Influence of Elevated Temperatures on Resistance Against Phoma Stem Canker in Oilseed Rape
K. Dale NoelAiming QiLakshmi Harika GajulaCraig PadleySteffen RietzYongju HuangBruce D.L. FittHenrik U. Stotz
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Cultivar resistance is an important tool in controlling pathogen-related diseases in agricultural crops. As temperatures increase due to global warming, temperature-resilient disease resistance will play an important role in crop protection. However, the mechanisms behind the temperature-sensitivity of the disease resistance response are poorly understood in crop species and little is known about the effect of elevated temperatures on quantitative disease resistance. Here, we investigated the effect of temperature increase on the quantitative resistance of Brassica napus against Leptosphaeria maculans. Field experiments and controlled environment inoculation assays were done to determine the influence of temperature on R gene-mediated and quantitative resistance against L. maculans; of specific interest was the impact of high summer temperatures on the severity of phoma stem canker. Field experiments were run for three consecutive growing seasons at various sites in England and France using twelve winter oilseed rape breeding lines or cultivars with or without R genes and/or quantitative resistance. Stem inoculation assays were done under controlled environment conditions with four cultivars/breeding lines, using avirulent and virulent L. maculans isolates, to determine if an increase in ambient temperature reduces the efficacy of the resistance. High maximum June temperature was found to be related to phoma stem canker severity. No temperature effect on stem canker severity was found for the cultivar ES Astrid (with only quantitative resistance with no known R genes). However, in the controlled environmental conditions, the cultivar ES Astrid had significantly smaller amounts of necrotic tissue at 20°C than at 25°C. This suggests that, under a sustained temperature of 25°C, the efficacy of quantitative resistance is reduced. Findings from this study show that temperature-resilient quantitative resistance is currently available in some oilseed cultivars and that efficacy of quantitative resistance is maintained at increased temperature but not when these elevated temperatures are sustained for a long period.Keywords:
Leptosphaeria maculans
Canker
Phoma
Blackleg
Blackleg
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Blackleg
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Abstract Leptosphaeria maculans is a fungal pathogen that causes blackleg disease in canola ( Brassica napus ), resulting in significant yield and economic losses in Canada and many parts of the world. Plant NAC transcription factors play critical roles in plant development and response to biotic or abiotic stress. In this study, we identified and characterized a BnNAC19 gene from Brassica napus . The overexpression of BnNAC19 in transgenic canola plants contributed to the improvement of seedling resistance against L. maculans . The mycelial growth of a green fluorescent protein‐tagged strain of L. maculans and production of pycnidiospores were shown to be inhibited in the transgenic canola plants overexpressing BnNAC19 . In addition, the canola transgenic line overexpressing BnNAC19 showed increased disease resistance in the adult plant, which was determined by quantitative resistance. Both increased seedling and adult plant resistance in transgenic canola plants overexpressing BnNAC19 indicate that the BnNAC19 gene plays a positive role against L. maculans . The expression pattern of genes BnNAC19 upstream and downstream of BnNAC19 that participate in plant defence pathways were investigated to elucidate the B. napus resistance mechanisms to L. maculans infection, and hence to aid the long‐term blackleg disease‐resistant breeding programmes.
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Abstract The survival and impact on canola of Leptosphaeria maculans, the causal agent of blackleg, might be reduced by agents that decompose canola stubble. Fifty-six fungal isolates from wood or canola were evaluated for their ability to overcome several biological and (or) physical constraints to their effectiveness in decomposing and (or) eliminating L. maculans from canola stubble. Relative to fungi from canola, wood decay fungi were more tolerant of reduced water potential but somewhat less tolerant of lower temperature. Wood decay fungi were no better than those isolated from canola at decomposing sterile canola stubble and less able than Coprinus spp. and Cyathus olla in colonizing and surviving in nonsterile stubble. None of the isolates were effective in eliminating L. maculans from stubble pieces or causing significant decomposition of nonsterile stubble either under laboratory conditions or in the field. These results suggest that considerable effort would be required to find isolates effective in managing blackleg disease through stubble decomposition. Wood decay fungi would seem to have little potential as biological control agents in this pathosystem because of their inability to colonize and decompose nonsterile canola stubble. Keywords: Leptosphaeria maculans blacklegcanoladecomposition
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A limited survey was conducted to determine the pathogenicity groupings (PG) of Leptosphaeria maculans associated with canola (Brassica napus var. oleifera) blackleg in south central Kentucky. In addition, multiyear ascospore discharge patterns were determined for infested canola stubble from seven fields. Strains PG3 and PG4 of L. maculans were most commonly associated with blackleg in Kentucky (18.4 and 79% of the isolates tested, respectively). One weakly virulent PG1 isolate (2.6%) and no PG2 isolates were found. L. maculans-infested canola stubble discharged ascospores within I week of harvest in 1990 and 1991. Early discharge patterns were not determined in 1989. Stubble collected each year released significant quantities of ascospores in the fall and winter months following crop harvest. However, few ascospores were released from infested stubble after the first year following crop production. The implications of these data in blackleg management are discussed.
