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

Authors


E-mail: sheau-fang.hwang@gov.ab.ca

Abstract

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.

Introduction

The obligate parasite Plasmodiophora brassicae causes clubroot, an important disease of crucifers worldwide. Clubroot was first identified on canola (Brassica napus) in Alberta in 2003 (Tewari et al., 2005), and has since spread to many fields throughout the province (Cao et al., 2009; Strelkov et al., 2012). The disease now presents a major threat to canola production in Alberta, and has recently been identified in the neighbouring province of Saskatchewan, suggesting that clubroot could eventually become widespread in the Canadian prairies. Clubroot infection can severely reduce the yield and quality of the canola crop (Pageau et al., 2006), and has caused yield losses of up to 100% in Alberta (Howard et al., 2010).

Once P. brassicae becomes established in a field, it builds up rapidly in the presence of susceptible crops via the formation of resting spores, which can persist in the soil for more than 15 years in the absence of a susceptible host (Wallenhammar, 1996). Therefore, crop rotations based on standard agronomic recommendations for Canada (one year of canola in four) will not significantly reduce resting spore populations (Strelkov et al., 2006), and even substantially longer rotations may not be effective because several common and endemic weed species such as horseradish (Armoracia rusticana), white mustard (B. hirta), wild mustard (B. kaber), camelina/false flax (Camelina sativa), shepherd’s purse (Capsella bursa-pastoris) and stinkweed (Thlaspi arvense) are also susceptible hosts (Howard et al., 2010).

On vegetable brassicas, the main clubroot management option is amendment of the soil with lime to increase its pH. However, this is not a practical or cost- effective strategy for use in the extensive production of field crops (Myers & Campbell, 1985; Webster & Dixon, 1991; Murakami et al., 2002). Reduction in clubroot severity is also possible with the application of fungicides and soil amendment products, but the rates used for horticultural crops are not cost-effective for canola (Hwang et al., 2011d, 2012). As a result, a range of alternative management strategies are being assessed for their usefulness in canola production, including timing of seeding (Gossen et al., 2009; Hwang et al., 2012), biological control (Peng et al., 2011) and the use of bait crops (Kroll et al., 1983; Ikegami, 1985; Murakami et al., 2001; Ahmed et al., 2011). While some of these management approaches hold some promise, the most effective method to control clubroot on canola is through the deployment of genetically resistant cultivars (Strelkov et al., 2011). A number of clubroot-resistant canola cultivars have recently become available on the Canadian market.

Ideally, the cropping of resistant cultivars should be accompanied by a reduction in the viable P. brassicae resting spore populations in the soil. However, clubroot resistant crops are not necessarily free of susceptible plants. Susceptible volunteers will continue to be present in infested fields for many years. As noted above, susceptible weed species may also be present, and there may be a small percentage of susceptible canola off-types associated with hybrid production. Data are needed on the effects of seeding resistant canola cultivars in infested fields. The cropping of resistant cultivars may stimulate resting spore germination, thereby reducing the viable spore population in the soil, while no (or few) viable resting spores will be produced on the resistant plants to contribute to soil inoculum loads. However, these benefits from the cropping of a resistant cultivar may be offset by the presence of susceptible hosts within the crop, as noted above. Furthermore, the deployment of resistant cultivars may create selection pressure on the pathogen to co-evolve and overcome resistance (Hwang et al., 2011b; LeBoldus et al., 2012). It is not known whether non-host crops or fallow reduce clubroot populations or create conditions that promote resting spore longevity.

The main objective of this study was to evaluate the effect of cropping susceptible or resistant canola on P. brassicae resting spore populations in the soil, and on subsequent clubroot disease development on a susceptible canola cultivar.

