Variation in frequency of avirulence genes in Leptosphaeria maculans in western Canada

Non-Peer Reviewe

Effects of a penthiopyrad and picoxystrobin fungicide mixtureon phoma stem canker (Leptosphaeria spp.) on UK winteroilseed rape

© Koninklijke Nederlandse Planteziektenkundige Vereniging 2016. This is a pre-copyedited, author-produced PDF of an article accepted for publication in European Journal of Plant Pathology following peer review. The final publication [Sewell, T.R., Moloney, S., Ashworth, M. et al., European Journal of Plant Pathology (2016) 145: 675-685, first published online April 5, 2016] is available at Springer via doi: http://dx.doi.org/10.1007/s10658-016-0916-8In the UK, fungicides are often used to controlphoma stem canker on winter oilseed rape. Field trialswere established near Boxworth, Cambridgeshire for fourcropping seasons (2011/2012, 2012/2013, 2013/2014 and2014/15) to test the efficacy of a new fungicide mixtureRefinzar® (penthiopyrad + picoxystrobin) by comparisonto an existing fungicide Proline 275® (prothioconazole)against phoma stem canker (Leptosphaeria spp.) andthe effect on winter oilseed rape (cv. Catana) yield. Ineach season, weather data were collected from a weatherstation at Boxworth and the release of ascospores wasmonitored using a nearby Burkard spore sampler. Thepatterns of ascospore release differed between seasonsand related to weather conditions. Fungicidespenthiopyrad + picoxystrobin and prothioconazole wereapplied in October/November when 10 % of plants hadphoma leaf spotting (T1, early), 4/8 weeks after T1 (T2,late) or at both T1 and T2 (combined). When phoma leafspot symptoms were assessed in autumn/winter,penthiopyrad + picoxystrobin and prothioconazole bothdecreased numbers of phoma leaf spots caused byL. maculans; there were few leaf spots caused byL. biglobosa. Penthiopyrad + picoxystrobin andprothioconazole both reduced phoma stem canker severitybefore harvest compared to the untreated control butdid not increase yield in these seasons when epidemicswere not severe. In 2013/2014, the presence ofL. maculans and L. biglobosa in upper stem lesions orstem base cankers was determined by species-specificPCR. The proportions of stems with L. maculans DNAwere much greater than those with L. biglobosa DNA forboth upper stem lesions and basal stem cankers. Theseresults suggest that both penthiopyrad + picoxystrobinand prothioconazole can decrease phoma stem cankerseverity on winter oilseed rape in severe disease seasons.Peer reviewe

Identification and QTL mapping of resistance to Turnip yellows virus (TuYV) in oilseed rape, Brassica napus

Key message Partially dominant resistance to Turnip yellows virus associated with one major QTL was identified in the natural allotetraploid oilseed rape cultivar Yudal. Abstract Turnip yellows virus (TuYV) is transmitted by the peach-potato aphid (Myzus persicae) and causes severe yield losses in commercial oilseed rape crops (Brassica napus). There is currently only one genetic resource for resistance to TuYV available in brassica, which was identified in the re-synthesised B. napus line ‘R54’. In our study, 27 mostly homozygous B. napus accessions, either doubled-haploid (DH) or inbred lines, representing a diverse subset of the B. napus genepool, were screened for TuYV resistance/susceptibility. Partial resistance to TuYV was identified in the Korean spring oilseed rape, B. napus variety Yudal, whilst the dwarf French winter oilseed rape line Darmor-bzh was susceptible. QTL mapping using the established Darmor-bzh × Yudal DH mapping population (DYDH) revealed one major QTL explaining 36% and 18% of the phenotypic variation in two independent experiments. A DYDH line was crossed to Yudal, and reciprocal backcross (BC1) populations from the F1 with either the susceptible or resistant parent revealed the dominant inheritance of the TuYV resistance. The QTL on ChrA04 was verified in the segregating BC1 population. A second minor QTL on ChrC05 was identified in one of the two DYDH experiments, and it was not observed in the BC1 population. The TuYV resistance QTL in ‘R54’ is within the QTL interval on Chr A04 of Yudal; however, the markers co-segregating with the ‘R54’ resistance are not conserved in Yudal, suggesting an independent origin of the TuYV resistances. This is the first report of the QTL mapping of TuYV resistance in natural B. napus

Extent and structure of linkage disequilibrium in canola quality winter rapeseed (Brassica napus L.)

