Importance of Leptosphaeria biglobosa as a cause of phoma stem canker on winter oilseed rape in the UK

Phoma stem canker is a major disease of oilseed rape in the UK, leading to annual yield losses worth more than £100M. The disease is caused by two closely related species, Leptosphaeria maculans and L. biglobosa. L. maculans is generally considered more damaging, causing stem base canker; L. biglobosa is generally less damaging, causing upper stem lesions. Therefore, previous work has mainly focused on L. maculans and there has been little work on L. biglobosa. This work investigated the contribution of L. biglobosa to stem canker epidemics by assessing the amounts of DNA of L. maculans and L. biglobosa in upper stem lesions or stem base cankers on winter oilseed rape cultivars with different types of resistance against L. mac ulans. Diseased upper stem and stem base samples were collected from nine oilseed rape cultivars in a 2011/2012 field experiment at Rothamsted. The presence of L. maculans or L. biglobosa in each stem sample was detected by speciesspecific PCR. The abundance of L. maculans or L. biglobosa in each stem sample was measured by quantification of L. maculans DNA and L. biglobosa DNA using quantitative PCR (qPCR). The amounts of L. biglobosa DNA were greater than those of L. maculans DNA in both upper stem and stem base samples. These results suggest that the severe upper stem lesions and stem base cankers in the 2011/2012 cropping season were mainly caused by L. biglobosa, suggesting that L. biglobosa can sometimes cause considerable yield loss in the UK. There were differences between cultivars in the amounts of L. maculans DNA and L. biglobosa DNA, with the susceptible cultivar Drakkar having more L. maculans DNA than L. biglobosa DNA while resistant cultivars had less L. maculans DNA than L. biglobosa DNA. These results suggest that L. biglobosa can be an important cause of phoma stem canker on oilseed rape in the UK.Peer reviewedFinal Published versio

Disease management in oilseed rape: insights into the Brassica napus-Pyrenopeziza brassicae pathosystem

Final Published versio

Effects of plant age and inoculum concentration on light leaf spot disease phenotypes on oilseed rape

© The Author(s). All rights reserved.Light leaf spot is caused by the fungal pathogen Pyrenopeziza brassicae and is the mosteconomically damaging disease of oilseed rape (Brassica napus) in the UK. Current controlrelies on repeated fungicide applications; however, pathogen fungicide-insensitivitydevelopment highlights the need for non-chemical controls like host resistance. A study wasdone to assess light leaf spot disease phenotype on the susceptible B. napus cultivar Charger indifferent treatment conditions; factors studied included plant age and inoculum concentration.Results showed that older plants grown in a controlled-environment cabinet produced the mostvisible symptoms. Plants that received a greater inoculum concentration (105spores/ml) weresignificantly shorter by 5 cm than those inoculated with a smaller inoculum concentration (104spores/ml), suggesting possible correlations between fungal inoculum concentration and plantgrowth. Additionally, > 20 P. brassicae field isolates were collected from leaf samples acrossEngland through single-spore isolation and will be screened for virulence

Improved understanding of novel sources of resistance against the light leaf spot pathogen, Pyrenopeziza brassicae

Chinthani Shanika Karandeni Dewage, Kavithra Jayani Wijerathna, Henrik U. Stotz, and Bruce D. L. Fitt, 'Improved understanding of novel sources of resistance against the light leaf spot pathogen, Pyrenopeziza brassicae', paper presented at the Association of Applied Biologists Conference Crop Production in Southern Britain 2017, 15 - 16 February 2017, Peterborough, UK. Proceedings available online at: http://www.aab.org.uk/contentok.php?id=501.In this work, the endophytic growth phase of the light leaf spot pathogen Pyrenopeziza brassicae in selected lines from a doubled haploid (DH) population of oilseed rape, which is known to segregate for resistance against P. brassicae, was characterised using controlled environment (CE) experiments. Fungal staining techniques and pathogen-specific quantitative polymerase chain reactions (qPCR) were used to observe and quantify the pathogen biomass, respectively. The qPCR results showed that the resistant lines contained little P. brassicae DNA and there seemed to be little to no change in the amount of DNA over time. In contrast, there was a considerable increase in pathogen DNA in susceptible lines from 0 to 24 days post inoculation (dpi). These results were also reflected in observations made by a fungal staining method. In addition, leaf samples of these DH lines, collected at three different times from winter oilseed rape field experiments, were analysed using qPCR. The resistant lines had a considerably smaller amount of P. brassicae DNA in leaf samples collected later in the cropping season than that in susceptible lines

