Canadian Canola Clubroot Cluster Pillar 3: Host-pathogen biology and interaction

Project Summary

Overview

The explosion of new, virulent pathotypes of Plasmodiophora brassicae (the clubroot pathogen) on canola crops in Alberta indicates that producers need management options for situations where no single source of genetic resistance is available to effectively manage all of the pathotypes of clubroot in their field.

The goal of this research is to develop and validate best management practices for managing clubroot in canola fields where strong genetic resistance is not available and for slowing the spread of these pathotypes into new areas. The study examines factors that affect resting spore survival, germination and infection, in both controlled environments and field trials. Sources of quantitative (non-pathotype specific or horizontal) resistance, which has not previously been studied in detail, are also being identified and assessed to determine if quantitative resistance might be used to increase the durability of genes that confer strong genetic resistance to clubroot. This study will also evaluate strategies for deployment of clubroot resistance genes, with the aim of identifying approaches that will maximize the durability of resistance. Finally, studies of changes in plant hormones in canola caused by clubroot will be examined to determine if this host-pathogen interaction is similar to that studied previously in Arabidopsis.

Objectives

The specific project objectives are to:

• Validate and enhance the efficacy of integrated pest management (IPM) strategies to reduce resting spore concentration in clubroot-infested fields and slow spread of clubroot via evaluation of factors that affect resting spore survival, germination, infection. (Gossen / McDonald)
• Identify lines of Brassica spp. that carry horizontal (non-pathotype specific) resistance to clubroot and compare the pattern of clubroot development with lines carrying strong pathotype-specific resistance (McDonald, Gossen)
• Examine the mechanism of and potential role of QR in IPM for clubroot (Peng et al.)
• Examine the hormonal changes in canola during clubroot development (Strelkov / Hwang)

Potential impact

Rapid breakdown of clubroot resistance (CR) over the last three years (prior to the start of this project) represents a rapidly increasing constraint to canola production on the Canadian prairies. Sources of resistance are limited, so approaches that have the potential to extend the durability of CR genes and allow growers to maintain economic yields in the face of the steady expansion of acreage affected by clubroot are needed. The goal of this research is to develop and validate best management practices for producers to manage clubroot in canola fields when no single source of resistance effectively manages all of the pathotypes of clubroot in the region (adoption by producers). The goal of a successful IPM program is reduction in spore numbers below economic thresholds.

New assessment technologies have recently been used to monitor the viability of resting spores (Al-Daoud et al. 2017) and the decline in spore concentration in soil. This technology demonstrated the benefit of crop rotation in reducing spore populations (Peng et al. 2014b, 2015). Identification of other IPM approaches that speed reduction of spore concentration over time is needed. Studies of factors affecting the maturation, infectivity and survival of resting spores of P. brassicae will identify the most effective IPM strategies to reduce spore concentration and manage clubroot. Also, the efficacy of new approaches, i.e., elicitors, will be assessed as they become available. Reduction of spore concentration in soil will extend the productive life of CR genes by reducing selection for virulent pathotypes. It could also reduce the rate of pathogen spread in new areas (adoption by producers).

Identifying lines of Brassica spp. with quantitative resistance (as opposed to lines with weak, partial resistance) will provide an opportunity to identify and assess the mode(s) of action that underlie this generally durable form of resistance. If quantitative resistance (QR) is consistent and durable, CR genes could eventually be stacked into a line with QR to increase the durability of CR resistance. Evaluation of multi-gene deployment strategies will identify the approach (stacking vs. gene rotation) that results in the greatest durability of CR resistance (adoption by seed companies, implemented by producers). The hormonal changes in canola during clubroot development have never been studied in detail. Detailed knowledge of these changes could eventually be used for manipulation of plant hormone homeostasis and signaling that could be used to develop cultivars with increased tolerance to clubroot (adoption by plant breeders).

Research activities

This research project is made up of multiple studies or research activities, which are led by different researchers, as provided below.

1. Reduction of resting spores (Gossen / McDonald)

New analytic techniques for estimation of the concentration of resting spores provide substantially improved estimates of viable spores in soil compared with direct spore counts. In addition, the project will continue to search for and evaluate novel biocontrol agents, plant defense elicitors, fungicides and soil amendments for efficacy against clubroot.

The initial assessments of interaction of inoculum density x treatment will be conducted in replicated, inoculated growth room studies. Treatments that show promise in the controlled environment studies will be tested in the field at a dedicated research site. If they continue to show promise, they will be included in larger-scale trials in commercial fields.

2. More durable disease reaction using QR (Gossen / McDonald)

Genotypes of B. napus, B. rapa and B. oleracea have been identified that have an intermediate reaction when screened for CR. Such lines are generally not selected for use in breeding programs. The reaction of these lines to a range of pathotypes will be assessed under controlled conditions. If QR is detected, microscopy and pathogen quantification using qPCR will be used to investigate the pattern of pathogen development within the root. This approach will be used to determine if each of the QR lines identified reacted similarly. Also, the durability of reaction for each line over time will be assessed.

3. Understanding the mechanisms of QR (Peng, Gossen, Hwang, Pageau, McDonald, Gossen)

The mode of action for QR will be studied on representative candidates with unique genetic backgrounds. A new group of plant protection products, known as plant defense elicitors, will be assessed as a seed treatment with the QR to improve the efficacy and stability of resistance. It is possible that these products can enhance the performance of QR.

Potential candidate lines with QR will be screened initially against selected pathotypes, followed by testing against a wide range of pathotypes found on the prairies for the versatility of resistance. Top candidates will be assessed for improved resistance durability in replicated field trials.

4. Physiology of clubroot development (Strelkov / Hwang)

Plant hormones affect clubroot symptom development of infected plants. This study will determine if previous hormone studies in this pathosystem (conducted on Arabidopsis) accurately represent the physiological system in canola. The data will be used to identify hormones playing a significant role in canola, and the stage(s) they are important. Pathotypes of P. brassicae will be selected to compare the effect of virulent and avirulent inoculum, which will allow the researchers to identify the changes that are associated with clubbing and susceptibility/resistance.

References

• Al-Daoud F, Gossen B D, McDonald M R. 2017. Plant Dis. 101: 442–447. http://dx.doi.org/10.1094/PDIS-05016-0715-RE.
• Peng G, Lahlali R, Hwang SF, Pageau D, Hynes RK, McDonald MR, Gossen BD, Strelkov SE. 2014b. Can. J. Plant Pathol. 36 (Suppl. 1): 99–112.
• Peng G, Pageau D, Strelkov SE, Gossen BD, Hwang SF, Lahlali R. 2015. Eur. J. Agron. 70: 78–84.