Linkage disequilibrium was investigated in canola quality winter rapeseed to analyze (1) the prospects for whole-genome association analyses and (2) the impact of the recent breeding history of rapeseed on linkage disequilibrium. A total of 845 mapped AFLP markers with allele frequencies ≥0.1 were used for the analysis of linkage disequilibrium in a population of 85 canola quality winter rapeseed genotypes. A low overall level of linkage disequilibrium was found with a mean r2 of only 0.027 over all 356,590 possible marker pairs. At a significance threshold of P = 2.8 × 10−7, which was derived by a Bonferroni correction from a global α-level of 0.1, only 0.78% of the marker pairs were in significant linkage disequilibrium. Among physically linked marker pairs, the level of linkage disequilibrium was about five times higher with more than 10% of marker pairs in significant linkage disequilibrium. Linkage disequilibrium decayed rapidly with distance between linked markers with high levels of linkage disequilibrium extending only for about 2 cM. Owing to the rapid decay of linkage disequilibrium with distance association analyses in canola quality rapeseed will have a significantly higher resolution than QTL analyses in segregating populations by interval mapping, but much larger number of markers will be necessary to cover the whole genome. A major impact of the recent breeding history of rapeseed on linkage disequilibrium could not be observed

QTL meta-analysis of root traits in Brassica napus under contrasting phosphorus supply in two growth systems

A high-density SNP-based genetic linkage map was constructed and integrated with a previous map in the Tapidor x Ningyou7 (TNDH) Brassica napus population, giving a new map with a total of 2041 molecular markers and an average marker density which increased from 0.39 to 0.97 (0.82 SNP bin) per cM. Root and shoot traits were screened under low and ‘normal’ phosphate (Pi) supply using a ‘pouch and wick’ system, and had been screened previously in an agar based system. The P-efficient parent Ningyou7 had a shorter primary root length (PRL), greater lateral root density (LRD) and a greater shoot biomass than the P-inefficient parent Tapidor under both treatments and growth systems. Quantitative trait loci (QTL) analysis identified a total of 131 QTL, and QTL meta-analysis found four integrated QTL across the growth systems. Integration reduced the confidence interval by ~41%. QTL for root and shoot biomass were co-located on chromosome A3 and for lateral root emergence were co-located on chromosomes A4/C4 and C8/C9. There was a major QTL for LRD on chromosome C9 explaining ~18% of the phenotypic variation. QTL underlying an increased LRD may be a useful breeding target for P uptake efficiency in Brassica

Evolution of Linked Avirulence Effectors in Leptosphaeria maculans Is Affected by Genomic Environment and Exposure to Resistance Genes in Host Plants

Brassica napus (canola) cultivars and isolates of the blackleg fungus, Leptosphaeria maculans interact in a ‘gene for gene’ manner whereby plant resistance (R) genes are complementary to pathogen avirulence (Avr) genes. Avirulence genes encode proteins that belong to a class of pathogen molecules known as effectors, which includes small secreted proteins that play a role in disease. In Australia in 2003 canola cultivars with the Rlm1 resistance gene suffered a breakdown of disease resistance, resulting in severe yield losses. This was associated with a large increase in the frequency of virulence alleles of the complementary avirulence gene, AvrLm1, in fungal populations. Surprisingly, the frequency of virulence alleles of AvrLm6 (complementary to Rlm6) also increased dramatically, even though the cultivars did not contain Rlm6. In the L. maculans genome, AvrLm1 and AvrLm6 are linked along with five other genes in a region interspersed with transposable elements that have been degenerated by Repeat-Induced Point (RIP) mutations. Analyses of 295 Australian isolates showed deletions, RIP mutations and/or non-RIP derived amino acid substitutions in the predicted proteins encoded by these seven genes. The degree of RIP mutations within single copy sequences in this region was proportional to their proximity to the degenerated transposable elements. The RIP alleles were monophyletic and were present only in isolates collected after resistance conferred by Rlm1 broke down, whereas deletion alleles belonged to several polyphyletic lineages and were present before and after the resistance breakdown. Thus, genomic environment and exposure to resistance genes in B. napus has affected the evolution of these linked avirulence genes in L. maculans