Mapping spot blotch resistance genes in four barley populations

Bipolaris sorokiniana (teleomorph: Cochliobolus sativus) is the fungal pathogen responsible for spot blotch in barley (Hordeum vulgare L.) and occurs worldwide in warmer, humid growing conditions. Current Australian barley varieties are largely susceptible to this disease and attempts are being made to introduce sources of resistance from North America. In this study we have compared chromosomal locations of spot blotch resistance reactions in four North American two-rowed barley lines; the North Dakota lines ND11231-12 and ND11231-11 and the Canadian lines TR251 and WPG8412-9-2-1. Diversity Arrays Technology (DArT)-based PCR, expressed sequence tag (EST) and SSR markers have been mapped across four populations derived from crosses between susceptible parental lines and these four resistant parents to determine the location of resistance loci. Quantitative trait loci (QTL) conferring resistance to spot blotch in adult plants (APR) were detected on chromosomes 3HS and 7HS. In contrast, seedling resistance (SLR) was controlled solely by a locus on chromosome 7HS. The phenotypic variance explained by the APR QTL on 3HS was between 16 and 25% and the phenotypic variance explained by the 7HS APR QTL was between 8 and 42% across the four populations. The SLR QTL on 7HS explained between 52 to 64% of the phenotypic variance. An examination of the pedigrees of these resistance sources supports the common identity of resistance in these lines and indicates that only a limited number of major resistance loci are available in current two-rowed germplasm

Pathogen Populations Evolve to Greater Race Complexity in Agricultural Systems – Evidence from Analysis of Rhynchosporium secalis Virulence Data

Fitness cost associated with pathogens carrying unnecessary virulence alleles is the fundamental assumption for preventing the emergence of complex races in plant pathogen populations but this hypothesis has rarely been tested empirically on a temporal and spatial scale which is sufficient to distinguish evolutionary signals from experimental error. We analyzed virulence characteristics of ∼1000 isolates of the barley pathogen Rhynchosporium secalis collected from different parts of the United Kingdom between 1984 and 2005. We found a gradual increase in race complexity over time with a significant correlation between sampling date and race complexity of the pathogen (r20 = 0.71, p = 0.0002) and an average loss of 0.1 avirulence alleles (corresponding to an average gain of 0.1 virulence alleles) each year. We also found a positive and significant correlation between barley cultivar diversity and R. secalis virulence variation. The conditions assumed to favour complex races were not present in the United Kingdom and we hypothesize that the increase in race complexity is attributable to the combination of natural selection and genetic drift. Host resistance selects for corresponding virulence alleles to fixation or dominant frequency. Because of the weak fitness penalty of carrying the unnecessary virulence alleles, genetic drift associated with other evolutionary forces such as hitch-hiking maintains the frequency of the dominant virulence alleles even after the corresponding resistance factors cease to be used

Combining ability in sweet pepper for resistance to powdery mildew Capacidade de combinação em pimentão para resistência a oídio

With the increase of the production of pepper in protected environment, pathogens began to cause serious damages to producers such as Leveillula taurica (Lév.) Arn., fungus that causes powdery mildew. The systemic fungicides have not shown very satisfactory results. Thus, considering that protected cropping of pepper in Brazil is significant and growing, the incorporation of genes that confer resistance to powdery mildew is important for the maintenance of this cropping system. In this research, carried out in 2007 in Campinas, São Paulo State, Brazil, the agronomic performance and resistance to powdery mildew was determined by triple hybrids of sweet pepper through general and specific combining ability of their parents. For agronomic performance of the triple hybrid, it was evaluated the weight length and width average of the fruit, length and width ratio of the fruit and wall thickness. For severity of powdery mildew a scale of scores from 1 to 5 was used. For statistical and genetic analysis, it was adopted Griffing's method two, model I, adapted for partial diallel. The experimental design was completely randomized blocks, with 17 treatments, including ten experimental hybrids and seven parents, eight replications and four plants per plot. The additive effects were greater than the non-additive effects for all agronomic characters; 'P36-R' and 'Platero' were highlighted as good combiners; 'Quantum-R x HV-12', 'Rubia x HV-12' and 'P36-R x HV-12' presented the best specific combining ability; the mean square of the specific combining ability for the severity of the disease was significant indicating the importance of genes with dominant and epistatic effect; triple hybrids obtained by crosses with 'Quantum-R' and 'Rubia-R' had negative general combining ability and the best reactions to powdery mildew.<br>Com o aumento do cultivo protegido de pimentão, patógenos como Leveillula taurica (Lév.) Arn., fungo causador do oídio, passaram a causar sérios prejuízos aos produtores. Os fungicidas sistêmicos não têm apresentado resultados muito satisfatórios. Assim, levando-se em conta que o cultivo protegido de pimentão no Brasil é importante e crescente, a incorporação de genes que conferem resistência ao oídio é relevante para a manutenção desse sistema de cultivo. Nesse trabalho, realizado em 2007 em Campinas-SP, determinou-se o potencial agronômico e de resistência a oídio de híbridos triplos de pimentão por meio da capacidade geral e específica de combinação dos genitores. Para determinar o desempenho agronômico dos híbridos triplos, avaliaram-se o peso, comprimento e largura média do fruto, relação entre comprimento e largura do fruto e espessura da polpa. Para a avaliação da severidade da doença utilizou-se escala de notas de 1 a 5. Para as análises estatístico-genéticas, adotou-se o método dois, modelo I de Griffing, adaptado para dialelo parcial. O delineamento experimental utilizado foi o de blocos ao acaso, com 17 tratamentos, incluindo dez híbridos experimentais e sete genitores, oito repetições e quatro plantas por parcela. Os efeitos aditivos foram superiores aos efeitos nãoaditivos para todos os componentes agronômicos avaliados; 'P36-R' e 'Platero' destacaram-se como bons combinadores para os caracteres agronômicos avaliados; 'Quantum-R x HV-12', 'Rubia-R x HV-12' e 'P36-R x HV-12' apresentaram os maiores efeitos da capacidade específica de combinação para a maioria dos caracteres avaliados; o quadrado médio da capacidade específica de combinação significativo para a severidade da doença indicou a importância de genes com efeito de dominância e epistasia; híbridos triplos obtidos do cruzamento com 'Quantum-R' e 'Rubia-R' apresentaram capacidade geral de combinação negativa e as melhores reações de resistência ao oídio