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 to manage clubroot in canola fields when no single source of resistance effectively manages all of the pathotypes of clubroot in the region and to slow the spread of these pathotypes into new areas.

To do this, the research team will:

• Examine factors that affect resting spore survival, germination and infection, in both controlled environments and field trials.
• Identify and assess sources of quantitative (non-pathotype specific or horizontal) resistance, which has not previously been studied in detail, to determine if quantitative resistance might be used to increase the durability of genes that confer strong genetic resistance to clubroot.
• Evaluate strategies for deployment of clubroot resistance genes, with the aim of identifying approaches that will maximize the durability of resistance.
• Examine plant hormones changes in canola caused by clubroot to determine if this host-pathogen interaction is similar to that studied previously in Arabidopsis.

This study complements the efforts of identifying quantitative resistance in Canadian Canola Clubroot Cluster Pillar 1: Integrated disease management and Canadian Canola Clubroot Cluster Pillar 2: Developing novel resistance resources and strategies to address the new threat of clubroot canola production on the Prairies.

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)

Background

Rapid breakdown of clubroot resistance (CR) over the three years leading up to the start of this project represent 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.

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.

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)

Short-duration lab studies indicated that grasses, cereals and even field pea plants reduced the population of resting spores of P. brassicae in soil. (This reduction was in addition to the natural reductions that occur in bare soil, likely via root exudates stimulating spore germination.) The incremental change each year is likely to be fairly small, but may continue over many years, speeding the reduction in spore concentration. High spore concentration is believed to be an important factor in the breakdown of genetic resistance to clubroot, so even slow change could be important in reducing selection pressure for new pathotypes.

A paper entitled Grasses and field crops reduce the concentration of resting spores of Plasmodiophora brassicae in soil under controlled conditions, which summarized the effect of cereal and perennial grass crops on resting spore concentration under controlled conditions, was published. Also see the ‘Reduction of resting spores of Plasmodiophora brassicae with wheat and lime‘ poster for more information.

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

A repetition trial has been completed (under controlled conditions) on resistance durability of canola varieties carrying two stacked CR genes and a single CR gene against a field population of pathotype 5X. The repetition ran five generational cycles of exposure, and the results showed that the partial resistance based on two stacked CR genes held up better than a single CR gene alone. As a result, the double-gene variety appeared to return less amounts of inoculum, relative to the single-gene variety, into the soil. The results indicate a value with the gene stacking for improved CR performance and durability.

In the final year of the project, studies were completed on the effect of grasses on spore numbers in field trials, on the efficacy of solarization and / or fumigants for reduction of clubroot severity, on the interaction of boron x calcium on clubroot severity, and on techniques to improve whole-genome sequencing of P. brassicae. Read more in ‘Seed treatment of canola (Brassica napus) with the endomycorrhizal fungus Piriformospora indica does not reduce clubroot‘.

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

A transcriptome study investigated resistance mechanisms between the double and single-gene varieties against the pathotype 5X. The results showed the similar trend as that found in the initial analysis; similar biological processes were activated in both varieties relative to a susceptible variety carrying a single CR gene, but more plant defense-related genes were activated in the double-gene variety and at higher expression levels. This indicates the additional benefit for stacking the two CR genes in canola varieties for stronger and a broader range of effectivity.

4. Physiology of clubroot development (Strelkov / Hwang)

Plant hormones affect clubroot symptom development of infected plants. This study investigated if previous hormone studies in this pathosystem (conducted on Arabidopsis) accurately represent the physiological system in canola.

Auxin regulation of root gall formation – Although the plant hormone auxin is believed to play an important role in clubroot disease development, no clear change in auxin levels could be detected between inoculated plants and non-inoculated controls of either cultivar. Arabidopsis auxin receptor mutants also showed no clear pattern of variation in disease development.

Ethylene regulation of clubroot disease development – Ethylene is an important regulator of plant responses to microbial pathogens. Previous studies indicated that ethylene may also play a role in club development in infected plants. In this study, there was a clear increase of ethylene precursor (ACC) level in infected plants compared to non-inoculated controls at 21 DAI, indicating a role of ethylene in symptom development.

Recommendations

The field studies demonstrated that the population of resting spores of P. brassicae in soil dropped quickly in the first two years after a susceptible crop, then declined slowly for many years (which was different from previous research that reported a gradual decline over time instead). This means that crop rotation could have an impact on clubroot, and that a 2 to 3 year interval between canola crops was almost as effective as much longer intervals for clubroot management.

The short-duration lab studies indicated that grasses, cereals and even field pea plants reduced the population of resting spores of P. brassicae in soil. This reduction occurred on top of the natural reductions that occur in bare soil, likely via root exudates stimulating spore germination.

Further research into all of the objectives outlined in this project to gain a strong understanding of the multi gene resistance and hormonal shifts in canola following disease affected plants would be beneficial for the industry.

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.