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Stem canker or blackleg of brassicas, caused by Leptosphaeria maculans , is one of the most damaging diseases of winter oilseed rape in the UK. Airborne ascospores, released in autumn and winter, initiate leaf infections which may lead to colonization of the petiole and, later in the season, formation of stem lesions and cankers. Although isolates of the pathogen differ in ability to cause damaging stem cankers, this is not readily apparent from leaf spotting or stem lesion symptoms. However, several cultural, biochemical and genetic characteristics appear to be associated with the ability to form damaging stem cankers and isolates can be assigned to one of two groups, termed A and B, on the basis of differences in these characteristics. To investigate the relationship between leaf spotting symptoms and subsequent stem canker formation, and to improve understanding of the epidemiology of this pathogen, it is desirable to differentiate between the stem canker forming A group and the less damaging B group of L. maculans . Characterization of isolate type is also important in seed testing and crop breeding programs, particularly in countries such as Canada and Poland where the A type is not ubiquitous. This article reviews methods, including plant assays, assessments of growth characteristics in vitro , isozyme analyses, secondary metabolite profiling, serology, and nucleic acid analyses, that can be used to differentiate the A and B groups.
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Blackleg (Phoma leaf spot), caused by the ascomycete Leptosphaeria spp. (anamorph: Phoma lingam), is a serious yield-limiting factor in canola. Leptosphaeria spp. populations are highly diverse and display differential interactions with canola cultivars, ranging from hypersensitive-like reactions to partial resistance and susceptibility. Leptosphaeria biglobosa encompasses weakly aggressive isolates and can be found late in the season towards maturity stages of the crop while other isolates from Leptosphaeria maculans are highly aggressive and can be detected throughout the season. In an earlier study, we showed the primary involvement of lignin in the containment of the progress of Phoma leaf spot symptoms around the infection sites by comparing the reaction of three different cultivars with a set of isolates with various levels of aggressiveness. The present investigation reports on further characterization of the host responses to L. biglobosa and L. maculans isolates used individually or subsequently in the infection. Hydroxycinnamates were the major phenolics that differentially accumulated locally or systemically from the infection sites in response to either pre- or co-inoculation with the weakly aggressive isolate tested. Given the involvement of these hydroxycinnamates as precursors for the synthesis of lignin and phenylamide phytoalexins, their detected amounts in response to the priming with a weakly aggressive isolate could explain the restricted development of further inoculated highly aggressive isolates. Key words: canola, Leptosphaeria maculans, L. biglobosa, blackleg, phenolics, phenylamides, hydroxycinnamates, ferulates, phytoalexins. Le chancre noir (phoma), causé par l'ascomycète Leptosphaeria spp. (forme imparfaite : Phoma lingam), constitue un facteur limitant au rendement du canola. Les populations de Leptosphaeria spp. sont très diversifiées et montrent des interactions différentielles, variant de l'hypersensibilité à la résistance partielle ou la sensibilité, vis-à-vis des cultivars de canola. Leptosphaeria biglobosa se compose d'isolats peu agressifs qui n'apparaissent que tard dans la saison quand les plantes ont atteint leur maturité. Leptosphaeria maculans comporte des isolats très agressifs et peut être détecté durant toute la saison de production. Dans une étude précédente comparant les réactions différentielles de trois cultivars à une série d'isolats ayant des niveaux différents d'agressivité, nous avions montré le rôle de la lignine comme composante principale dans la restriction de la progression des symptômes de la maladie autour des sites d'infection. La présente étude fait davantage état de la caractérisation des réponses de l'hôte aux isolats de L. biglobosa et de L. maculans, inoculés soit séparément soit l'un après l'autre. Les dérivés hydroxycinnamiques sont les principaux composés phénoliques accumulés de manière différentielle, que ce soit localement autours des sites d'infection ou systémiquement, en réaction à la pré- ou à la co-inoculation avec l'isolat peu agressif étudié. Étant donné le rôle de ces dérivés hydroxycinnamiques comme précurseurs dans la synthèse de la lignine et de celle des phytoalexines de nature phénylamidique, les quantités induites en réaction à l'isolat peu agressif pourraient expliquer le taux relativement limité de symptômes causés par l'inoculation ultérieure d'isolats beaucoup plus agressifs. Mots-clés : canola, Leptosphaeria maculans, L. biglobosa, chancre noir, composés phénoliques, phénylamides, dérivés hydroxycinnamiques, dérivés féruliques, phytoalexines.
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Blackleg, caused by Leptosphaeria maculans, is a major threat to canola production in Canada. With the exception of China, L. maculans is present in areas around the world where cruciferous crops are grown. The pathogen can cause trade barriers in international canola seed export due to its potential risk as a seed contaminant. The most recent example is China restricting canola seeds imported from Canada and Australia in 2009. Therefore, it is important to assess the level of Blackleg infection in Canadian canola seed lots and dockage (seeds and admixture). In this study, canola seed lots and dockage samples collected from Western Canada were tested for the presence of the aggressive L. maculans and the less aggressive L. biglobosa. Results showed that both L. maculans and L. biglobosa were present in seed lots and dockage samples, with L. biglobosa being predominant in infected seeds. Admixture separated from dockage had higher levels of L. maculans and L. biglobosa infection than samples from seed lots. Admixture appears to harbour higher levels of L. maculans infection compared to seeds and is more likely to be a major source of inoculum for the spread of the disease than infected seeds.
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