In recent years soybean rust, caused by Phakopsora pachyrhizi has become one of the most serious threats to soybean production in Brazil. Breeding lines and varieties have been selected for resistance to soybean rust in Asia. However, differences in virulence between Asian and Brazilian rust populations should be considered in order to select and use resistant resources from Asia. Here, we suggest criteria for distinguishing resistant from susceptible types by the analysis of four resistance characters: frequency of lesions having uredinia, number of uredinia per lesion, frequency of open uredinia, and sporulation level, determined by the utilization of 63 genotypes. Under growth chamber conditions, a set of 13 soybean varieties were exposed to three rust populations-one from Japan and two from Braziland evaluated for the resistance characters mentioned above. The Japanese and Brazilian populations clearly differed in virulence, as did the two Brazilian populations. Only two resistance genes, Rpp4 from PI459025 and Rpp5 from Shiranui, commonly conferred resistance on all three rust populations. The number of resistant varieties or resistance genes useful in both countries appears limited. Therefore, a resistant cultivar that is universally effective against soybean rust should be developed by pyramiding some major resistance genes and by introducing horizontal resistance. Keywords: Phakopsora pachyrhizi, lesion type, pathogenicity, resistant variety. RESUMO Desenvolvimento de critério de classificação da resistência à ferrugem asiática da soja e diferenças de virulência entre populações do Japão e do BrasilNos últimos anos a ferrugem asiática, causada pelo fungo Phakopsora pachyrhizi tornou-se uma das mais sérias ameaças a produção de soja Brasileira. Linhagens melhoradas e variedades têm sido selecionadas para a resistência à ferrugem da soja na Ásia, entretanto para a seleção e utilização dessas fontes de resistência, diferenças de virulência entre populações Asiáticas e Brasileiras desse fungo devem ser consideradas. Neste trabalho sugerimos um critério para se distinguir resistência de susceptibilidade pela análise de quatro caracteres de resistência: freqüência de lesões contendo urédias, número de urédias por lesão, freqüência de urédias abertas e nível de esporulação determinados pela utilização de 63 genótipos. Sob condições controladas em câmaras de crescimento, treze variedades de soja foram expostas a três populações de fungos -uma população proveniente do Japão e duas populações provenientes do Brasil-e avaliadas quanto aos caracteres de resistência mencionados acima. As populações Brasileiras diferiram entre si claramente quanto a virulência e em relação à população de isolados do Japão. Apenas dois genes de resistência, Rpp4 presente na variedade PI459025 e Rpp5 presente na variedade Shiranui conferiram resistência as três populações da ferrugem. O número de variedades ou genes resistentes úteis em ambos os países parece ser limitado. Assim, um cultivar universalmente efetivo contra a ferru...

Asian soybean rust (ASR) is caused by the fungal pathogen Phakopsora pachyrhizi Sydow & Sydow. It was first identified in Brazil in 2001 and quickly infected soybean areas in several countries in South America. Primary efforts to combat this disease must involve the development of resistant cultivars. Four distinct genes that confer resistance against ASR have been reported: Rpp1, Rpp2, Rpp3, and Rpp4. However, no cultivar carrying any of those resistance loci has been released. The main objective of this study was to genetically map Rpp2 and Rpp4 resistance genes. Two F(2:3) populations, derived from the crosses between the resistant lines PI 230970 (Rpp2), PI 459025 (Rpp4) and the susceptible cultivar BRS 184, were used in this study. The mapping populations and parental lines were inoculated with a field isolate of P. pachyrhizi and evaluated for lesion type as resistant (RB lesions) or susceptible (TAN lesions). The mapping populations were screened with SSR markers, using the bulk segregant analysis (BSA) to expedite the identification of linked markers. Both resistance genes showed an expected segregation ratio for a dominant trait. This study allowed mapping Rpp2 and Rpp4 loci on the linkage groups J and G, respectively. The associated markers will be of great value on marker assisted selection for this trait.

