Susceptible Canola Research Articles

Studying temperature’s impact on Brassica napus resistance to identify key regulatory mechanisms using comparative metabolomics

To investigate the effects of temperature on Brassica napus (canola) resistance to Leptosphaeria maculans (LM), the causal agent of blackleg disease, metabolic profiles of LM infected resistant (R) and susceptible (S) canola cultivars at 21 °C and 28 °C were analyzed. Metabolites were detected in cotyledons of R and S plants at 48- and 120-h post-inoculation with LM using UPLC-QTOF/MS. The mock-inoculated plants were used as controls. Some of the resistance-related specific pathways, including lipid metabolism, amino acid metabolism, carbohydrate metabolism, and aminoacyl-tRNA biosynthesis, were down-regulated in S plants but up-regulated in R plants at 21 °C. However, some of these pathways were down-regulated in R plants at 28 °C. Amino acid metabolism, lipid metabolism, alkaloid biosynthesis, phenylpropanoid biosynthesis, and flavonoid biosynthesis were the pathways linked to combined heat and pathogen stresses. By using network analysis and enrichment analysis, these pathways were identified as important. The pathways of carotenoid biosynthesis, pyrimidine metabolism, and lysine biosynthesis were identified as unique mechanisms related to heat stress and may be associated with the breakdown of resistance against the pathogen. The increased susceptibility of R plants at 28 °C resulted in the down-regulation of signal transduction pathway components and compromised signaling, particularly during the later stages of infection. Deactivating LM-specific signaling networks in R plants under heat stress may result in compatible responses and deduction in signaling metabolites, highlighting global warming challenges in crop disease control.

Introgression of Resistance to Multiple Pathotypes of Plasmodiophora brassicae from Turnip (Brassica rapa ssp. rapifera) into Spring B. napus Canola

Clubroot disease resistance has been introgressed from Brassica rapa into canola following different approaches including a B. napus × B. rapa interspecific cross; however, the details of this cross are not available. To extend our knowledge of this cross for the introgression of resistance to multiple pathotypes, we crossed a clubroot-resistant turnip with a susceptible canola and backcrossed the F1′s to canola. The backcross population was subjected to pedigree breeding with selection for clubroot resistance and canola-quality traits; selection for resistance to pathotype 3H and 3A was performed in the early and advanced generations, respectively. The advanced generation lines were also evaluated for resistance to 3H, 3A, 2B, and 5x, and for field resistance to clubroot, and agronomic and seed quality traits. Following this, we developed canola lines carrying resistance to multiple pathotypes and nuclear DNA content similar to B. napus. Resistance to 3H showed a weak correlation with other pathotypes whereas resistance to 3A showed a strong positive correlation with 5x indicating that resistance to these pathotypes was under a similar genetic control. Thus, the results demonstrate that resistance to multiple pathotypes can be introgressed from a turnip into a canola even when selection for resistance to single pathotype is performed in early generations.

A Proteome-Level Investigation Into Plasmodiophora brassicae Resistance in Brassica napus Canola.

Clubroot of Brassicaceae, an economically important soil borne disease, is caused by Plasmodiophora brassicae Woronin, an obligate, biotrophic protist. This disease poses a serious threat to canola and related crops in Canada and around the globe causing significant losses. The pathogen is continuously evolving and new pathotypes are emerging, which necessitates the development of novel resistant canola cultivars to manage the disease. Proteins play a crucial role in many biological functions and the identification of differentially abundant proteins (DAP) using proteomics is a suitable approach to understand plant–pathogen interactions to assist in the development of gene specific markers for developing clubroot resistant (CR) cultivars. In this study, P. brassicae pathotype 3 (P3H) was used to challenge CR and clubroot susceptible (CS) canola lines. Root samples were collected at three distinct stages of pathogenesis, 7−, 14−, and 21-days post inoculation (DPI), protein samples were isolated, digested with trypsin and subjected to liquid chromatography with tandem mass spectrometry (LC-MS/MS) analysis. A total of 937 proteins demonstrated a significant (q-value < 0.05) change in abundance in at least in one of the time points when compared between control and inoculated CR-parent, CR-progeny, CS-parent, CS-progeny and 784 proteins were significantly (q < 0.05) changed in abundance in at least in one of the time points when compared between the inoculated- CR and CS root proteomes of parent and progeny across the three time points tested. Functional annotation of differentially abundant proteins (DAPs) revealed several proteins related to calcium dependent signaling pathways. In addition, proteins related to reactive oxygen species (ROS) biochemistry, dehydrins, lignin, thaumatin, and phytohormones were identified. Among the DAPs, 73 putative proteins orthologous to CR proteins and quantitative trait loci (QTL) associated with eight CR loci in different chromosomes including chromosomes A3 and A8 were identified. Proteins including BnaA02T0335400WE, BnaA03T0374600WE, BnaA03T0262200WE, and BnaA03T0464700WE are orthologous to identified CR loci with possible roles in mediating clubroot responses. In conclusion, these results have contributed to an improved understanding of the mechanisms involved in mediating response to P. brassicae in canola at the protein level.