Materials and methods

Resting spore contribution from susceptible canola under mini-plot conditions

Mini-plots consisting of plastic tubs (50 × 35 × 22 cm) were filled with 30 L of a black chernozemic soil that was naturally infested with P. brassicae (approximately 1 × 108 resting spores g−1), collected from a commercial field, and diluted with soilless mix (Sunshine Mix 4; pH 6·5; SUN GRO® Horticulture Canada Ltd) at ratios of 1:100, 1:50, 1:25, 1:10, 1:5, 1:3 and 1:2, resulting in an estimated resting spore gradient of 9·0 × 105, 1·9 × 106, 9·0 × 106, 1·6 × 107, 2·5 × 107, 3·3 × 107 and 5 × 107 spores mL−1, respectively. Each tub was sown with a total of 40 seeds of the susceptible canola cultivar 45H26 (Pioneer Hi-Bred), in eight rows across each tub. The tubs were placed outdoors at a field site near Edmonton, AB in a randomized complete block (RCB) design with six replicates (tubs) on 28 June 2011. Irrigation was provided as required with an overhead sprinkler, in order to provide sufficient moisture for plant growth and clubroot development. Six weeks after planting, clubbed roots (galls) were collected from 10 of the 40 plants and weighed. In order to determine the amount of resting spores contributed to the soil, the remaining plants were harvested on 21 September (approximately 12 weeks after planting), when the crop was fully mature and the galls were beginning to disintegrate. A 1 g aliquot of the gall from each replicate was placed in 20 mL of water and macerated in a laboratory blender. The resulting suspension was then sieved through eight layers of cheesecloth, and the resting spores were quantified with a haemocytometer under a microscope. Three slides were observed for each replicate. The data were converted to resting spores contributed per millilitre of soil mix prior to conducting any statistical analysis. The experiment was conducted twice. Analysis of variance of the data was performed to determine the effect of inoculum concentration on gall mass and the amount of resting spores contributed to the soil after one crop of susceptible canola. Means separation was conducted using Duncan’s multiple range test (P ≤ 0·05). In addition, regression analysis was performed to examine the relationships among the initial resting spore concentration in the soil, gall mass and the amount of resting spores contributed by one crop of susceptible canola.

Resting spore populations after cropping resistant and susceptible canola under field conditions

Clubroot-resistant (45H29) and susceptible (45H26) canola cultivars were grown in a field near Edmonton, AB that was heavily infested with P. brassicae resting spores (1 × 108 spores g−1). A single-factor experiment was set up in a RCB design with four replications. On 16 August, when the crop had matured and just prior to when the clubbed roots (galls) began to decay, 20 plants in each plot were uprooted, washed gently under tap water, and the fresh weight of the galls was recorded. A small aliquot (1 g) of galls from each plot was macerated in a blender for 3–5 min and sieved through eight layers of cheesecloth. The resting spore concentration of the resultant suspension was estimated with a haemocytometer under a microscope, in order to determine the number of inoculum units contributed to the soil. The experiment was repeated in a separate field, also near Edmonton, AB.

There was no main effect of repetition or treatment × repetition interaction in an analysis of variance of the data when combined across repetitions (main effect: P = 0·76 for gall mass; P = 0·94 for resting spores, interaction: P = 0·68 for gall mass; P = 0·66 for resting spores). Therefore, the repetitions were pooled for subsequent analysis, where differences between the resistant and susceptible cultivars were assessed using a paired t-test.

Effects of cropping resistant and susceptible canola cultivars on clubroot severity in subsequent crops

The canola cultivars 45H29 (resistant) and 45H26 (susceptible) were grown in soilless mix (Sunshine Mix 4; pH 6·5; SUN GRO® Horticulture Canada Ltd) in plastic containers (35 × 23 × 13·5 cm), each containing 10 L of soilless mix, under greenhouse conditions (20 ± 2°C). The soilless mix was inoculated with P. brassicae resting spores collected from root galls and adjusted to a concentration of 1 × 106 spores mL−1 of soilless mix. Four rows of 25 seeds each were seeded in each container. The containers were placed on water-filled trays (water pH 6·5) for the first 2 weeks after seeding, in order to ensure high soil moisture, and were then watered as needed for four additional weeks, for a total growing period of 6 weeks. The plants were fertilized with a 0·1% solution of NPK (20-20-20) once a week (commencing at 2 weeks after seeding) until the end of each cycle of plantings. A fallow treatment (a container filled with clubroot-inoculated soilless mix but not seeded with any crop) was maintained alongside the canola-seeded treatments.

