Association mapping of a locus that confers southern stem canker resistance in soybean and SNP marker development.

BACKGROUND: Southern 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. RESULTS: We 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. CONCLUSIONS: Our 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.

Genetic control of soybean (Glycine max) yield in the absence and presence of the Asian rust fungus (Phakopsora pachyrhizi)

Soybean is one of the most important crops in Brazil and continuously generates demands for production technologies, such as cultivars resistant to diseases. In recent years, the Asian rust fungus (Phakopsora pachyrhizi Syd. & P. Syd 1914) has caused severe yield losses and the development of resistant cultivars is the best means of control. Understanding the genetic control and estimating parameters associated with soybean (Glycine max) resistance to P. pachyrhizi will provide essential information for cultivar selection. We investigated quantitative genetic control of P. pachyrhizi and estimated parameters associated to soybean yield in the absence and presence of this phytopathogen. Six cultivars and their 15 diallel derived F2 and F3 generations were assessed in experiments carried out in the absence and presence of P. pachyrhizi. The results indicated that soybean yield in the presence and absence of P. pachyrhizi is controlled by polygenes expressing predominantly additive effects that can be selected to develop new cultivars resistant or tolerant to P. pachyrhizi. These cultivars may prove to be a useful and more durable alternative than cultivars carrying major resistance genes.

Fungicidas no controle da ferrugem asiática (Phakopsora pachyrhizi) e produtividade da soja / Fungicides on the control of soybean rust (Phakopsora pachyrhizi) and soybean yield

Neste trabalho, avaliou-se o efeito de fungicidas no desenvolvimento da ferrugem asiática, causada pelo fungo Phakopsora pachyrhizi, e na produtividade da soja. Os fungicidas foram pulverizados quando as plantas estavam no estádio de desenvolvimento R4. Utilizou-se a cultivar Fepagro-RS 10 e delineamento blocos ao acaso com quatro repetições e dez tratamentos. A avaliação da severidade da doença foi feita 30 dias após a pulverização, utilizando-se uma escala de notas de 0 a 5, considerando-se a porcentagem da área foliar infectada. Os tratamentos com os fungicidas azoxystrobin, difenoconazole + propiconazole, fluquinconazole, myclobutanil, pyraclostrobin + epoxiconazole, tebuconazole e trifloxistrobina + propiconazole, controlaram o patógeno em relação à testemunha, mostrando notas médias de severidade igual ou inferior a 2 e não diferiram entre si. O tratamento com difenoconazole (nota 3,05) controlou o patógeno, mas com nota inferior ao tratamento com carbendazin (nota 3,85) e à testemunha (nota 4,5). Todos os tratamentos mostraram produtividades superiores à testemunha, variando entre 14,5 por cento e 27,3 por cento. O peso de cem sementes foi superior ao da testemunha para todos os tratamentos (11,8 por cento a 14,4 por cento), com exceção do carbendazin (5,2 por cento).