Clubroot Severity Research Articles

Melatonin and copper oxide nanoparticles synergistically mitigate clubroot disease and enhance growth dynamics in Brassica rapa

Clubroot, a devastating soil borne disease affecting 30%∼50% of Brassicaceae crops worldwide, lacks effective control measures. In the present study, we explored the potential of melatonin (MT) and copper oxide nanoparticle (CuO-NPs) in mitigating clubroot severity in the Brassica rapa ssp. pekinensis. Following 18 h priming with MT, CuO-NPs, or both seeds were grown in controlled environment using synthetic potting mix. Inoculated with Plasmodiophora brassicae spores on 5th day, followed by a soil drench phyto-nano treatment with a week interval. Plants were assessed for various health and growth indices including disease, biometrics, photosynthesis, reactive oxygen species (ROS), antioxidant enzyme activity, hormones and genes expression at onset of secondary clubroot infection using established protocols. Statistical analysis employed ANOVA with Fisher's LSD for significance assessment (P < 0.05). Our results revealed that seed priming with both MT (50 μMol/L) and CuO-NPs (200 mg/L), followed by soil drenching significantly reduced clubroot incidence (38%) and disease index (57%), compared to control treatments. This synergistic effect was associated with enhanced plant growth (shoots: 48% and roots: 59%). Plants treated with both MT and CuO-NPs showed robust antioxidant defenses, significantly increased superoxide dismutase (SOD (25/29%)), catalase (CAT (83/55%)), and ascorbate peroxidase (APX (83/46%)) activity in both shoots/roots, respectively, compared to infected control. Notably, salicylic acid and jasmonic acid levels doubled in treated plants, while stress hormone abscisic acid (ABA) decreased by 80% in roots and 21% in shoots. Gene expression analysis corroborated these findings, showing that the combined treatment activated antioxidant defense genes (SOD, APX and CAT) by 1.9–7.2-fold and upregulated hormone signaling genes JAZ1 (7.8-fold), MYC2 (3.9-fold) and SABP2 (36-fold). Conversely, ABA biosynthesis genes (ABA1 and NCED1) were downregulated up to 7.2-fold, while plant resistance genes NPR1, PRB1 and PDF1.2 were dramatically increased by up to 6.3-fold compared to infected plants. Overall, our combined treatment approach significantly reduces clubroot severity in B. rapa via enhanced antioxidant defenses, improved ROS scavenging, coordinated hormonal regulation and increased pathogen response genes. This study offers promising strategy for developing effective control measures against clubroot in susceptible cruciferous crops.

Biological Control of Rapeseed Clubroot (Plasmodiophora brassicae) Using the Endophytic Fungus Didymella macrostoma P2.

Didymella macrostoma P2 was isolated from rapeseed (Brassica napus), and it is an endophyte of rapeseed and an antagonist of three rapeseed pathogens, Botrytis cinerea, Leptosphaeria biglobosa, and Sclerotinia sclerotiorum. However, whether P2 has a suppressive effect on infection of rapeseed by the clubroot pathogen Plasmodiophora brassicae remains unknown. This study was conducted to detect production of antimicrobials by P2 and to determine the efficacy of the antimicrobials and P2 pycnidiospores in suppression of rapeseed clubroot. The results showed that cultural filtrates (CFs) of P2 in potato dextrose broth and the substances in pycnidiospore mucilages exuded from P2 pycnidia were inhibitory to P. brassicae. In the indoor experiment, seeds of the susceptible rapeseed cultivar Zhongshuang No. 9 treated with P2 CF and the P2 pycnidiospore suspension (P2 SS, 1 × 107 spores/ml) reduced clubroot severity by 31 to 70% on the 30-day-old seedlings compared with the control (seeds treated with water). P2 was reisolated from the roots of the seedlings in the treatment of P2 SS; the average isolation frequency in the healthy roots (26%) was much higher than that (5%) in the diseased roots. In the field experiment, seeds of another susceptible rapeseed cultivar, Huayouza 50 (HYZ50), treated with P2 CF, P2 CE (chloroform extract of P2 CF, 30 μg/ml), and P2 SS reduced clubroot severity by 29 to 48% on 60-day-old seedlings and by 28 to 59% on adult plants (220 days old) compared with the control treatment. The three P2 treatments on HYZ50 produced significantly (P < 0.05) higher seed yield than the control treatment on this rapeseed cultivar, and they even generated seed yield similar to that produced by the resistant rapeseed cultivar Shengguang 165R in one of the two seasons. These results suggest that D. macrostoma P2 is an effective biocontrol agent against rapeseed clubroot.