Effects of cultivar resistance and fungicide application on stem canker of oilseed rape (Brassica napus) and potential interseasonal transmission of Leptosphaeria spp. inoculum

© 2021 The Authors. Plant Pathology published by John Wiley & Sons Ltd on behalf of British Society for Plant Pathology. This is an open access article under the terms of the Creative Commons Attribution License, https://creativecommons.org/licenses/by/4.0/Phoma stem canker is a damaging disease of oilseed rape (Brassica napus) that causesannual yield losses to UK oilseed rape growers worth approximately £100 million,despite the use of fungicides. In the UK, oilseed rape is sown in August/Septemberand harvested in the following July. The disease epidemics are initiated by ascosporesreleased from Leptosphaeria spp. pseudothecia (ascocarps) on stem stubble in theautumn/winter. Control of this disease is reliant on the use of cultivars with “fieldresistance” and azole fungicides. This study investigated the effects of cultivar resistanceand application of the fungicide prothioconazole on the severity of stem cankerbefore harvest and the subsequent production of pseudothecia on the infected stubbleunder natural conditions in the 2017/2018, 2018/2019, and 2019/2020 croppingseasons. The application of prothioconazole and cultivar resistance decreased theseverity of phoma stem canker before harvest, and the subsequent production ofLeptosphaeria spp. pseudothecia on stubble in terms of pseudothecial density. Resultsshowed that stems with less severe stem cankers produced fewer mature pseudotheciaof Leptosphaeria spp. on the infected stubble. This investigation suggests that themost sustainable and effective integrated control strategy for phoma stem canker inseasons with low quantities of inoculum is to use cultivars with medium or good fieldresistance and apply only one spray of prothioconazole when required.Peer reviewe

Chemical warfare: the fungal quest to conquer oilseed rape

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Leptosphaeria biglobosa inhibits the production of sirodesmin PL by L. maculans

© 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/BACKGROUND: Phoma stem canker is caused by two coexisting pathogens, Leptosphaeria maculans and L. biglobosa. They coexist because of their temporal and spatial separations, which are associated with the differences in timing of their ascospore release. L. maculans produces sirodesmin PL, while L. biglobosa does not. However, their interaction/coexistence in terms of secondary metabolite production is not understood. RESULTS: Secondary metabolites were extracted from liquid cultures, L. maculans only (Lm only), L. biglobosa only (Lb only), L. maculans and L. biglobosa simultaneously (Lm&Lb) or sequentially 7 days later (Lm+Lb). Sirodesmin PL or its precursors were identified in extracts from ‘Lm only’ and ‘Lm+Lb’, but not from ‘Lm&Lb’. Metabolites from ‘Lb only’, ‘Lm&Lb’ or ‘Lm+Lb’ caused significant reductions in L. maculans colony area. However, only the metabolites containing sirodesmin PL caused a significant reduction to L. biglobosa colony area. When oilseed rape cotyledons were inoculated with conidia of ‘Lm only’, ‘Lb only’ or ‘Lm&Lb’, ‘Lm only’ produced large gray lesions, while ‘Lm&Lb’ produced small dark lesions similar to lesions caused by ‘Lb only’. Sirodesmin PL was found only in the plant extracts from ‘Lm only’. These results suggest that L. biglobosa prevents the production of sirodesmin PL and its precursors by L. maculans when they grow simultaneously in vitro or in planta. CONCLUSION: For the first time, L. biglobosa has been shown to inhibit the production of sirodesmin PL by L. maculans when interacting simultaneously with L. maculans either in vitro or in planta. This antagonistic effect of interspecific interaction may affect their coexistence and subsequent disease progression and management.Peer reviewe

Current understanding of phoma stem canker and light leaf spot on oilseed rape in the UK

© The Author(s). All rights reserved.Oilseed rape is the third most important arable crop in the UK. Phoma stem cankerand light leaf spot are two economically important diseases of this crop. These twodiseases cause annual yield losses of winter oilseed rape worth > £100M, despite theuse of fungicides. Phoma stem canker is caused by two closely related fungalpathogens Leptosphaeria maculans and L. biglobosa, whereas light leaf spot is causedby the fungal pathogen Pyrenopeziza brassicae. Epidemics of both diseases areinitiated in autumn by ascospores released from crop debris from the previous croppingseason. However, phoma stem canker is a monocyclic disease, while light leaf spot isa polycyclic disease. Understanding the pathogen biology, disease epidemiology andhost resistance are essential for effective control of these two diseases. This minireview summarises current understanding of these two diseases in relation to pathogenbiology, disease epidemiology and host resistance