At the end of the 6-week growth period, all canola plants were uprooted, the roots of the plants were macerated in water, filtered through eight layers of cheesecloth and re-incorporated back into the soil from which the plants came, after adjusting the water volume to 250 mL for each container. The water was poured evenly over the soil surface and then the spore suspension was thoroughly mixed throughout the soil in the container using a garden trowel. The soil was allowed to dry for 7 days. A new crop of the same cultivar was replanted into this soil. After three successive 6-week cycles (resistant-resistant-resistant (RRR); or susceptible-susceptible-susceptible (SSS)), the same susceptible canola cultivar was grown in the soil from both cropping sequences, as well as in the soil used for fallow treatments (FFF). The inoculum potential in the re-inoculated soilless mix was estimated by assessing root weight, plant height, clubroot incidence and severity, and by counting resting spore populations in the soilless mix after each cycle. Disease severity was assessed on a 0–3 scale, where 0 = no galls, 1 = a few small galls, 2 = moderate galling and 3 = severe galling (Kuginuki et al., 1999). Severity ratings for each experimental unit were converted to an index of disease (ID) using the formula of Strelkov et al. (2006).

The experiments were conducted under greenhouse conditions with a 16 h photoperiod (20 ± 2°C day/16 ± 2°C night). The experiment was set up in a RCB design with six replications. The experiment was conducted twice.

The inoculum potential was estimated by counting resting spores in the soilless mix after each cropping cycle with a haemocytometer under a microscope. The resting spore extraction procedure was adopted from the method of Castlebury et al. (1994) as modified by Hwang et al. (2011a). Briefly, soilless mix samples were collected after completion of each cropping cycle. Five cores of soilless mix samples from each replicate tray of each of the treatments were collected with a cork borer (13 mm diameter) and air-dried at room temperature (20 ± 2°C), thoroughly mixed and stored in a coldroom (5°C) until use. For the extraction of resting spores, 0·5 g of soilless mix sample from each replication of a treatment was added to 20 mL of water and mixed for 1 min in a laboratory blender (Waring Commercial). The slurry was filtered through eight layers of cheesecloth. A 12 mL aliquot of the suspension was centrifuged at 3900 g for 15 min and the supernatant was discarded. The spores were suspended in 6 mL of 50% (w/v) sucrose solution, agitated in a vortex mixer for 2 min, and then centrifuged for 5 min at 1700 g. The supernatant was transferred into a 50 mL tube, and water was added to 45 mL, mixed with a vortex mixer and centrifuged at 3900 g for 15 min. The supernatant was discarded and the pellet was mixed with 5 mL of water using a vortex mixer, centrifuged at 3900 g for 15 min. The supernatant was again discarded and the pellet was suspended in 2 mL of water and refrigerated at 4°C until used for counting.

Data analysis

Prior to analysis of variance, all of the data sets were tested for homogeneity of variance using a normal probability plot. Analysis of variance of the data was performed separately for each cropping cycle using the GLM procedure of SAS (SAS Inc.). As there was no significant effect of repetition, the data from the two repeats of each cycle of cropping were combined for subsequent analyses. The treatment effects were compared using a protected LSD test at  0·05. In addition, repeated measures analysis of variance was performed to determine whether the changes in the root mass, plant height, disease severity and resting spore populations resulting from the cropping cycles were significantly different.

Results

Resting spore contribution from susceptible canola under mini-plot conditions

Gall tumour weight increased with increasing inoculum concentration (Fig. 1a) across the high levels of disease pressure examined in this trial. The resting spore contribution also increased as inoculum concentration increased. There were strong log linear relationships between initial resting spore concentration and gall mass, and initial resting spore concentration and the amount of resting spores contributed to the soil, which can be best explained by the linear equations: y (gall mass) = 1·5112x−7·404, R2 = 0·95 and y (log10 of the contributed numbers of resting spores) = 0·2386+ 6·3307, R2 = 0·97, respectively, where x is the log10 of the initial number of resting spores. The numbers of resting spores contributed to the soil by a single susceptible crop were in the range of 5·6–13·2 × 107 spore mL−1 of soil mix. However, the rate of increase in resting spore populations slowed as the initial resting spore numbers increased (Fig. 1b). The declining rate of spore proliferation was described as y = −0·7614+ 6·3307, where x is the log10 of the initial resting spore number and y is the log10 of the contributed spore numbers divided by the original spore numbers.

Figure 1.

 Gall mass and contribution of resting spores after one crop of a susceptible canola cultivar in mini-plots (plastic tubs filled with Plasmodiophora brassicae infested soil with different inoculum levels) maintained outdoors at CDC north, Edmonton, Alberta in 2011 (a). The number of resting spores contributed was calculated based on the mean gall mass produced at each initial inoculum level. Relationship between the initial number of spores in the substrate and number of spores contributed by fresh clubroot galls (b).

