Identification, mapping, and evaluation of unique sources of soybean rust resistance
Harris, Donna Karen
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Soybean rust (SBR), caused by Phakopsora pachyrhizi Syd., is considered the most destructive foliar disease of soybean Glycine max L. Merr and could become a serious threat to soybean production in the USA. Six rust resistance loci (Rpp1, 2, 3, 4, 5, and 6) have been reported, however none of these genes is resistant to all isolates of the pathogen. One objective of this research was to map QTLs conditioning variation in SBR-induced canopy damage using a population of recombinant inbred lines (RILs) from the cross of Benning x PI416937. The population was rated for canopy severity damage during SBR epidemics for three years. Multiple interval mapping models identified four QTL on soybean chromosomes Gm08, Gm15, Gm13, and Gm16 conditioning variation in canopy severity damage. A second objective of this research was to investigate the effect of lesion type, Red-Brown (RB) vs. TAN and canopy severity (low vs. high severity) on yield loss. Tan lesions are the susceptible phenotype to infection by P. pachyrhizi and produce proficient sporulation, while the RB resistant lesion type has little to no sporulation. Eight RILs were chosen that had either RB or TAN lesions types and low or high canopy severity. A split plot design was used for field evaluations with the main plots being the absence or presence of fungicide applications and the subplots being the eight RILs. Results indicated that both low and high severity TAN lines yielded significantly higher in fungicide treated plots, while the RB low and high canopy severity lines had no significant yield losses in the fungicide plots versus unprotected plots. The last objective of this research was to identify unique sources of resistance to SBR. Seventy-five SBR-resistant plant introductions (PIs) were identified and crosses made between these PIs and a susceptible line. Through a combination of bulked segregant analysis (BSA), a set of P. pachyrhizi isolates collected from different areas of the world, and haplotype analysis at the fine-mapped intervals of Rpp1, Rpp3, and Rpp4, nine PI’s with putatively unique sources of resistance were identified and the genomic locations of these genes are being fine mapped.