Interactions in the Brassica napus-Pyrenopeziza brassicae pathosystem and sources of resistance to P. brassicae (light leaf spot)

Funding Information: This research was funded by AHDB Cereals and Oilseeds (project no. 2140010), the UK Biotechnology and Biological Sciences Research Council (BBSRC/BB/P00489X/1), Innovate UK (102641), the Department for Environment, Food and Rural Affairs (OREGIN, CH0104), the Gen Foundation, and the University of Hertfordshire. We acknowledge in‐kind contributions from Mark Nightingale (Elsoms Seeds UK Ltd) and Dr Vasilis Gegas (Limagrain UK Ltd) by providing advice and support for this study. We thank Dr Rachel Wells (John Innes Centre, Norwich, UK) for providing seeds of Q DH lines and Cabriolet. Publisher Copyright: © 2021 The Authors. Plant Pathology published by John Wiley & Sons Ltd on behalf of British Society for Plant PathologyAbstract: Pyrenopeziza brassicae, cause of light leaf spot (LLS), is an important pathogen of oilseed rape and vegetable brassicas and has a wide geographic distribution. Exploitation of host resistance remains the most sustainable and economically viable solution for disease management. This study evaluated 18 oilseed rape cultivars or breeding lines for host resistance against P. brassicae in glasshouse experiments. Selected cultivars/lines were inoculated with eight single‐spore isolates of the pathogen obtained from three different regions in England. Analysis of P. brassicae infection‐related changes on host plants identified leaf deformation as a characteristic feature associated with P. brassicae infection, this showed poor correlation to LLS severity measured as the amount of pathogen sporulation on infected plants. Resistant host phenotypes were identified by limitation of P. brassicae sporulation, with or without the presence of a necrotic response (black flecking phenotype). Investigation of this pathosystem revealed significant differences between cultivars/lines, between isolates, and significant cultivar/line‐by‐isolate interactions. In total, 37 resistant and 16 moderately resistant interactions were identified from 144 cultivar/line‐by‐isolate interactions using statistical methods. Most of the resistant/moderately resistant interactions identified in this study appeared to be nonspecific towards the isolates tested. Our results suggested the presence of isolate‐specific resistant interactions for some cultivars. Several sources of resistance have been identified that are valuable for oilseed rape breeding programmes.Peer reviewe

QTL mapping for resistance against Pyrenopeziza brassicae derived from a Brassica napus secondary gene pool

© 2022 Karandeni Dewage, Cools, Stotz, Qi, Huang, Wells and Fitt. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). https://creativecommons.org/licenses/by/4.0/Use of host resistance is the most economical and environmentally safe way to control light leaf spot disease of oilseed rape (Brassica napus). The causal organism of light leaf spot, Pyrenopeziza brassicae, is one of the most economically damaging pathogens of oilseed rape in the United Kingdom and it is considered to have a high potential to evolve due to its mixed reproduction system and airborne ascospores. This necessitates diverse sources of host resistance, which are inadequate at present to minimize yield losses caused by this disease. To address this, we screened a doubled haploid (DH) population of oilseed rape, derived from a secondary genepool (ancestral genomes) of B. napus for the introgression of resistance against P. brassicae. DH lines were pheno typed using controlled-environment and glasshouse experiments with P. brassicae populations obtained from three different geographic locations in the United Kingdom. Selected DH lines with different levels of resistance were further studied in a controlled-environment experiment using both visual (scanning electron microscope – SEM) and molecular (quantitative PCR) assessment methods to understand the mode/s of host resistance. There was a clear phenotypic variation for resistance against P. brassicae in this DH population. Quantitative trait locus (QTL)analysis identified four QTLs with moderate to large effects, which were located on linkage groups C1, C6, and C9. Of these, the QTL on the linkage group C1 appeared to have a major effect on limiting P. brassicae asexual sporulation. Study of the sub-cuticular growth phase of P. brassicae using qPCR and SEM showed that the pathogen was able to infect and colonise both resistant and susceptible Q DH lines and control B. napus cultivars. However, the rate of increase of pathogen biomass wassignificantly smaller in resistant lines, suggesting that the resistance segregating in this DH population limits colonisation/sporulation by the pathogen rather than eliminating the pathogen. Resistance QTLs identified in this study provide a useful resource for breeding cultivar resistance for effective control of light leaf spot and form a starting point for functional identification of the genes controlling resistance against P. brassicae that can contribute to our knowledge on mechanisms of partial resistance of pathogens.Peer reviewedFinal Published versio