Resting spore populations after cropping resistant and susceptible canola under field conditions

The gall mass produced by the susceptible canola cultivar under field conditions was 14-fold higher than that of the resistant cultivar (0·19 vs 2·9 g mean gall mass per plant; Fig. 2). The resting spore populations contributed to the soil by the susceptible canola per gram of gall (1 × 1010 spores) were also greater than the resistant canola (6 × 109 spores). In the case of the resistant canola cultivar, 14% of the plants were infected with clubroot, while 100% of the susceptible canola plants were infected (data not shown).

Figure 2.

 Contribution of resting spores to the soil after one crop of a clubroot resistant (45H29, Pioneer Hi-Bred) or susceptible (45H26, Pioneer Hi-Bred) canola cultivar under field conditions. Data are the means of two trials and four replications. The means were significantly different at P > |t| ≤0·0001 by t-test. The plants were harvested at crop maturity when the galls started to decay.

Effects of cropping resistant and susceptible canola cultivars on clubroot severity in subsequent crops

In the assessment at the end of the cropping sequences, plants were smaller, root mass was larger and index of disease was greater in the SSSS cropping sequence than following the RRRS or FFFS sequences (Fig. 3a–c). Plants were taller in the FFFS sequence compared to the RRRS sequence and were taller in the RRRS sequence compared to the SSSS sequence. The large root mass in the susceptible cultivar resulted from gall formation.

Figure 3.

 Effect of sequential cropping of resistant (45H29, Pioneer Hi-Bred) and susceptible (45H26, Pioneer Hi-Bred) canola cultivars on fresh root weight (a), plant height (b) and index of disease (c) caused by Plasmodiophora brassicae on a final susceptible canola crop. Bars in each cycle with the same letter do not differ at  0·05.

In each cropping cycle, clubroot severity was higher in the susceptible cultivar than in the resistant cultivar (Fig. 3c). Severity in the resistant cultivar remained relatively constant over the three cycles, whereas severity increased when the susceptible canola was grown sequentially (Fig. 3c). At the end of the final assessment phase (fourth cropping cycle) with the susceptible cultivar, clubroot severity was 10-fold greater in the SSSS cropping sequence relative to the RRRS sequence, but there was no difference in severity between the RRRS and FFFS sequences (Fig. 3c).

Repeated measures analysis of variance also showed a significant cultivar effect and cropping cycles effect (time) on root mass, plant height and disease severity ( 0·05), indicating that root mass, plant weight and disease severity were different for resistant and susceptible cultivars and changes over the cropping cycles (Fig. 4). Significant cultivar × cropping cycle interactions on root mass and disease severity revealed that root mass and disease severity changes over time, and that the pattern of these changes was different for the resistant and susceptible cultivars (Fig. 4).

Figure 4.

 Change in root mass (a), clubroot severity (b) and plant height (c) over cropping cycles with resistant (R; 45H29, Pioneer Hi-Bred) and susceptible (S; 45H26, Pioneer Hi-Bred) canola cultivars.

Similarly, the soil resting spore concentration was higher following the SSSS sequence than the RRRS or FFFS sequences. The resting spore concentration increased after each cycle of cropping of the susceptible canola cultivar (S, SS, SSS and SSSS), but decreased in the FFFS and RRRS sequences (Figs 5 and 6). Repeated measures analysis indicated that these changes in the resting spore populations over the cropping cycles were significant ( 0·05).

Figure 5.

 Effect of sequential cropping of resistant or susceptible canola cultivars or a fallow condition on resting spore concentration in the soil. A different pattern of bars was used to distinguish each cycle denoted on the x-axis. Bars in each cycle topped with the same letter do not differ at  0·05 by Fisher’s protected least significance difference test.

Figure 6.

 Changes in resting spore concentration over cropping cycles with resistant (R; 45H29, Pioneer Hi-Bred) or susceptible (S; 45H26, Pioneer Hi-Bred) canola cultivars, or a fallow (F) treatment. The fallow condition was maintained over the same period in which three cycles of resistant or susceptible canola was cropped.

