title: A trade off between mlo resistance to powdery mildew and increased susceptibility of barley to a newly important disease, Ramularia leaf spot.
abstract: Ramularia leaf spot (RLS), caused by the fungus Ramularia collo-cygni, is a serious, recently emerged disease of barley in Europe and other temperate regions. This study investigated the trade off between strong resistance to powdery mildew conferred by mlo mutant alleles and increased susceptibility to RLS. In field trials and seedling tests, the presence of mlo alleles increased severity of RLS. Genetic analysis of a doubled-haploid population identified one quantitative trait locus for susceptibility to RLS, colocalizing with the mlo-11 allele for mildew resistance. The effect of mlo-11 on RLS severity was environmentally sensitive. Analysis of near-isogenic lines of different mlo mutations in various genetic backgrounds confirmed that mlo alleles increased RLS severity in seedlings and adult plants. For mlo resistance to mildew to be fully effective, the genes ROR1 and ROR2 are required. RLS symptoms were significantly reduced on mlo-5 ror double mutants but fungal DNA levels remained as high as in mlo-5 single mutants, implying that ror alleles modify the transition of the fungus from endophytism to necrotrophy. These results indicate that the widespread use of mlo resistance to control mildew may have inadvertently stimulated the emergence of RLS as a major disease of barley.
authors.affiliation: Department of Crop Genetics, John Innes Centre, Norwich Research Park, Norwich, NR4 7UH, UK.
authors.forename: Graham R D
authors.initials: GR
authors.lastname: McGrann
authors.forename: Anna
authors.initials: A
authors.lastname: Stavrinides
authors.forename: Joanne
authors.initials: J
authors.lastname: Russell
authors.forename: Margaret M
authors.initials: MM
authors.lastname: Corbitt
authors.forename: Allan
authors.initials: A
authors.lastname: Booth
authors.forename: Laetitia
authors.initials: L
authors.lastname: Chartrain
authors.forename: William T B
authors.initials: WT
authors.lastname: Thomas
authors.forename: James K M
authors.initials: JK
authors.lastname: Brown
mesh_terms: D000483:Alleles; D001203:Ascomycota / Q000502:physiology*; D002874:Chromosome Mapping; D032461:Chromosomes, Plant / Q000235:genetics; D060467:Disease Resistance / Q000235:genetics*; D005060:Europe; D001467:Hordeum / Q000235:genetics* / Q000276:immunology / Q000382:microbiology / Q000502:physiology; D054884:Host-Pathogen Interactions; D009154:Mutation; D010935:Plant Diseases / Q000382:microbiology*; D018515:Plant Leaves / Q000235:genetics / Q000276:immunology / Q000382:microbiology / Q000502:physiology; D010940:Plant Proteins / Q000235:genetics* / Q000378:metabolism; D040641:Quantitative Trait Loci; D036226:Seedlings / Q000235:genetics / Q000276:immunology / Q000382:microbiology / Q000502:physiology
chemical_list: C104922:MLO protein, Hordeum vulgare; D010940:Plant Proteins
keywords: Biotrophic pathogens; Blumeria graminis; Hordeum vulgare; ROR genes.; Ramularia collo-cygni; disease resistance; necrotrophic pathogens; plant breeding