Plasmodiophora brassicae, the pathogen responsible for clubroot, is the most damaging protist pest affecting canola in Northeastern North Dakota and globally. The pathogen infects roots, forming galls that contain billions of resting spores, which persist in soil for extended periods and facilitate rapid disease spread. Clubroot has been detected annually in North Dakota since its introduction in 2013. Growers have primarily relied on Mendelian resistance sources; however, prolonged use of this gene has resulted in clubroot populations capable of overcoming resistance. Clubroot prevalence in North Dakota has been monitored over the past ten years. Recent annual surveys in 21 counties have revealed a breakdown of first-generation resistance in canola cultivars in Cavalier County for the second consecutive year. This breakdown in resistance poses a significant threat to canola production. To mitigate the spread of clubroot, growers are recommended to implement longer crop rotations (one in four years) in acidic soils, use multi-gene clubroot-resistant cultivars, and maintain equipment sanitation in affected areas. Continued evaluation of resistant cultivars and annual monitoring of clubroot prevalence and resistance breakdown in North Dakota are essential.

Plasmodiophora brassicae is a biotrophic protist that induces significant developmental and physiological reprogramming within its host plant. Numerous research projects focus on identifying the genetic basis of plant tolerance and resistance to clubroot disease; however, there remains a vast unexplored territory covering the biological aspects that influence this interaction. By applying basic plant biology expertise, we can unravel the complex relationship between this intriguing organism and the plants it affects. However, to achieve that, a comprehensive understanding of the interaction model is essential. This presentation aims to provide fundamental information to elucidate the complexity of this model, which may be useful in designing and interpreting research related to clubroot disease. Let’s dive into the fascinating world of P. brassicae–plant interaction biology.

Monitoring, Management and Microbiome of Clubroot

The McDonald lab has conducted a range of clubroot research, including effects of temperature, pH, calcium, crop rotation, cover crops and biocontrols, and also methods to assess only viable resting spores. Studies on the interaction of temperature and pH demonstrated that there was low disease development below ~14 °C, and confirmed the optimum temperatures of 24–26 °C. Repeated freezing and thawing of clubs in lab conditions greatly reduced the viability of resting spores, but freezing and thawing in the field had a much smaller effect. Resting spores suspended in water survived up to five months; 60% were viable in frozen suspension, and 30% when subjected to freezing and thawing. The use of propidium monoazide with qPCR amplifies only the DNA of viable resting spores to better assess the effects of these and other studies. Increasing pH resulted in a consistent reduction in clubroot severity but there was still 40% severity at pH 8.0 when temperature and moisture were optimum. Increasing soil pH with amendments suppressed clubroot, while amendments that increased calcium without affecting pH did not. Some cover crops and cereal rotation crops could reduce the population of resting spores by stimulating germination in the absence of a host. The endophyte, Beauveria bassiana suppressed clubroot on cabbage under controlled environment conditions. In the field, treated plants had higher yields, but clubroot suppression was less consistent. Some soilless mixes do not support the development of clubroot even under optimum conditions for disease development. Studies on the soil microbiome in infested fields found that the microbiome was different between bulk soil and rhizosphere soil, but not very different between plants with or without clubroot. Identification of pathotypes can be useful for selecting clubroot-resistant canola cultivars. The Canadian Clubroot Differential set was used to identify the prevalence of highly virulent pathotypes, that could overcome first-generation resistance in canola, in P. brassicae in northern Ontario. Work on molecular methods for studying P. brassicae continues.

Nearly a century has elapsed since populations of Plasmodiophora brassicae were first classified according to their interaction patterns with Brassica species. Subsequently, numerous differential host sets have been established and implemented globally to investigate the pathogenic diversity within P. brassicae populations. These systems comprise a variety of classification frameworks, including several globally- or country-specific clubroot differential sets. Some of these systems are subjected to continuous refinement and are periodically updated to accommodate newly identified pathogen variants. This presentation will address the concept of pathotypes, elucidate the principal distinctions among pathotyping systems, and discuss their respective advantages and limitations. Furthermore, comparative analyses of isolates classified under different systems will be presented. Participants are invited to deepen their understanding of the significance of P. brassicae pathotypes in plant pathology and in the development of clubroot-resistant cultivars. The presentation will comprise additional aspects, including the distribution of specific P. brassicae pathotypes in Poland in comparison with other countries. Various molecular detection approaches will be presented and critically compared. Furthermore, the unique biological features of the pathogen will be discussed, alongside both currently utilized and novel potential sources of resistance. Particular attention will be given to the first HO-CR and HOLL-CR breeding lines developed in Poland. Moreover, the speaker will share her experience with the audience in studying clubroot using rhizotrons and automated methods for phenotyping the root systems of healthy and P. brassicae-infected Brassica plants.

Small Patch Detection of Clubroot (Plasmodium brassicae) in Canola (Brassica napus) using Drone-mounted Hyperspectral Cameras.