Discussion

Several strategies have been examined for the management of clubroot on canola, including manipulation of the timing of seeding (Gossen et al., 2009; Hwang et al., 2012), biological control (Peng et al., 2011) and the use of bait crops (Kroll et al., 1983; Ikegami, 1985; Murakami et al., 2001; Ahmed et al., 2011), but so far genetic resistance is the only control option that has proven to be consistently effective.

The impact of a single crop of clubroot-susceptible canola on the numbers of resting spores added to the soil was assessed in mini-plot and field plot studies. Both types of studies revealed that one crop of a susceptible canola cultivar resulted in the addition of huge numbers of resting spores to the soil. A single generation of infected plants increased the spore load in soil infested with 1 × 106 resting spores per mL by over 60-fold, while the increase in spore load declined to less than threefold as the initial spore load approached 5 × 107 spores mL−1. Therefore, the inoculum potential of the soil clearly increases at a greater rate when the soil is infested at lower levels, relative to the rate of increase when the soil is already infested at high levels.

When a clubroot resistant cultivar was included in the field study, the susceptible cultivar produced a much greater gall mass than the resistant cultivar, as expected. Moreover, galls in the resistant cultivar produced fewer resting spores per gram than in the susceptible cultivar, which further reduced the relative contribution of inoculum by the resistant cultivar. It is concluded that cropping of resistant cultivars will greatly reduce the resting spore contribution to the soil, relative to cropping of susceptible cultivars.

The study on the effect of repeated cropping of resistant and susceptible cultivars also demonstrated that not all of the plants in the resistant cultivar 45H29 were resistant. However, repeated cropping of the resistant cultivar (RRR) or continuous fallow (FFF) reduced resting spore populations and also reduced clubroot severity in the subsequent susceptible canola crop, compared to repeated cropping of the susceptible canola (SSS). There were no differences between FFF and RRR. This suggests that while the resistant cultivar does not increase soil inoculum loads, it also does not act as a bait crop (increasing the germination of resting spores), because inoculum levels after three successive resistant crops were not significantly different from the fallow treatment.

Taking the regression equation for fallow ground from Figure 6, a spore half-life of 7·14 cycles can be calculated. The length of each cycle was 49 days, and if it is assumed that most spore metabolic activity occurs on days on which the temperature exceeds 20°C for at least a short period and that there are an average of 79·6 days in each growing season in which these conditions exist near Edmonton (Environment Canada, 2012), the half-life of the spores in the Edmonton area should be 4·4 growing seasons, which is similar to the 3·6-year half-life calculated by Wallenhammar (1996) under conditions in Sweden. The small difference between the values could reflect the cooler conditions and shorter growing season in Edmonton. However, the equations for susceptible and resistant cultivars cannot be extrapolated in a similar manner, because in both cases new spores are being added to the soil after each cycle.

In the field trial, 14% of the plants of the resistant canola cv. 45H29 developed some root galling. These galls were larger than those formed in the greenhouse study, where most of the galls from the resistant cultivar were rated as 1 on a 0–3 scale. The genetic background of 45H29 (and the other clubroot-resistant canola cultivars that are commercially available) is proprietary information, and so little is known about the nature of the resistance sources involved. The susceptible portion of the plants may have been susceptible off-types, or resulted from segregation or incomplete gene penetrance. Alternatively, the low levels of disease may have been a consequence of an as yet unidentified factor. Nonetheless, gall mass and resting spore production in the resistant cultivar was very low compared to the susceptible cultivar. Assuming a plant population of 100 plants m−2 and an even distribution of spores through the cultivated layer of soil (c. 10 cm deep), the resistant cultivar would have added 1·1 × 1011 spores m−2 to an existing field population of 1·0 × 1013 spores m−2, while the susceptible cultivar would have added 2·9 × 1012 spores to the pre-existing population. To compare this to the greenhouse experiment, an initial concentration of 1·0 × 106 spores mL−1 of soil would correspond to 1·0 × 1011 spores m−2. The number of added spores would have been 5·6 × 1012 spores m−2; at an initial spore concentration of 5·0 × 107 spores (5·0 × 1012 spores m−2), the susceptible cultivar would have added 1·3 × 1013 spores m−2. The higher numbers are attributable to the much higher plant densities in the greenhouse experiment (100 plants in 10 L of greenhouse soil would equate to 1000 plants m−2 of field soil).