BackgroundSouthern stem canker (SSC), caused by Diaporthe aspalathi (E. Jansen, Castl. & Crous), is an important soybean disease that has been responsible for severe losses in the past. The main strategy for controlling this fungus involves the introgression of resistance genes. Thus far, five main loci have been associated with resistance to SSC. However, there is a lack of information about useful allelic variation at these loci. In this work, a genome-wide association study (GWAS) was performed to identify allelic variation associated with resistance against Diaporthe aspalathi and to provide molecular markers that will be useful in breeding programs.ResultsWe characterized the response to SSC infection in a panel of 295 accessions from different regions of the world, including important Brazilian elite cultivars. Using a GBS approach, the panel was genotyped, and we identified marker loci associated with Diaporthe aspalathi resistance through GWAS. We identified 19 SNPs associated with southern stem canker resistance, all on chromosome 14. The peak SNP showed an extremely high degree of association (p-value = 6.35E-27) and explained a large amount of the observed phenotypic variance (R2 = 70%). This strongly suggests that a single major gene is responsible for resistance to D. aspalathi in most of the lines constituting this panel. In resequenced soybean materials, we identified other SNPs in the region identified through GWAS in the same LD block that clearly differentiate resistant and susceptible accessions. The peak SNP was selected and used to develop a cost-effective molecular marker assay, which was validated in a subset of the initial panel. In an accuracy test, this SNP assay demonstrated 98% selection efficiency.ConclusionsOur results suggest relevance of this locus to SSC resistance in soybean cultivars and accessions from different countries, and the SNP marker assay developed in this study can be directly applied in MAS studies in breeding programs to select materials that are resistant against this pathogen and support its introgression.

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2523 RESEARCH S everal legumes, when in symbiosis with bacteria collectively known as rhizobia, have the ability to assimilate atmospheric N 2 in a process called biological N fixation (BNF). Legume-rhizobia symbioses potentially contribute both to environmental and agricultural sustainability by reducing greenhouse gas emissions, nitrogenous water pollution, and fossil energy consumption (Serraj et al., 1999;Kaewsuralikhit et al., 2005;Hungria and Mendes, 2015).Soybean [Glycine max (L.) Merr.] is the most important legume cultivated worldwide. Symbiotic associations between soybean and Bradyrhizobium japonicum, Bradyrhizobium diazoefficiens, and Bradyrhizobium elkanii strains are economically and environmentally important; for example, savings on N fertilizer in Brazil with the soybean crop are estimated at about US$15 billion per cropping season (Hungria and Mendes, 2015). ABSTRACTAlthough biological N fixation (BNF) is a key process for economic cultivation of soybean [Glycine max (L.) Merr.], the trait is often neglected in breeding programs, mainly due to difficulties in evaluating nodulation and N fixation parameters. We used a genotyping-by-sequencing approach to provide a dense genome-wide marker coverage with 1448 single nucleotide polymorphisms (SNPs) distributed broadly across the chromosomes of a soybean population. The mapping population was composted of 113 F 7:8 recombinant inbred lines, obtained by singleseed descent method, derived from crossing of soybean cultivars 'Bossier' (high BNF capacity) and 'Embrapa 20' (medium BNF capacity). The traits evaluated were nodule number (NN), nodule dry weight (NDW), average NDW (ANDW; i.e., NDW/NN), and shoot dry weight (SDW). A genetic map was constructed with 1448 SNPs that generat ed 35 linkage groups totaling 1793 cM, thus covering ?72% of the genome based on the consensus linkage map. The estimated heritability for NN, NDW, ANDW, and SDW was 0.41, 0.30, 0.30, and 49.11, respectively. Inclusive composite interval mapping identified two significant quantitative trait loci (QTLs) for ANDW at 67 cM on chromosome 13, with a logarithm of odds (LOD) score of 4.66. The additive effect of this locus was −0.31 mg nodule −1 , and it explained 18.13% of the phenotypic variation. Another QTL was identified for SDW at 24 cM on chromosome 19 and exhibited an LOD score of 3.93. The additive effect was of 0.57 g plant −1 , and it accounted for 14.93% of the phenotypic variation. Additive genetic effects contribute to the final phenotype. In the present study, Embrapa 20 was the major contributor to ANDW, and Bossier was the major contributor to SDW. The latter was previously reported using simple sequence repeat markers.

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