Evaluation of lime products for clubroot (Plasmodiophora brassicae) management in canola (Brassica napus) cropping systems

Clubroot, caused by Plasmodiophora brassicae, is an important soilborne disease of canola (Brassica napus) in Alberta, Canada. As clubroot prefers acidic soils, the application of lime to increase soil pH may reduce disease. Replicated field plot experiments were conducted to evaluate the efficacy of hydrated lime for clubroot management. In 2017, moderate (11.4 t ha−1) to high rates (12.7 t ha−1) of hydrated lime reduced the index of disease (ID) by 35–91% in a susceptible canola cultivar 8 weeks after seeding. In contrast, no effect of lime treatment was observed in the 2018 field trials, due to several environmental factors. A greenhouse study compared the efficacy of hydrated lime and limestone in reducing ID in susceptible and resistant canola cultivars, at different application rates and inoculum concentrations. In treatments that did not receive lime, ID was very high (92–100%) in the susceptible and low (9–13%) in the resistant canola. The application of hydrated lime at 4.7, 8.1, 11.4 and 14.8 t ha−1 eliminated visible symptoms in both cultivars, whereas limestone reduced the ID only at the two lowest inoculum concentrations. Root tissues from the same study were analyzed by quantitative PCR to measure P. brassicae proliferation in planta. Inoculum concentration and the type and rate of lime significantly affected the amount of P. brassicae DNA present in the root tissue. Repeated trials with less virulent inoculum indicated similar trends. Hydrated lime may represent a more effective tool than limestone to manage P. brassicae on canola in highly infested soils.

Identification of lncRNAs in response to infection by Plasmodiophora brassicae in Brassica napus and development of lncRNA-based SSR markers.

Clubroot resistance in spring canola has been introgressed from different Brassica sources; however, molecular mechanism underlying this resistance, especially the involvement of long non-coding RNAs (lncRNAs), is yet to be understood. We identified 464 differentially expressed (DE) lncRNAs from the roots of clubroot-resistant canola, carrying resistance on chromosome BnaA03, and susceptible canola lines challenged with Plasmodiophora brassicae pathotype 3. Pathway enrichment analysis showed that most of the target genes regulated by these DE lncRNAs belonged to plant-pathogen interaction and hormone signaling, as well as primary and secondary metabolic pathways. Comparative analysis of these lncRNAs with 530 previously reported DE lncRNAs, identified using resistance located on BnaA08, detected 12 lncRNAs that showed a similar trend of upregulation in both types of resistant lines; these lncRNAs probably play a fundamental role in clubroot resistance. We identified SSR markers within 196 DE lncRNAs. Genotyping of two DH populations carrying resistance on BnaA03 identified a marker capable of detecting the resistance in 98% of the DH lines. To our knowledge, this is the first report of the identification of SSRs within lncRNAs responsive to P. brassicae infection, demonstrating the potential use of lncRNAs in the breeding of Brassica crops.

Early fungicide treatment reduces blackleg on canola but yield benefit is realized only on susceptible cultivars under high disease pressure

