V. P. Manolii

Virulence and spread of Plasmodiophora brassicae [clubroot] in Alberta, Canada

Clubroot, caused by Plasmodiophora brassicae, has emerged as an important disease of canola (Brassica napus) in Alberta, Canada. Annual surveys from 2005 to 2008 revealed 405 clubroot-infested fields, mainly in central Alberta, but also in the southern part of the province. Within-field surveys suggested that the main mechanism of clubroot spread is through the movement of infested soil on farm machinery, as the frequency of infestation was highest at the field entrances and decreased sharply at distances of 150 and 300 m from the entrance. Characterization of P. brassicae populations from Alberta on the differentials of the European Clubroot Differential (ECD) series, P.H. Williams, and A. Somé et al. revealed that pathotype ECD –/15/12, 3, or P2, respectively, is predominant in the central part of the province, a finding consistent with earlier reports. However, novel virulence phenotypes were also observed. Two populations from southern Alberta were classified as pathotype 5 or P3 on the hosts of P.H. Williams and A. Somé et al., respectively, or as pathotypes ECD –/6/8 and –/4/0 on the ECD set, suggesting independent introductions of P. brassicae to this region or greater diversity than previously thought. Pathogen populations collected from infected canola plants from Quebec and Manitoba also yielded unique virulence patterns but were still virulent on most B. napus hosts, and an Ontario population induced significant disease only on a few of the vegetable differentials. Virulence levels overall were highest for the P. brassicae populations from central Alberta, which may help to explain the rapid increase in clubroot observed in this province.

Emergence of new virulence phenotypes of Plasmodiophora brassicae on canola (Brassica napus) in Alberta, Canada

Clubroot, caused by Plasmodiophora brassicae, is an important soilborne disease of canola (Brassica napus) in Alberta, Canada. Genetic resistance is the most effective clubroot management tool, and resistant cultivars are grown extensively in affected regions. In 2013, relatively severe symptoms of clubroot were observed in some fields of resistant canola. In greenhouse tests, four populations of P. brassicae from two of these fields caused significantly increased levels of clubroot on the cultivars from which they had been first recovered; these included three populations (L-G1, L-G2 and L-G3) recovered from the cultivar ‘L135C’, and one population (D-G3) recovered from ‘D3152’. Further testing showed that L-G1, L-G2 and L-G3 were highly virulent on a suite of six resistant canola cultivars (‘45H29’, ‘D3152’, ‘74–47CR’, ‘1960’, ‘L135C’ and ‘6056CR’) representing a cross-section of products available in Canada, while a seventh cultivar (‘9558c’) was moderately resistant to moderately susceptible. Bioassays of field soil with a dozen clubroot-resistant host genotypes confirmed that in most cases, resistance was no longer effective. Host responses to the population D-G3 were more variable, with most cultivars developing intermediate levels of disease. All four P. brassicae populations were classified as pathotypes 5, P3 and 16/6/8 on the differentials of Williams, Somé et al., and the European Clubroot Differential set. The pathotype classifications, however, do not reflect the increased virulence of these populations on clubroot-resistant canola. The identification of new virulence phenotypes of P. brassicae capable of overcoming genetic resistance underscores the need for increased stewardship of resistance sources.

Interaction of pH and temperature affect infection and symptom development of Plasmodiophora brassicae in canola

Abstract The occurrence of clubroot on canola (Brassica napus), caused by the soilborne protozoan Plasmodiophora brassicae, in western Canada is currently centred in a region of slightly acidic soils near Edmonton, AB. Warm temperatures and slightly acidic conditions are known to favour the development of clubroot. The current study was conducted as part of a larger project to assess the risk that P. brassicae will spread to other areas in the prairie region, e.g., where soil pH is neutral or alkaline. The interaction of temperature (10, 15, 20, 25, 30 °C) and pH (5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0) on root hair infection (RHI) and clubroot symptom development in canola roots was studied under optimum moisture conditions and moderate (1–5 × 106 resting spores per seedling) inoculum levels under controlled conditions. The highest levels of RHI (max = 61%) and clubroot incidence and severity (max. = 100%) developed at pH 5.0–6.5 × 20–25 °C. Clubroot levels were intermediate at pH 7.0–8.0 × 20–25 °C, and very low at 10 and 15 °C, regardless of pH. Surveys of clubroot-infested canola fields in Alberta demonstrated that there was only a weak correlation between soil pH and clubroot level (r = −0.30 for incidence and r = −0.33 for severity, based on 267 fields). The absence of a strong correlation supports the results of the controlled environment study. We conclude that moderate levels of clubroot can develop at pH levels well above its pH optimum when temperature and moisture are suitable. This may be the underlying cause of failures in clubroot control that occasionally occur in infested vegetable fields treated with lime. This study indicates that there is a substantial risk that moderate levels of clubroot will develop in regions where soils are neutral or slightly alkaline if other conditions (temperature, moisture, inoculum load) are favourable for disease development.

