1a. Objectives (from AD-416):
The overall objective of the project “Identification and Utilization of Resistance to Soybean Rust” is to identify and develop soybean germplasm with broad resistance to Phakopsora pachyrhizi, the cause of soybean rust (SBR). Characterization of virulence diversity among P. pachyrhizi populations and the reactions of resistant lines are important to guide breeding for effective and durable resistance. Specific objectives will be to (1) identify resistant plant introductions (PIs) from the USDA Soybean Germplasm Collection; and (2) map and transfer resistance genes to germplasm adapted to the U.S.
1b. Approach (from AD-416):
Objective 1 will be addressed by testing soybean germplasm accessions from the USDA-ARS Soybean Germplasm Collection and breeding lines developed from crosses made with those accessons for resistance to soybean rust (Phakopsora pachryhizi) in the field and greenhouse. Due to demonstrated ability of the rust pathogen to evolve and to evidence that multiple pathotypes and virulence groups already exist in the southern U.S., it will be important to confirm the resistance of historically resistant accessions as well as identifying additional sources of novel resistance genes. This will be done annually in locations in the southern U.S. through the assistance of collaborators. The Rpp (Resistance to Phakopsora pachyrhizi) genes responsible for the resistance in these accessions would then need to be characterized by developing lines and populations that can be used to map their locations in the genome and study their effects both singly and in combination with other Rpp genes. This information will be important to allow breeders to decide which genes to combine to achieve broader and more durable resistance. Assessments of genetic and pathogenic diversity within and among populations of the rust fungus are important to develop effective breeding and management strategies, so the third objective of this project will be to collect and characterize isolates of the rust fungus that have been collected in different years and locations. The germplasm screening nurseries established to meet Objective 1 would provide sources of contemporary isolates, and colleagues in areas that experience soybean rust epidemics are likely to submit local samples of SBR as well. Isolates from field populations will be pathotyped using a set of differential lines and will also be characterized genetically using DNA markers.
3. Progress Report:
1. Greenhouse evaluation of the reactions of selected soybean germplasm accessions to Georgia isolates of soybean rust: Seedlings of 130 plant introductions (PIs) and susceptible cultivar checks were inoculated with a mixture of Georgia isolates of Phakopsora pachryhizi in a greenhouse in Griffin, GA. The PIs included in the assay had previously shown resistance to field populations of the fungus at locations in the southern U.S. Fifty-nine accessions did not develop any lesions in the six replications. Twenty-nine PIs appeared immune in some replications, but developed the type of reddish-brown (RB) lesions associated with incomplete resistance in others. Thirteen resistant lines not previously reported to be resistant to rust have been crossed with susceptible parents to produce segregating mapping populations. 2. USDA soybean PI screening for soybean rust resistance: A total of 125 soybean PIs and susceptible cultivar checks will be planted in Attapulgus, GA in August 2012 to study their reactions to the local population of the rust fungus (P. pachyrhizi). Most of these were also planted in 2011, but due to unusually hot and dry weather, a rust epidemic failed to develop before a mid-November early frost destroyed the plants. 3. Identification of putatively unique sources of soybean rust resistance genes: Phenotypic and genotypic data collected by University of Georgia colleagues indicated that five of the segregating populations that they evaluated have a putatively unique source of resistance not previously reported in the literature. Early data from 15 other populations derived from crosses between a rust-resistant parent and an elite susceptible parent suggest that some of the resistant parents also have unique resistance genes. Mapping populations from an additional 22 resistant parents were developed and are in various stages of testing. 4. Development of backcross lines with major rust resistance (Rpp) genes: At the University of Georgia, the Rpp1, Rpp2, Rpp3 and Rpp4 rust resistance genes were backcrossed into the high-yielding breeding line G00-3213. Similar lines are being developed that carry the Rpp5 or Rpp6 resistance genes.