Blackleg [Leptosphaeria maculans (Desm.) Ces. & de Not.] is the most widespread disease of canola (Brassica napus L.) on the Canadian prairies. It has a noticeably increased incidence in recent years possibly due to shifts in the pathogen population and erosion in cultivar resistance. This study was conducted to assess foliar-applied fungicides for mitigating the risk of blackleg. Field trials were conducted at five locations in the Black or Dark-Brown Soil Zones for four years using the susceptible canola ‘Westar’. The fungicides pyraclostrobin (Headline®), azoxystrobin (Quadris®), propiconazole (Tilt®), and azoxystrobin + propiconazole (Quilt®) were applied at the 2–4 leaf stage against early infection. For comparisons, pyraclostrobin was also applied at bolting of ‘Westar’ and at the 2–4 leaf stage of two resistant cultivars (‘43E01’, ‘45H29’). These early treatments, except propiconazole, significantly reduced the mean disease incidence (MDI) and disease severity index (DSI) on ‘Westar’, relative to untreated control, reducing the impact of disease on yield by 16.5–26.9%. Late application of pyraclostrobin at bolting was ineffective. Two-application treatments, with pyraclostrobin at the 2–4 leaf stage and propiconazole at the bolting stage, or vice versa, provided no further efficacy or yield benefit relative to the early application of pyraclostrobin alone. None of the treatments showed substantial disease reduction or yield benefit in two low-disease station years (MDI <30%). On the resistant cultivars, pyraclostrobin reduced MDI and DSI, but showed no yield benefit. As resistant canola cultivars are used commonly on the prairies, a routine application of foliar fungicide is not recommended.

Infection of canola by the root pathogen Plasmodiophora brassicae increases resistance to aboveground herbivory by bertha armyworm, Mamestra configurata Walker (Lepidoptera: Noctuidae)

Infection of plants by pathogens can result in the upregulation of induced defenses; plants may be more or less susceptible to attack by insect herbivores following infection. We investigated the interaction between canola, Brassica napus L., plants infected with clubroot, Plasmodiophora brassicae Woronin, and a generalist herbivore the bertha armyworm (BAW) Mamestra configurata Walker using two canola cultivars that varied in susceptibility to clubroot disease. Volatile organic compounds released from experimental plants differed with infection and female adult BAW could discriminate between canola plants inoculated with P. brassicae and disease-free plants. Adult female moths preferentially laid eggs on disease-free plants of the susceptible cultivar to P. brassicae. Inoculation of resistant canola with P. brassicae, however, did not influence oviposition by female BAW. The fitness of BAW larvae was reduced when they were reared on susceptible canola inoculated with P. brassicae. Salicylic acid and its conjugates in susceptible canola plants were induced following P. brassicae inoculation as compared to disease-free susceptible plants. We conclude that suppression of BAW oviposition and offspring fitness may result in part from a change in the volatile profile of the plant as a result of inoculation and the induction of defenses in inoculated susceptible canola.

Influence of resistant cultivars and crop intervals on clubroot of canola

Clubroot, caused by Plasmodiophora brassicae, is an important constraint on canola (Brassica napus) production in Canada. Rotations of clubroot-resistant (CR) canola cultivars in various sequences and planting intervals between canola with non-host crops and fallow periods were evaluated to determine their effects on clubroot severity and P. brassicae resting spore populations under field and micro-plot conditions. Under micro-plot conditions, the rotation sequences including CR canola, continuous fallow, and the non-host barley reduced gall weight by 63%–100% and clubroot severity by 34%–100% compared with continuous planting of susceptible canola. No visible clubroot symptoms developed following continuous fallow or the non-host crop. Under field conditions, clubroot severity was very high (78% disease index) in the continuous susceptible canola sequence. Most of the CR canola rotation sequences significantly reduced clubroot severity by 12%–23%, but continuous fallow, continuous barley, and alternating the CR canola cultivars ‘45H29’ or ‘73-47’ with ‘TC72429-10’ reduced clubroot severity by 32%–36%. A comparison of intervals between canola crops and four cropping sequences (continuous susceptible canola, alternating canola with barley or pea, a 2-yr non-host interval between canola crops, and a 3-yr non-host interval between canola crops) was conducted over 5 yr. A 2- or 3-yr non-host interval improved plant height, plant biomass, and seed yield, and reduced gall mass, P. brassicae propagules in the soil, and clubroot severity. A significant yield increase of more than 3600% was observed in a 3-yr non-host interval.

