1a. Objectives (from AD-416):
To provide support for collaborative field and greenhouse research to identify and characterize resistance to soybean rust and other economically important diseases in the USDA-ARS Soybean Germplasm Collection and in breeding populations developed from crosses to sources of resistance. Soybean germplasm accessions and breeding lines will be assayed for resistance or tolerance to soybean rust and to other diseases for which natural epidemics occur in the field. Genes associated with resistance will be mapped and resistant breeding lines will be selected for promising agronomic appearance and performance.
1b. Approach (from AD-416):
Plant introductions (PIs) from the USDA-ARS Soybean Germplasm Collection in Urbana, IL will be planted in small, replicated field plots at the North Florida Research and Education Center (NFREC) to test their resistance to the local population of Phakopsora pachyrhizi, the primary fungus that causes soybean rust. In seasons when the local rust epidemic is negligible, supplementary inoculations with spores collected from infected plants at the NFREC may be used to promote disease pressure in the experimental plots. Natural infections with pathogens causing foliar diseases like Cercospora blight and frogeye leaf spot, or soilborne diseases like Phytophthora root and stem rot will be used to identify germplasm with apparent resistance to those pathogens, and breeding populations segregating for disease symptoms. After resistance has been confirmed with assays conducted under controlled conditions, DNA markers will be used to map resistance genes in segregating populations and inbred lines segregating for differential resistance reactions will be developed as tools for studying genes and proteins involved in resistance to the pathogens.
3. Progress Report:
Soybean plots were planted at the North Florida Research and Education Center (NFREC) in June and in August, 2012, and were harvested after they matured. Due to a strong effect of environmental conditions on the annual soybean rust epidemics in the southeastern U.S., the level of disease pressure and the rate at which a rust epidemic develops can vary considerably in different years. Plots planted in August have a higher chance of getting soybean rust early (caused by the fungus Phakopsora pachyrhizi) in development than those planted in June, but plots must be planted at the earlier date to allow accurate assessment of agronomic value and yield potential. The June-planted plots included an experiment to evaluate tolerance to soybean rust and more than 1,500 breeding lines from populations segregating for resistance to soybean rust. Six cultivars developed by southeastern U.S. university breeding programs were evaluated for tolerance to soybean rust. A tolerant cultivar or breeding line is one that is susceptible, but on which the percent yield loss caused by the disease is significantly less than the losses that occur on most other susceptible lines or cultivars. When this study was conducted in previous years with a larger number of cultivars, the University of Georgia line G00-3209 consistently had a lower percent yield loss than other entries tested, so the subset of entries chosen for the 2012 experiment included G00-3209, its sister line G00-3213, the two cultivars that it was derived from, and two cultivars which had consistently shown less tolerance to soybean rust. The experiment was a split plot design in which fungicide protected and unprotected plots were grown side by side, with two border rows separating them to prevent fungicide from drifting onto plants in the unprotected treatment. The plots consisted of four rows each, but only the middle two rows were harvested. To determine percent yield loss, the seed yields of the unprotected plots (which had been infected with rust) were compared to the yields of the fungicide-protected plots, which did not have the disease. Differences in loss percentages were compared statistically. The 2012 data and data from previous years that the study was conducted are being compiled into a manuscript. The other June-planted plots included 1,050 rows of breeding lines and F2 populations derived from crosses between a rust-resistant Asian soybean Plant Introduction (PI) and a cultivar or elite breeding line from the U.S. The F2 populations were grown in a single row, whereas later generations were grown in hill plots, 5’ rows, or 10’ rows, depending on the generation. F2 and F3 plants from 22 other populations were grown in hand-planted hill plots and short rows. Plants were selected on the basis of agronomic appearance (lodging and shattering resistance, pod set, and seed appearance), as well as resistance to soybean rust and other foliar diseases that were present in the plots, like bacterial pustule. Additional plots intended primarily to evaluate soybean germplasm and breeding lines for resistance to rust were planted in mid-August to optimize the likelihood that they would become infected with rust before reaching maturity. One experiment, which has been planted annually since 2006, was conducted to evaluate the resistance of 114 PIs from the USDA Soybean Germplasm Collection compared to 11 susceptible cultivars. The plants in these plots were not rated for agronomic traits, since they flowered and matured early due to the late planting date. The 2012 data have been compiled with rust reaction data from 2009 and 2011 into a manuscript entitled ‘Evaluation of soybean germplasm accessions for resistance to Phakopsora pachyrhizi populations in the southeastern United States, 2009-2012’, which was recently submitted to Crop Science. A manuscript with the title ‘First report of Phakopsora pachyrhizi adapting to soybean genotypes with Rpp1 and Rpp6 rust resistance genes in field plots in the United States’ was accepted in early May for publication in Plant Disease. In addition, 340 breeding lines from several sources were also planted in August to be screened for soybean rust resistance. These included 83 backcross lines that ARS researchers in Urbana had developed from crosses between Glycine max and an Australian accession of the perennial soybean species G. tomentella. These interspecific backcross lines are unique in the world, due to the difficulty of obtaining viable progeny from such crosses. Ten lines were screened for an ARS Research Geneticist in Stoneville, MS, in addition to 214 additional lines from an Urbana ARS soybean breeding program. Selected lines are being re-evaluated in 2013 to confirm their resistance to the local soybean rust population.In addition to soybean rust, there was a high incidence of downy mildew in the 2012 field plots, so there was a high probability that plants lacking symptoms of that disease were resistant to it. Data were collected on the presence of the disease to allow selection against susceptible plants. Other diseases, including damping off from soil pathogen infections, were not as common in 2012 as they have been in some previous years.