This presentation will discuss research investigations aimed at determining whether clubroot (Plasmodium brassicae), an economically important disease of canola (Brassica napus), can be detected from the air using drone-mounted hyperspectral cameras. Early detection, when the disease is limited to small, localized patches prior to spreading, would be of significant economic value; clubroot symptoms don’t generally develop until after the onset of flowering, when detailed scouting is difficult. Twenty-three research and commercial canola fields were imaged across Alberta and Saskatchewan Canada, during early to late flowering from 2021 to 2023 using a DJI M600 remotely piloted aircraft system (RPAS) outfitted with a Headwall nano-hyperspec camera. Follow-up ground surveys were conducted post-harvest by plant pathologists on each sampled field. One commercial research facility proved especially valuable for training a predictive classification model since plant pathologists associated with research operations were willing to share disease severity indices for imaged areas at the conclusion of their research trials. This data offered a diverse selection of cultivars for supervised training involving various classification algorithms. Early attempts at predictive model development using conventional statistics looked promising but lacked repeatability due to uncertainties in data mapping. Follow-up investigations, relying on more deliberate mapping, predictive analytics, and machine learning, reinforced earlier detected trends while providing additional data insights. Stochastic gradient boosting offered the most consistently reliable result outperforming binary logistic regression, classification and regression trees, and random forests, with test data misclassification rates often less than five percent. In a blind comparison of small patch hyperspectral data sets collected over three years and hundreds of kilometres of geographical expanse, predictive classification yielded 100 percent agreement with post-swathing pathology reports at the field level and > 90 percent agreement when analysis was limited to the small patch scale.

How Plasmodiophora brassicae is hiding in its host: can we use this knowledge to control the disease with an endophytic fungus?

Plasmodiophora brassicae can infect all important Brassica crop species as well as the model plant Arabidopsis thaliana. However, the biology of the clubroot pathogen P. brassicae in its host is still not well understood. The genome sequence of P. brassicae has opened up novel approaches to study the protist, among them the identification of potential effectors. We identified several genes encoding putative plant hormone metabolizing enzymes, such as a SABATH-type methyltransferase which is involved in the downregulation of the defense hormone salicylic acid. We recently added a compound that was described as an inductor for systemic acquired resistance, N-hydroxypipecolic acid, to the substrates effectively converted by the methyltransferase. Since one of the pathways the protist interferes with during disease development is defense signaling, we hypothesized that the induction of defense pathways by microbes could be one way of reducing club formation, and evaluated the potential of microbes able to induce immune responses to combat the protist. One such endophytic fungus is Acremonium alternatum, which was studied using different host plants. Interestingly, the response of different hosts in reducing disease symptoms varied by species and also at the cultivar level. The potential for possible applications in practical settings will be discussed, together with different methods to apply A. alternatum.

Clubroot, caused by Plasmodiophora brassicae, is a major problem for canola production worldwide. The continual emergence of new pathotypes challenges the durability of resistant cultivars. This webinar will highlight how integrating transcriptomics, proteomics, metabolomics, and advanced microscopy is improving our understanding of the mechanisms underlying clubroot resistance in Brassica napus. Using resistant and susceptible, doubled-haploid, and near-isogenic lines inoculated with P. brassicae, we characterized host responses across multiple stages of infection. Our analyses identified key resistance-associated genes, particularly on chromosomes A03 and A08, involved in cell wall remodelling, lignin biosynthesis, calcium and reactive oxygen species signalling, and defence pathways. These molecular changes were supported by the accumulation of defence-related metabolites and are being further validated using Fourier-transform infrared (FT-IR) spectroscopy and transmission electron microscopy (TEM) to reveal structural and chemical changes in resistant cell walls. The findings demonstrate that resistance is associated with coordinated molecular and structural reinforcement of the cell wall and identify candidate genes and biomarkers that can support the development of durable clubroot-resistant canola cultivars.

RenSeq and whole-genome sequencing uncover allelic diversity of clubroot resistance genes in commercial breeding canola lines

Clubroot disease, caused by the obligate biotrophic pathogen Plasmodiophora brassicae, is a major threat to canola (Brassica napus) production worldwide. Clubroot-resistant (CR) cultivars remain the most effective disease-management strategy, but the genetic basis of resistance in commercial canola remains poorly understood because many resistance sources are proprietary and associated genotypic information is rarely accessible. Although nucleotide-binding leucine-rich repeat (NLR) immune receptors account for most cloned CR genes, no pan-NLRome has incorporated CR lines used in commercial canola breeding. Here, we combined whole-genome sequencing and resistance gene enrichment sequencing (RenSeq) to assemble and annotate the NLR repertoires of five homozygous CR inbred lines (IH1–IH5) used for commercial breeding and displaying contrasting resistance profiles against predominant Canadian P. brassicae pathotypes. We integrated these NLRomes with the susceptible cultivar Westar to construct a comparative pan-NLRome for canola. Across the five CR lines, total NLR content was highly conserved, ranging from 504 to 517 genes, with TIR-NLRs representing the predominant class. C-JID-containing TIR-NLRs accounted for more than 30% of each NLR repertoire, and integrated-domain analysis identified conserved and genotype-specific NLR-IDs, including previously unreported domains in IH4. Pan-NLRome analysis resolved 366 NLR orthogroups (OGs), 60.7% of which were core, and identified resistant-line-enriched OGs absent from Westar as candidate CR-associated loci. Unexpectedly, a homolog of the functionally characterized CR gene CRa was detected in all five CR lines. Moreover, a homolog of another CR gene, Crr1a, was detected in both resistant and susceptible lines, indicating that the presence or absence of a gene alone does not predict resistance. Instead, structural variation affecting LRR and C-JID regions suggests that allele-level diversity within conserved NLR loci contributes to CR-associated variation, with implications for allele-specific marker development and durable CR deployment.