Comparison of techniques for estimation of resting spores of Plasmodiophora brassicae in soil

Clubroot (Plasmodiophora brassicae) is an important disease of canola (Brassica napus) and other brassica crops. Accurate estimation of inoculum load in soil is important for evaluating producer risk in planting a susceptible crop, but also for evaluation of management practices such as crop rotation. This study compared five molecular techniques for estimating P. brassicae resting spores in soil: quantitative polymerase chain reaction (qPCR), competitive positive internal control PCR (CPIC‐PCR), propidium monoazide PCR (PMA‐PCR), droplet digital PCR (ddPCR) and loop‐mediated isothermal DNA amplification (LAMP). For ddPCR and LAMP, calibrations were developed using spiked soil samples. The comparison was carried out using soil samples collected from a long‐term rotation study at Normandin, Québec, with replicated plots representing 0‐, 1‐, 2‐, 3‐, 5‐ and 6‐year breaks following susceptible canola infested with clubroot. CPIC‐PCR and ddPCR provided repeatable estimates of resting spore numbers in soil compared with estimates from qPCR or LAMP alone. CPIC‐PCR provided the most robust measurement of spore concentration, especially in the 2 years following a crop of susceptible canola, because it corrected for effects of PCR inhibitors. PMA‐PCR demonstrated that a large proportion of the DNA of P. brassicae detected in soil after the susceptible canola crop was derived from spores that were immature or otherwise not viable. Each assay provided a similar pattern of spore concentration in soil, which supported the conclusion of a previous study at this site that resting spore numbers declined rapidly in the first 2 years after a susceptible crop, but much more slowly subsequently.

Virulence and inoculum density‐dependent interactions between clubroot resistant canola (Brassica napus) and Plasmodiophora brassicae

To mitigate the impact and dissemination of clubroot in western Canada, canola (Brassica napus) producers have relied on clubroot resistance traits. However, in 2013 and 2014, new strains of the clubroot pathogen, Plasmodiophora brassicae, emerged that are virulent on most clubroot‐resistant (CR) canola genotypes. Novel strains of the pathogen were inoculated onto two susceptible canola cultivars, one resistant line and six CR cultivars. Although all cultivars/lines showed a susceptible response to inoculation with the new strains of P. brassicae, the severity of disease reaction, root hair infection rates and the amount of P. brassicae DNA present in each canola genotype varied depending on the strain. In addition, the effect of inoculum density on disease severity and gall formation was recorded for one of these new strains on a universally susceptible Chinese cabbage cultivar and one susceptible and 10 resistant canola genotypes. Although root galls were observed at an inoculum density of 103 spores per mL of soil, clear differentiation of susceptible and resistant reactions among canola cultivars/lines was not observed until the inoculum density reached 105 spores mL−1. At a spore density of 106 spores mL−1 and above, all cultivars/lines developed susceptible reactions, although there was some differentiation in the degree of reaction. This study shows the potential to develop a unique disease profile for emergent clubroot pathotypes and shows a useful range of spore densities at which to study new P. brassicae strains.

Infection and Gene Expression of the Clubroot Pathogen Plasmodiophora brassicae in Resistant and Susceptible Canola Cultivars.

Infection by the clubroot pathogen Plasmodiophora brassicae on resistant and susceptible canola cultivars was investigated at various times following inoculation. Primary infection occurred on more than 90% of root hairs in both cultivars at 7 days after inoculation (dai), and thereafter declined to less than 20% at 14 to 35 dai. The amount of primary infection on the two cultivars was similar at each time point. Secondary infections were rare in both cultivars at 5 and 7 dai but became common after 14 dai. At 14 to 28 dai, the level of secondary infection was greater in the resistant cultivar than in the susceptible one. The in planta expression of 12 selected P. brassicae genes was investigated by reverse-transcription quantitative polymerase chain reaction. All genes were upregulated at 5 or 7 dai in the resistant cultivar. In the susceptible cultivar, the 12 genes could be classified into three groups according to their expression patterns: 2 genes showed an expression peak at 14 dai, 3 showed two expression peaks at 14 and 35 dai, and the others showed an expression peak at 35 dai. Results from this study will be useful in breeding for resistance and in selecting candidate pathogenicity genes for further studies.