Exploring the influence of rapeseed cultivar and pathogen isolate on Acremonium alternatum's efficacy in clubroot disease control

Clubroot disease, caused by Plasmodiophora brassicae, ranks among the most significant diseases affecting rapeseed cultivars, leading to substantial annual yield losses. Current control methods are limited to a small selection of chemical or biological treatments. Using biocontrol organisms presents a promising strategy for reducing disease severity and promoting plant vigour. However, their efficacy is strongly dependent on biotic and abiotic factors during the growing season, as well as the specific application conditions. In the present study, we evaluated the efficacy of the biocontrol fungus Acremonium alternatum in reducing clubroot disease symptoms across different susceptible and resistant rapeseed cultivars (Brassica napus) under various experimental greenhouse settings employing different types of P. brassicae inoculum: a uniform single spore isolate e3 and two German field isolates P1 and P1 ( +). We found that A. alternatum can reduce clubroot disease symptoms in susceptible rapeseed cultivars Visby, Ability and Jenifer, but not cv. Jumbo, when inoculated with the aggressive single spore isolate P. brassicae e3 at moderate (106 spores mL−1) and high (107 spores mL−1) densities. A. alternatum enhanced plant vitality and shoot biomass in cv. Visby inoculated with field isolates P1 or P1 ( +) but did not considerably reduce clubroot severity there. The clubroot-resistant cv. Mentor displayed a reduction in clubroot symptoms after A. alternatum treatment. In conclusion, A. alternatum holds some promise in managing moderate P. brassicae levels in the soil and could serve as an option in integrated pest management of clubroot disease when combined with resistant cultivars.

Bacterial consortium based on Pseudomonas fluorescens, Lysinibacillus xylanilyticus and Bacillus velezensis reduces clubroot disease in broccoli

Clubroot disease caused by Plasmodiophora brassicae is the most devastating disease in Brassicaceae plants. Control of clubroot is limited because of the survival of resting spores of the pathogen in the soil for years. Crop rotation, liming of the soil, fungicides and resistant cultivars have not been totally efficient in reducing the damages of the disease or pathogen spread. Although biocontrol with microorganisms has shown potential against clubroot, the variable results when using individual strains have made implementation difficult. Thus, the aim of this work was to determine whether a consortium of selected beneficial microorganisms is more efficient in controlling clubroot in broccoli than single strains. Single strains of bacteria from the Lysinibacillus, Bacillus and Pseudomonas genera (5 × 107 cells ml−1) and Trichoderma spp. (1 × 106 conidia ml−1) were evaluated in an initial screening. Four strains showing the best performance on plant growth and reduction of clubroot were evaluated as consortia in a further experiment. Treatments were applied in nursery (4 ml plant−1), and to the soil one week before transplant and the day of transplant (60 ml plant−1 each time). Plant shoot and root dry weight and the incidence and severity of clubroot were recorded. Supernatant from liquid cultures of Pseudomonas migulae Pf014 and Lysinibacillus xylanilyticus Br042, and washed cells of Bacillus velezensis Bs006 and B. pumilus Br019 promoted shoot growth of the plants. Single applications of Trichoderma asperellum Th034, Bs006, Br042 and Pseudomonas fluorescens Ps006 showed biocontrol potential against clubroot. However, the consortia built of Bs006, Br042 and Ps006 showed the highest reduction of clubroot incidence (78%) and severity (74%). A synergistic interaction by this consortium was found compared with the efficacy of single strains. To our knowledge, this work reports for the first time a consortium based on three rhizobacteria with high potential to control clubroot disease.

Integrated Management of Clubroot in Zhejiang Province, China

Clubroot, caused by Plasmodiophora brassicae, is a destructive soil-borne disease significantly harming global Brassica crop production. This study employed the Williams and European Clubroot Differential (ECD) and Williams systems to identify the pathotypes of P. brassicae collected from Hangzhou City, Yuhang District and Quzhou City, Kaihua County in Zhejiang Province. Greenhouse and field trials were conducted to evaluate the effects of plastic film covering and four chemical agents on the growth parameters and clubroot severity of the Chinese cabbage cultivar ‘Granaat’. Potential treatment mechanisms on clubroot were explored through a qPCR analysis of the resting spore density and pH measurement of the soil. Furthermore, treatment with 1-napthaleneacetic acid (NAA), a synthetic auxin, was also evaluated for its potential role in suppressing clubroot. The results indicate that the pathotypes of P. brassicae in the two districts were P1, ECD20/31/12, and P3, ECD20/15/4. While an individual application of plastic film covering could not effectively control clubroot, calcium cyanamid, dazomet and ammonium bicarbonate demonstrated significant efficacy in its management. These three agents significantly reduced the resting spore density in the soil, with calcium cyanamid and ammonium bicarbonate also increasing soil alkalinity. Additionally, ammonium bicarbonate promotes lateral root development in ‘Granaat,’ helping infected plants access adequate water and nutrients. However, NAA exhibited no efficacy in clubroot control. Therefore, sustained lateral root development is crucial for effectively resisting P. brassicae invasion. Considering application costs and environmental friendliness, we propose the field application of ammonium bicarbonate as the optimal method for clubroot disease management in Zhejiang Province.

The potential of PGPR and Trichoderma-based bioproducts and resistant cultivars as tools to manage clubroot disease in cruciferous crops.