Histological analysis of spindle and spheroid root galls caused by Plasmodiophora brassicae

Clubroot of crucifers, caused by the obligate parasite Plasmodiophora brassicae, is characterized by the formation of conspicuous root galls. These galls usually have a club- or spindle-shaped morphology, and interfere with water and nutrient uptake by infected plants. Smaller galls, historically regarded as resistance structures and distinct from the typical spindle-shaped galls, have also been identified and termed ‘spheroid galls’ because of their spherical or nearly spherical form. An assessment of various Brassica species and varieties revealed that spheroid galling could be observed in all genotypes investigated, but occurred regularly only in a few particular host/P. brassicae combinations. While spindle gall formation was coincident with the expansion of the stele and infection of secondary tissues by P. brassicae, spheroid galls typically had a region of proliferating tissue that corresponded to the secondary cortex and periderm of the healthy plants, with the outer proliferating tissue less infected than the inner portions. The underlying host tissue showed limited secondary tissue development, was largely uninfected, and, where infection occurred, a continuous stele was maintained. An active host defensive reaction, in the form of cell lignification or the hypersensitive response, was not observed, while pathogen resting spores were visible in one longitudinal section of a spheroid gall. These findings suggest that while the proliferation of P. brassicae is restricted in spheroid galls, these structures are not indicative of complete resistance to clubroot.

Spread of Plasmodiophora brassicae on canola in Canada, 2003–2014: Old pathogen, new home

Abstract Clubroot caused by Plasmodiophora brassicae has been reported at sites across North America on brassica vegetables for more than 50 years. However, it had not been reported on canola (Brassica napus) on the Canadian prairies until the initial discovery of a cluster of 12 infested fields near Edmonton AB in 2003. The purpose of this review is to consolidate and summarize the data on the spread of P. brassicae on canola in Canada since 2003, to compare this pattern of distribution with observations from an infested site in Ontario, and to draw inferences about the relative importance of short- and long-distance transmission of the pathogen on clubroot distribution in the prairie region. Over the last decade, P. brassicae has spread across central Alberta, with the leading edge of the epidemic moving at about 20 km per year, resulting in more than 1850 fields confirmed infested. DNA of the pathogen has also been detected from soil collected at sites across Saskatchewan and Manitoba, and very slight clubroot symptoms have been observed at isolated sites across the prairies. Transport of resting spores in soil carried on farm equipment has been shown to be an important mechanism of short-distance dissemination in this region. Dispersal of resting spores with wind-borne soil may also have an important role in both short- and long-distance dissemination. Dispersal on seed does not appear to be an important factor in clubroot spread. In contrast to the rapid spread observed in Alberta, P. brassicae is spreading very slowly, if at all, at the site in Ontario. This likely reflects the relatively small size and strength of the inoculum source and the absence of susceptible hosts nearby at the site in Ontario, relative to the thousands of hectares of heavily infested fields that provide a large, strong inoculum source in central Alberta.

Virulence and pathotype classification of Plasmodiophora brassicae populations collected from clubroot resistant canola (Brassica napus) in Canada

Abstract Clubroot, caused by Plasmodiophora brassicae Wor., is an important soilborne disease of canola (Brassica napus L.) in Canada that is managed mainly by planting clubroot-resistant (CR) cultivars. Populations of P. brassicae representing 106 fields in Alberta were obtained from galled roots of CR canola plants collected in 2014–2016 and characterized for virulence on seven CR canola cultivars. Sixty-one of these populations could overcome resistance in at least one CR cultivar and were evaluated further by inoculation on 13 Brassica hosts termed the Canadian Clubroot Differential (CCD) Set. The CCD Set included the differentials of Williams and Somé et al., selected hosts of the European Clubroot Differential Set, and the B. napus cultivars ‘Brutor’, ‘Mendel’, ‘Westar’ and ‘45H29’. Each unique virulence pattern on the CCD Set represented a distinct pathotype and was identified with a letter. Five reference isolates, obtained prior to the introduction of CR canola, also were assessed. A total of 17 pathotypes were detected using the CCD Set, compared with five pathotypes using the system of Williams and two with the system of Somé et al., suggesting that the CCD Set has a greater differentiating capacity. Pathotype A, a variant of pathotype 3 (as per Williams) which is able to overcome the resistance in CR B. napus, was predominant. The original pathotype 3, which is avirulent on CR canola, was classified as CCD pathotype H. An integrated strategy, combining other tools in addition to resistance, will be needed for the sustainable management of clubroot.

A molecular marker for the specific detection of new pathotype 5-like strains of Plasmodiophora brassicae in canola

Clubroot of crucifers, caused by Plasmodiophora brassicae, is managed in canola (Brassica napus) by the deployment of resistant cultivars. Recently, however, new strains of P. brassicae have been detected in Alberta, Canada, that can overcome this resistance. Some of these strains are classified as pathotype 5 on the differential system of Williams, but are distinguished by their ability to overcome host resistance. In order to expedite the identification of these new pathotype 5-like strains, three primer sets were developed based on the 18S-ITS region of the pathogen. With primers P5XF3 and P5XR3, a 127 bp product was amplified from all new pathotype 5-like strains following optimized PCR analysis. A TaqMan probe-based quantitative assay was also developed. These protocols could be used to detect as little as 0.5 pg P. brassicae DNA, and as few as 10 mL 1 pathogen resting spores; infection of host tissues could be detected as soon as 4 days after inoculation. The PCR and qPCR assays described in this study represent useful tools for the rapid and reliable diagnosis and quantification of new pathotype 5-like strains of P. brassicae.