A >2-year crop rotation reduces resting spores of Plasmodiophora brassicae in soil and the impact of clubroot on canola

Resting spores of Plasmodiophora brassicae (clubroot) can survive for many years in soil, so crop rotation has not been considered as a major tool for clubroot management. However, it was not known if 2- to 5-year crop rotations would reduce the pathogen inoculum in soil and alleviate disease impact. Two field trials were conducted at a heavily infested site. In 2012, susceptible, moderately susceptible and resistant canola cultivars were seeded to plots with a 0- to 4-year break from susceptible canola. Resting spores in soil were estimated using a quantitative PCR assay. In the second trial on adjacent plots in 2013, three resistant cultivars were evaluated. A break from susceptible canola ≥2 years resulted in a >90% reduction in resting spores relative to continuous canola or a 1-year break, which, however, failed to reduce the clubroot severity substantially on susceptible or moderately susceptible cultivars. Plants of these cultivars died before maturity in plots of continuous canola or a 1-year break, and were stunted in plots of 2- to 4-year breaks. A ≥2-year break increased the yield of all cultivars relative to continuous canola, but the yield was low (<400kgha−1) for susceptible or moderately susceptible cultivars with all rotation intervals. The yield for resistant cultivar was >1230kgha−1 in continuous canola, and a 2- to 4-year break increased the yield by 32–76%. It was showed here that, for the first time, most P. brassicae resting spores disintegrate within first two years in soil. We also demonstrate that a 2- to 4-year break under heavy infestation alleviate the impact of clubroot on susceptible or moderately susceptible cultivars only marginally, but increase the yield of resistant cultivars substantially. Therefore, a >2-year break from canola is recommended with use of resistant cultivars for clubroot management on canola.

Mapping of clubroot (Plasmodiophora brassicae) resistance in canola (Brassica napus)

Clubroot disease, caused by Plasmodiophora brassicae, has become a major problem in the production of cruciferous crops worldwide. In this study, a population of 121 doubled haploid (DH) lines derived from a crossing between a resistant and a susceptible canola (Brassica napus) genotype was subjected to phenotypic and genotypic studies to determine the inheritance and location of the resistance gene(s). After inoculation with pathotype 3 of P. brassicae, the lines showed a 1:1 segregation ratio for resistance, indicating that resistance in this population is controlled by a single gene. Fifteen PCR‐based markers that were known to be linked to clubroot resistance (CR) genes were screened against genomic DNA from parents and resistant and susceptible bulks. Marker GC1680, linked to the CR gene CRa, exhibited polymorphism between the parents and between the resistant and susceptible bulks. CRa target primers were used to amplify fragments from the two parents and the resultant sequences were compared. A high degree of sequence similarity was found between the parents in the nucleotide binding site domain of CRa. In contrast, sequence polymorphisms were detected in the leucine‐rich repeat (LRR) domain. One pair of primers that amplify a band from the LRR region of the resistant parent but not the susceptible parent was used to screen the DH population. Amplicons were obtained from 60 of the 61 resistant lines and two of the 60 susceptible lines; thus, three recombinants were found. Based on these results, a resistance locus linked to CRa was found.

Effect of host and non‐host crops on Plasmodiophora brassicae resting spore concentrations and clubroot of canola

Plasmodiophora brassicae, causal agent of clubroot of crucifers, poses a serious threat to Canadian canola production. The effects of fallow (F) periods and bait crops (clubroot‐susceptible canola (B) and perennial ryegrass (R)) on clubroot severity and P. brassicae resting spore populations were evaluated in five sequences: R–B, B–R, R–F, B–F and F–F. Both host and non‐host bait crops reduced clubroot severity in a subsequent crop of a susceptible canola cultivar compared with fallow. Resting spore and P. brassicae DNA concentrations decreased in all treatments, but were lowest for the R–B and B–R bait crop sequences. In addition, two studies were conducted in mini‐plots under field conditions to assess the effect of rotation of susceptible or resistant canola cultivars on clubroot severity and P. brassicae resting spore populations. One study included three crops of susceptible canola compared with a 2‐year break of oat–pea, barley–pea, wheat–wheat or fallow–fallow. The other study assessed three crops of resistant canola, two crops of resistant canola with a 1‐year break, one crop of resistant canola and a 2‐year break, and a 3‐year break with barley followed by a susceptible canola. The rotations that included non‐host crops of barley, pea or oat reduced clubroot severity and resting spore concentrations, and increased yield, compared with continuous cropping of either resistant or susceptible canola. Growing of a susceptible canola cultivar contributed 23–250‐fold greater gall mass compared with resistant cultivars.

