). 4. Rating Methods - Entries at 2006 locations were rated by visual examination of plants within each row and a numerical rating was given on the basis of disease severity, incidence, and extent of upward spread of disease through the canopy. Due to severe whitefly infestation and injury to some accessions caused by soil pathogens (primarily Fusarium and Phytophthora) in Quincy in 2007, entries were evaluated by examining five leaflets under a microscope at 20x and counting the number of SBR lesions in the field of view (Fig. 3). A mean was calculated from the four leaflets with the highest lesion densities. AccessionMGLesion count from Quincy in 2007 (Avg. no. of lesions in 1.0 cm 2 ) Disease rating Attapulgus, 2006 (Range 1-5) Disease rating Quincy, 2006 (Range 1-5) Disease rating Alexandria, 2006 (Range 1-9) Comments PI I ‘MN1302’I Cultivar PI BIII L III Rpp1 gene ‘Williams 82’III> Cultivar PI AV PI V PI V PI AV ‘5601T’V> Cultivar PI VI PI BVI ‘Dillon’VI> Cultivar PI VII> Rpp2 gene ‘FT-2’VII Brazil. Cv. ‘Benning’VII> Cultivar PI AVIII PI AVIII ‘Prichard’VIII> Cultivar Table 1. Average density of SBR lesions on leaves of selected USDA soybean germplasm accessions and cultivar checks from Quincy, FL in 2007 compared with 2006 disease ratings from Attapulgus (GA), Quincy (FL), and Alexandria (LA). Higher disease ratings indicate greater susceptibility to SBR. 5. Results and Discussion -Lesion density data from some of the PIs and cultivars evaluated in Quincy, FL in 2007 are compared with 2006 disease rating data from three locations in Table 1. Lesion counts preceded by a “>” sign indicate that at least one leaflet from the sample had >50 lesions that could not be counted accurately. -Some accessions (e.g., the Williams 82-derived line L which carries Rpp1, and PIs A and ) had substantially fewer lesions than North American cultivar checks from the same MG. -Occasional discrepancies between the 2007 SBR lesion counts and the 2006 field ratings (and among the 2006 ratings) illustrate the importance of evaluating germplasm in multiple environments. -Data from field evaluations across multiple locations and years help to identify accessions that appear to consistently show resistance to rust isolates from different regions of the Southeast. Fig. 1. Locations of USDA germplasm evaluations in 2007 Bossier City Alexandria Baton Rouge Fairhope Quincy Attapulgus Blackville Fig. 2. Mobile lighting towers used to extend natural photoperiod in Quincy, FL. Fig. 3. Sporulating lesions on underside of leaf from a susceptible soybean plant.