Lima bean is affected by severe downy mildew epidemics caused by the oomycete Phytophthora phaseoli. There are six documented races of P. phaseoli (A to F). Race F is currently predominant in the mid-Atlantic region, creating the need for resistant lima bean cultivars with desirable agronomic characteristics. In order to develop markers for detecting race F resistance, bulked segregant analysis (BSA) using genotyping-by-sequencing (GBS) was used on a biparental F population comprised of 216 lima bean progeny segregating for a dominant race F resistance phenotype. Data were analyzed using a custom bioinformatic analysis pipeline (redrep). Kompetitive allele-specific polymerase chain reaction assays were developed using 12 GBS markers associated with the race F resistance phenotype. Using these assays, the F population was used to map the race F resistance locus. Seven markers were in linkage and significantly associated with race F resistance that mapped between two markers located approximately 4.88 centimorgan (cM) apart. These assays were successfully used to genotype a newly acquired lima bean diversity panel consisting of 256 landraces, cultivars, and wild germplasm, and a haplotype consisting of two of the seven linked markers was demonstrated to accurately predict race F resistance. This confirmed the ability of our customized methods to accurately predict phenotypes in diverse lines of lima bean.
`Redcoat' soldier bean cultivar originated from off-type, virgarcus patterned seeds found in a foundation seed lot of `Red Hawk' dark red kidney bean (Phaseolus vulgaris L.). These off-type seeds were hypothesized to be the result of a single gene mutation. A mutation at either of two loci involved in bean seedcoat pattern expression, T or Z, could convert self-colored seedcoats to a virgarcus pattern. The results of test-crosses of `Redcoat' and `Red Hawk' to lines with known alleles at the seedcoat pattern loci indicate that the dominant T allele of `Red Hawk' mutated to recessive t in `Redcoat'. The mutant t gene prevents expression of red veins in wing petals due to v rkd, and enables expression of the z gene (and possibly other genes) carried cryptically by `Red Hawk'. On the basis of preliminary data, we speculate that the two types of virgarcus patterns observed (classic in `Redcoat' and standard in the tester) may be controlled by different Bip alleles as they interact with t z.
The oomycete Phytophthora phaseoli is one of the most threatening pathogens of lima bean (Phaseolus lunatus) in the humid Mid-Atlantic United States. In the last 60 years, P. phaseoli has evolved to overcome genetic resistance in the host and several physiological races have been identified during the last 6 decades. Six physiological races A, B, C, D, E, and F have been identified over the years. Only race F has been detected in the field over the past decade. Identifying and characterizing sources of resistance to this pathogen and determining the nature of resistance were the main objectives. Eight commercial cultivars and 35 germplasm accessions of P. lunatus were evaluated for their reaction to races E and F. Four commercial cultivars and four accessions with resistance to race E, and two cultivars and four accessions with resistance to race F were identified. None of the germplasm evaluated were resistant to both races. Five populations of F2 plants and a recombinant inbred line (RIL) population were produced and inoculated to investigate the inheritance of resistance to races E and F. Resistance to races E and F was determined to be conferred by single, independent, dominant genes.
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