Turnip mosaic virus: potential for crop losses in the grain belt of New South Wales, Australia

The potential of Turnip mosaic virus (TuMV) to infect and damage cool season crops in the grain belt of New South Wales, Australia, was investigated by serological tests on 24,689 dicot weed, grain, and forage specimens from 1999 to 2007 and infectivity/pathogenicity tests with six isolates. Natural infection by TuMV was common in Brassicaceae weeds. Infected grain crops included mustard (Brassica juncea), field pea (Pisum sativum), chickpea (Cicer arietinum), and coriander (Coriandrum sativum). Forage (turnip, Brassica rapa) was also infected. None of 9,816 canola (Brassica napus, at least 19 cultivars) or 1,967 faba bean (Vicia faba, three cultivars) plants were infected. Six isolates from weeds, mustard, and chickpea were inoculated on a range of weed and crop species including four B. napus pathotype differential lines. Inoculated Brassicaceae weeds, mustard, field pea cv. ‘Cressy Blue’, coriander, Chinese cabbage (B. rapa), and forage turnip (B. rapa) were usually infected. Field pea cv. Dundale and radishes (Raphanus sativus) were infected infrequently. Symptoms were severe in mustard, forage turnip, chickpea, and field pea. The reportedly susceptible canola cv. ‘Outback’ displayed only variable infectivity and mild symptoms for five isolates and no infectivity for one isolate. Faba bean, field pea cv. ‘Excell’, and two B. napus differentials appeared to be non-hosts. The results suggest that TuMV strains naturalised in Brassicaceae weeds in NSW in 1999–2007 could damage mustard, field pea, and forage turnip, but not canola or faba bean. These NSW strains appeared to be distinct from strains that damage canola in Europe, North America, and Asia.

Effects of root exudates and pH on Plasmodiophora brassicae resting spore germination and infection of canola (Brassica napus L.) root hairs

Clubroot, caused by Plasmodiophora brassicae, is emerging as a serious threat to canola production on the Canadian Prairies. Germination of soil-borne resting spores (RSs) and their subsequent infection of canola root hairs are important requirements for the establishment of the clubroot pathogen in canola. Stimulation of RS germination by certain non-host plant species and elevation of soil pH by application of calcium-rich compost may provide some level of clubroot control in the field. Experiments were conducted under controlled conditions to determine the effects of (i) host and non-host root exudates (REs) and different pH values on the germination of RSs, and (ii) different pH values either alone or in conjunction with a Ca-containing nutrient solution (NS) on root hair infection of a susceptible canola cultivar. The percentage of RS germination was significantly enhanced when incubated with REs prepared in NS from three plant species compared to a NS alone or sterile dH2O. Non-host REs from perennial ryegrass (Lolium perenne L.) produced higher spore germination than exudate preparations from canola (Brassica napus L.) and Chinese cabbage (Brassica rapa var. pekinensis, cv. Granaat) roots. The percentages of RS germination were higher at pH values ranging from 5.0 to <7.0, and lower at pH values >7.0 in both the presence and absence of REs. The effects of various pH values on the percent of root hair infection varied. Root hair infection was higher at relatively lower pH values (6.0–7.0) compared to either lower or higher pH values when a NS was added. It was however lower at pH values below 6.5 compared to those above 6.5 when the experiment was conducted in the absence of a NS. The possible impacts of REs, different pH values, and calcium levels in the soil on the severity of clubroot on canola are discussed.

Disruption of a gene encoding a hypothetical secreted proteinfrom Sclerotinia sclerotiorum reduces its virulence on canola (Brassica napus)

The phytopathogen Sclerotinia sclerotiorum has been extensively studied, but factors contributing to the pathogenicity of this fungus are not comprehensively understood. With the annotation of the S. sclerotiorum genome and its characterization at the transcriptome and proteome levels, hypothetical and predicted proteins can be targeted for more detailed studies, helping to elucidate fungal infection and virulence mechanisms. Accordingly, a gene (ssv263) encoding a hypothetical, novel protein from S. sclerotiorum that is orthologous to a secreted protein from Botrytis cinerea, and which appears to be unique to these pathogens, was targeted for further analysis. Mutant strains of S. sclerotiorum were generated by disruption of the ssv263 gene and characterized for virulence on a susceptible canola (Brassica napus) genotype. Based on the extent of symptom development, the virulence of the ssv263-disrupted mutants was significantly reduced relative to the wild-type. To our knowledge, this is the first study in which the gene encoding a hypothetical, secreted protein with unknown function has been identified as a possible virulence factor in S. sclerotiorum. These findings are discussed in the context of developing novel strategies for the detection of this pathogen and/or engineering durable disease tolerance.