Several clubroot-resistant canola cultivars have been registered recently for production in Canada. There are at least five pathotypes in the populations of P. brassicae in Alberta (Strelkov et al., 2006, 2007; Xue et al., 2008; Cao et al., 2009), and deployment of resistant cultivars may create selection pressure on the pathogen to co-evolve and overcome resistance (Diederichsen et al., 2009; Hwang et al., 2011c; LeBoldus et al., 2012). Previous reports indicate that clubroot resistance in canola is controlled by a combination of major genes and quantitative trait loci (Matsumoto et al., 1998; Suwabe et al., 2003, 2006; Hirai et al., 2004; Piao et al., 2004). Regardless of the nature of this resistance, it will be prudent to ensure that it is carefully managed in order to maintain its durability.

A previous study reported that gall formation was reduced when both susceptible and resistant radish cultivars were grown together, compared to the cropping of a susceptible cultivar alone (Kroll et al., 1983). Root hair infection in cabbage seedlings was also lower after cultivation of crucifers or ryegrass (Lolium perenne) than after cultivation of other non-crucifers (Friberg et al., 2006). These two studies appear to indicate that growing certain host crops (or even some non-hosts like ryegrass) may help to reduce spore populations in the soil, and thereby reduce clubroot development. However, the current study does not support these observations, but instead is consistent with a previous greenhouse study (Hwang et al., 2011b) indicating that growing a susceptible canola cultivar increased levels of resting spores in the soil compared to a resistant cultivar.

Root weight was greater but plants were shorter in the susceptible cultivar compared to the resistant cultivar. It appears that while gall production resulted in an increase in root mass, a diversion of plant resources to produce the root galls and resting spores, together with a disruption in nutrient and water uptake by infected roots, lead to a decrease in plant height (Dixon, 2006). Over the three cropping cycles, plant height and gall mass declined in both the resistant and susceptible cultivars, possibly indicating the presence of other deleterious factors in the soil, or a decline in the availability of trace nutrients over time. Incorporation of freshly macerated galls may have had a direct and deleterious impact on plant growth, possibly through stimulation of secondary pathogens or release of phytotoxic chemicals during decomposition. Clubroot severity increased over time in the susceptible cultivar, but remained low and relatively unchanged in the resistant cultivar. However, in the final assessment, in which all of the treatments were seeded with a susceptible cultivar, disease severity in soil that had previously been sown to a resistant cultivar or had been fallow was lower than in the initial cycle of the susceptible treatment, suggesting a decline in disease potential over time. It should be noted that in field crop situations, the resistant and susceptible cultivars are usually resistant to the same herbicides, so that under frequent rotations more susceptible volunteer canola plants from previous crops could be expected, and these would contribute to spore loading in subsequent crops.

As expected, the susceptible cultivar contributed more resting spores to the soil population than the resistant cultivar. Higher numbers of spores in the resistant treatment compared to the fallow treatment likely reflected the fact that about 14% of plants in the resistant cultivar treatment developed galls, contributing to the soil spore population. This may indicate that repeated cultivation of a resistant cultivar will result in selection for pathogen phenotypes that can overcome this source of resistance, or may indicate a problem with off-types or incomplete resistance in this line.

Although spore populations were greater in the resistant treatment compared to the fallow treatment, repeated cropping of resistant canola and continuous fallow both reduced soil resting spore populations and clubroot severity in a final cycle seeded to a susceptible cultivar, and also improved plant height relative to repeated cropping of a susceptible cultivar. In clubroot-infested fields, at least within a western Canadian context, it is concluded that cropping of clubroot-susceptible canola cultivars will result in an increase in P. brassicae resting spore populations in the soil over time, whilst cropping of clubroot-resistant cultivars will not contribute significantly to inoculum build-up, and would be equivalent to leaving the land fallow. However, under field conditions, it is likely that susceptible off-types, susceptible canola volunteers and cruciferous weeds would continue to increase resting spore populations if not well controlled.

Acknowledgements

The authors thank the Canola Agronomic Research Program (Alberta Canola Producers Commission, SaskCanola, Manitoba Canola Growers Assoc., Canola Council of Canada), the ACIDF (Alberta Crop Industry Development Fund) and the Clubroot Risk Mitigation Initiative (AAFC/CCC) for financial support, and Pioneer Hi-Bred for supplying the canola seed.

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