Summary Near‐isogenic Brassica napus lines carrying/lacking resistance gene Rlm6 were used to investigate the effects of temperature and leaf wetness duration on phenotypic expression of Rlm6 ‐mediated resistance. Leaves were inoculated with ascospores or conidia of Leptosphaeria maculans carrying the effector gene AvrLm6 . Incubation period to the onset of lesion development, number of lesions and lesion diameter were assessed. Symptomless growth of L. maculans from leaf lesions to stems was investigated using a green fluorescent protein (GFP) expressing isolate carrying AvrLm6 . L. maculans produced large grey lesions on Darmor (lacking Rlm6 ) at 5–25 °C and DarmorMX (carrying Rlm6 ) at 25°C, but small dark spots and ‘green islands’ on DarmorMX at 5–20°C. With increasing temperature/wetness duration, numbers of lesions/spots generally increased. GFP‐expressing L. maculans grew from leaf lesions down leaf petioles to stems on DarmorMX at 25°C but not at 15°C. We conclude that temperature and leaf wetness duration affect the phenotypic expression of Rlm6 ‐mediated resistance in leaves and subsequent L. maculans spread down petioles to produce stem cankers.
Fitness of virulent (avrLm1 and avrLm4) or avirulent (AvrLm1 and AvrLm4) isolates of Leptosphaeria maculans on Brassica napus without the corresponding resistance genes Rlm1 and Rlm4 was investigated in controlled environment (CE) and field experiments. Results indicate that there is a measurable fitness cost for virulent isolates compared to avirulent isolates in terms of number of lesions, size of lesions, distance grown through leaf tissue towards the petiole in CE experiments and in terms of systemic growth from leaf lesions to stems in field experiments. There were differences between the AvrLm1 and AvrLm4 loci in fitness cost. The optimal temperature for leaf infection was higher for AvrLm4 isolates than for AvrLm1 isolates. There was a cultivar effect on fitness cost of virulence at the AvrLm1 locus but not at the AvrLm4 locus. Results from field experiments suggest that on the same host without the corresponding Rlm1 and Rlm4 genes, AvrLm4 isolates were more fit than AvrLm1 isolates in warmer growing seasons. The fitness cost of virulence at the AvrLm1 locus was generally smaller than that at the AvrLm4 locus, suggesting that the corresponding resistance gene Rlm4 is more durable than Rlm1. Frequencies of avirulent AvrLm1 and AvrLm6 alleles in airborne inoculum for 2006/07, 2007/08 and 2008/09 growing seasons in the UK were investigated using quantitative PCR. There were differences in frequencies of AvrLm1 and AvrLm6 alleles in the three seasons. The detection of changes in frequencies of avirulent alleles provides essential information to guide deployment of cultivars with corresponding resistance genes to minimise the risk of breakdown of novel resistance.
A comprehensive linkage atlas for seed yield in rapeseed. Most agronomic traits of interest for crop improvement (including seed yield) are highly complex quantitative traits controlled by numerous genetic loci, which brings challenges for comprehensively capturing associated markers/genes. We propose that multiple trait interactions underlie complex traits such as seed yield, and that considering these component traits and their interactions can dissect individual quantitative trait loci (QTL) effects more effectively and improve yield predictions. Using a segregating rapeseed (Brassica napus) population, we analyzed a large set of trait data generated in 19 independent experiments to investigate correlations between seed yield and other complex traits, and further identified QTL in this population with a SNP-based genetic bin map. A total of 1904 consensus QTL accounting for 22 traits, including 80 QTL directly affecting seed yield, were anchored to the B. napus reference sequence. Through trait association analysis and QTL meta-analysis, we identified a total of 525 indivisible QTL that either directly or indirectly contributed to seed yield, of which 295 QTL were detected across multiple environments. A majority (81.5%) of the 525 QTL were pleiotropic. By considering associations between traits, we identified 25 yield-related QTL previously ignored due to contrasting genetic effects, as well as 31 QTL with minor complementary effects. Implementation of the 525 QTL in genomic prediction models improved seed yield prediction accuracy. Dissecting the genetic and phenotypic interrelationships underlying complex quantitative traits using this method will provide valuable insights for genomics-based crop improvement.
Effects of pretreatment of Brassica napus leaves with ascospores of Leptosphaeria biglobosa or chemical defence activators [acibenzolar‐ S ‐methyl (ASM) or menadione sodium bisulphite (MSB)] on infection by ascospores of Leptosphaeria maculans (phoma stem canker) and development of disease were studied in controlled‐environment (phoma leaf spot) and field (phoma leaf spot and stem canker) experiments. In controlled‐environment experiments, pretreatment of oilseed rape leaves (cv. Madrigal) with L. biglobosa , ASM or MSB delayed the appearance of L. maculans phoma leaf spot lesions. These pretreatments also decreased the phoma leaf spot lesion area in both pretreated leaves (local effect) and untreated leaves (systemic effect). In winter oilseed rape field experiments in the 2002/03 and 2003/04 growing seasons, pretreatment with L. biglobosa or ASM in October/November decreased not only the number of phoma leaf spot lesions per leaf caused by L. maculans in autumn/winter, but also the severity of phoma stem canker in the subsequent spring/summer. Effects were greater in 2002/03 (when natural L. maculans ascospore release began in September 2002) than in 2003/04 (when ascospore release began in December following a period of dry weather in August/September 2003). These results suggest that pretreatment with biological or chemical defence activators can induce local and systemic resistance to L. maculans , with both short‐term effects on the development of phoma leaf spotting and long‐term effects on the development of stem canker 8 months later.
Phoma stem canker is an economically important disease of oilseed rape, caused by two co-existing fungal pathogen species, Leptosphaeria maculans (Plenodomus lingam) and Leptosphaeria biglobosa (Plenodomus biglobosus). Leptosphaeria maculans produces a phytotoxin called sirodesmin PL. Our previous work showed that L. biglobosa has an antagonistic effect on the production of sirodesmin PL if it is simultaneously co-inoculated with L. maculans. However, the effects of sequential co-inoculation on interspecific interactions between the two pathogens are not understood.
Controlled environment (CE) and field experiments were done to investigate effects of fungicide on growth of L. maculans and L. biglobosa in relation to development of phoma leaf spots and phoma stem canker on oilseed rape. In CE experiments, for plants inoculated with L. maculans, fungicide treatment decreased lesion size and amount of L. maculans DNA in leaves; for plants inoculated with L. biglobosa, fungicide did not affect lesion size or amount of pathogen DNA. In field experiments in 2006/07 and 2007/08, fungicide treatment decreased phoma leaf spot incidence in autumn and stem canker severity at harvest, and increased yield. Fungicide treatment decreased stem canker severity more on cv. Courage, with a good yield response, than on cv. Canberra. In 2006/07, fungicide decreased the amount of L. maculans DNA more than that of L. biglobosa in stem tissues (measured by quantitative PCR). There was a linear relationship between the amount of L. maculans DNA in stems and the stem canker severity score at harvest, but there was no clear relationship between the amount of L. biglobosa DNA in stems and the stem canker severity score. These results suggest that effects of fungicides on interactions between L. maculans and L. biglobosa might affect severity of phoma stem canker and yield response.
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