Effects of fungicide, seeding date and seedling age on clubroot severity, seedling emergence and yield of canola

Hwang, S. F., Cao, T., Xiao, Q., Ahmed, H. U., Manolii, V. P., Turnbull, G. D., Gossen, B. D., Peng, G. and Strelkov, S. E. 2012. Effects of fungicide, seeding date and seedling age on clubroot severity, seedling emergence and yield of canola. Can. J. Plant Sci. 92: 1175–1186. The infestation of seeds by Plasmodiophora brassicae can result in the transmission of clubroot disease in canola. Five fungicides, including Dynasty 100 FS (azoxystrobin), Helix Xtra (thiamethoxam+difenoconazole+metataxyl+fludioxonil), NebijinTM5SC (flusulfamide), Prosper FX (clothianidin+carbathiin+trifloxystrobin+metalaxyl), and Vitavax RS (carbathiin+thiram), were evaluated under greenhouse conditions using artificially infested canola seeds for their efficacy in eliminating seed-borne inoculum. All of the fungicides significantly reduced clubroot relative to the non-treated control, but NebijinTM5SC and Dynasty 100 FS were the most effective. However, in field trials with Cruiser 5 FS (thiamethoxam), Helix Xtra, Dynasty, Prosper and Sedaxane (pyrazole anilide) applied alone or as a mixture, none of the treatments reduced clubroot severity or improved seedling emergence or yield compared with the insecticidal control (Cruiser 5 FS) in the susceptible cultivar. Clubroot severity was lower in early-seeded canola compared with the late-seeded crops in 2 site-years. The younger seedlings had greater disease severity and reduced plant height and yield than did older seedlings in both resistant and susceptible canola cultivars. We conclude that a combination of approaches including seed treatments and manipulation of seeding dates in conjunction with deployment of resistant cultivars is necessary for the sustainable management of clubroot in canola.

Effect of susceptible and resistant canola plants on Plasmodiophora brassicae resting spore populations in the soil

Clubroot, caused by Plasmodiophora brassicae, has become a serious threat to canola (Brassica napus) production in western Canada. Experiments were conducted to assess the effect of growing resistant and susceptible canola genotypes on P. brassicae soil resting spore populations under greenhouse, mini‐plot and field conditions. One crop of susceptible canola contributed 1·4 × 108 spores mL−1 soil in mini‐plot experiments, and 1 × 1010 spores g−1 gall under field conditions. Repeated cropping of susceptible canola resulted in greater gall mass compared to resistant canola lines. It also resulted in reduced plant height, increased clubroot severity in susceptible canola, and increased numbers of resting spores in the soil mix.

Infection and development of Plasmodiophora brassicae in resistant and susceptible canola cultivars

Commercial cultivars of canola with resistance to clubroot (Plasmodiophora brassicae) have been registered recently in Canada. However, little is known about how and when resistance is expressed. Time series assessments of root hair infection and cortical infection were made in four cultivars that differed in reaction to two pathotypes, P3 and P6 (Williams' system), of P. brassicae. The cultivars were ‘45H29’ (resistant), ‘InVigor 5030’ (moderately resistant), ‘46A76’ (susceptible) and ‘45H21’ (susceptible to P3, resistant to P6). For assessment of root hair infection (RHI), seedlings were harvested at 4, 8 and 12 days after inoculation (DAI). For assessment of cortical colonization (% root area occupied by the pathogen), plants were harvested at 16, 22 and 28 DAI. In compatible interactions (susceptible cultivar × virulent pathotype), RHI developed quickly and was slower in intermediate and incompatible interactions. The maximum RHI for both compatible and incompatible interactions was 65–70%; maximum RHI was slightly but significantly lower (59%) in the intermediate interaction type. At 28 DAI, cortical colonization was high in ‘46A76’ (P3 = 45%, P6 = 35%), intermediate in ‘InVigor 5030’ (P3 = 23%, P6 = 16%), and no colonization (0%) was observed for either pathotype in ‘45H29’. In ‘45H21’, cortical colonization by P3 was high (35%), but inoculation with P6 resulted in no colonization. In the compatible interactions, cortical colonization by P3 was consistently higher than by P6. There were small differences in the pattern of RHI associated with resistance, but resistance was most clearly expressed during secondary infection and development.