Monitoring hidden crop stress with plant electrophysiology: applications for canola and clubroot research

Plants use long-distance signalling to coordinate responses whenever stress occurs, and wherever it begins, including below ground. This presentation shows how continuous electrophysiological monitoring, combined with signal processing and machine learning, delivers real-time insights into plant responses to both biotic and abiotic challenges.

Published research demonstrates the detection of root-knot nematode stress within 3–5 days of infestation and differentiation of plant responses to preventative and curative nematicides. Canola studies reveal treatment- and hybrid-dependent differences in water status, nutrient dynamics, environmental resilience and responses to biostimulants.

Together, these results demonstrate an innovative approach to monitoring both above- and below-ground causes of crop stress. Applied to clubroot research, this could enable continuous comparison of physiological responses in resistant and susceptible genotypes, identification of early infection-associated signal patterns, and real-time measurement of whether — and when — candidate treatments alter the progression of plant stress.

Cytokinin metabolism in Plasmodiophora brassicae

Clubroot, caused by Plasmodiophora brassicae, remains one of the most damaging diseases of winter oilseed rape in Europe, with increasing importance due to changes in pathogen populations and virulence structure. In recent years, shifts in pathotype distribution and aggressiveness have been observed across Central Europe and Sweden, challenging the effectiveness of currently deployed resistance sources. This study summarizes recent findings on the virulence dynamics of P. brassicae populations collected from major winter oilseed rape-growing regions and compares their interaction patterns on differential host sets. The results indicate an ongoing adaptation of pathogen populations, including the emergence and increased frequency of highly virulent pathotypes capable of overcoming widely used resistance gene. In addition, agricultural policy developments linked to the EU Green Deal, such as increased Brassica cultivation as cover crops, may lead to higher inoculum densities in agricultural soils, further intensifying disease pressure. Consequently, future integrated pest management (IPM) strategies must account for both increasing pathogen pressure and the evolving virulence structure of P. brassicae. These changes highlight the need for more durable sources of resistance, improved crop rotation systems, and coordinated regional monitoring to ensure sustainable clubroot management in Europe.

Twenty Years of Clubroot in Western Canada

Emilee Storfie, Yoann Aigu, Sandra Velasco-Cuervo, Leonardo Galindo-González, Victor Manolii, Sheau-Fang Hwang, and Stephen Strelkov

Since its establishment in 2003, clubroot disease (Plasmodiophora brassicae) continues to threaten canola (oilseed rape; Brassica napus) production in the Canadian Prairies. To limit the spread and severity, an integrated management approach has been adopted, which includes field surveys, deployment of clubroot-resistant (CR) canola cultivars, and crop rotation. In Alberta, field surveys have been conducted over the past two decades to assess the spatial and temporal spread and dynamics of the clubroot outbreak. From its initial detection, the number of infested fields has steadily increased each year, due to local spread, reaching a cumulative total of 4,347 fields by 2025. Among the 668 field isolates collected between 2013 and 2023, and characterized using the Canadian Clubroot Differential set, 44 distinct pathotypes were assigned, demonstrating substantial virulence diversity. One explanation for this diversity was the selection pressure imposed on P. brassicae populations by the continuous cropping of first-generation CR cultivars. The most prevalent pathotypes are 3A (32.5%) and 3D (19.7%), which are ‘resistance-breaking’, followed by the ‘non-resistance-breaking’ pathotype 3H (13.9%). To complement the knowledge gained from the field, significant effort has been made to understand the effector biology and mechanisms underlying P. brassicae virulence. Specifically, genome and transcriptome analyses were conducted on the resistance-breaking pathotypes 3A and 5X. Long-read sequencing of their single-spore isolates revealed genome sizes of 27.4 and 23.8 Mbp, with estimated 12,666 and 11,919 predicted coding sequences, respectively. Furthermore, RNA sequencing found 53 putative effectors expressed during infection by both pathotypes. Downstream functional characterization on two of these effector candidates included signal peptide validation, subcellular localization, and activity assays. Ongoing field surveillance of pathogen distribution and virulence, together with the characterization of the P. brassicae effector repertoire, will continue to inform strategies for durable clubroot management.