The objective of this research was to determine the potential use of eco-friendly technologies to reduce the clubroot disease caused by Plasmodiophora brassicae, the main constraint of cruciferous crops worldwide. Two commercial bioproducts were evaluated in susceptible broccoli, one based on the PGPR consortium (Bacillus amyloliquefaciens, Bacillus pumilus, and Agrobacterium radiobacter K84) and the other one based on Trichoderma koningiopsis Th003 (Tricotec® WG). Additionally, the resistant broccoli cv. Monclano® was tested under two concentrations of resting spores (RS) of P. brassicae, 1 × 103 and 1 × 105 RS g-1 of soil. The first phase of evaluations with broccoli was carried out under a greenhouse, while susceptible broccoli, cauliflower, and red cabbage were included in a subsequent field phase. Tebuconazole + Trifloxystrobin mixture and Fluazinam were included as positive controls. The effectiveness of the bioproducts depended on the nature of the biocontrol agent, the concentration of P. brassicae, and the dose of treatment. Tricotec® showed consistent plant growth promotion but no biocontrol effect against clubroot, and the rhizobacteria-based bioproduct significantly reduced the disease in both greenhouse and field experiments. Higher disease severity was observed with the higher dose of Tricotec®. Under field conditions, the rhizobacteria reduced the incidence progress by 26%, 39%, and 57% under high, medium, and low pressure of the pathogen, respectively. However, no reduction of clubroot severity under high pressure of the pathogen was observed. Complete inhibition of club formation in roots was achieved via the fungicide, but a phytotoxic effect was observed under greenhouse conditions. Fungicides reduced the incidence progress of clubroot, but not the severity under high inoculum pressure in the field. The fungicides, the bacterial treatment, and the combination of bioproducts tended to delay the progress of the disease compared with the negative control and Tricotec alone. The resistant broccoli showed a low level of disease under high concentrations of P. brassicae (less than 10% incidence and up to 2% severity). These results suggested the overall potential of commercial tools based on the PGPR consortium and plant resistance to control P. brassicae. The integration of control measures, the role of Trichoderma spp. in P. brassicae-cruciferous pathosystems, and the need to recover highly infested soils will be discussed.

Evaluation of Amisulbrom Products for the Management of Clubroot of Canola (Brassica napus).

Clubroot, caused by Plasmodiophora brassicae, is an important disease of canola (Brassica napus). Amisulbrom, a quinone inside inhibitor (QiI), was evaluated for its effectiveness in clubroot management in Alberta, Canada. Resting spores of P. brassicae were treated in vitro with 0, 0.01, 0.1, 1, and 10% (w/v) amisulbrom to determine its effect on spore germination and viability. Amisulbrom inhibited resting spore germination by up to 79% and reduced viable spores by 31% relative to the control. Applications of a liquid solution (AL1000, 1000 g active ingredient (ai) ha-1) and granular formulations (AF700, 700 g ai ha-1; AF1000, 1000 g ai ha-1; AF1500, 1500 g ai ha-1) of amisulbrom were tested on the canola cultivars '45H31' (clubroot-susceptible) and 'CS2000' (moderately resistant) under greenhouse conditions and in field experiments in 2019 and 2020. In the greenhouse, the treatments were evaluated at inoculum concentrations of 1 × 105 or 1 × 107 resting spores g-1 soil. A trend of decreasing clubroot severity with an increasing amisulbrom rate was observed. At the lower spore concentration, treatment with AF1500 resulted in a clubroot disease severity index (DSI) <20% for both cultivars, while the lowest DSI under both low and high spore concentrations was obtained with AL1000. The field results indicated a significant reduction in DSI, with varied effects of rates and liquid vs. granular formulations. The greatest reductions (up to 58.3%) in DSI were obtained with AF1500 and AL1000 in 2020. These findings suggest that amisulbrom holds promise as part of an integrated clubroot management approach.

Effect of Mulching on Soil Temperatures and Its Impact on Plasmodiophora brassicae and Clubroot.

Clubroot caused by Plasmodiophora brassicae is a serious soilborne disease on cruciferous crops worldwide. Agricultural practice is a preferable clubroot management strategy because of its low investment requirement and environmental safety. Among the agricultural practices, solarization has been widely applied in the integrated management of other soilborne diseases. However, only few reports exist on the effect of solarization on clubroot management. In this study, we measured the effect of plastic mulching on soil temperature at different depths and on clubroot incidence and severity under greenhouse and field conditions. The pathogen density in the soil after solarization was measured by quantitative PCR analysis. Results indicated that the mulching treatment increased soil temperature especially in the soil layer ranges of 0 to 20 cm. Solarization with mulching also effectively reduced the incidence and severity of clubroot in the greenhouse assay and the field trial by decreasing the P. brassicae population in the soil. This study suggested that solarization with mulching can impair clubroot development and thus contribute to the sustainable management of clubroot.

New endophytic strains of Trichoderma promote growth and reduce clubroot severity of rapeseed (Brassica napus).

Rapeseed (Brassica napus L.) is the world's third most important edible oilseed crop after soybean and palm. The clubroot disease caused by Plasmodiophora brassicae poses a significant risk and causes substantial yield losses in rapeseed. In this study, 13 endophytic fungal strains were isolated from the healthy roots of rapeseed (B. napus) grown in a clubroot-infested field and molecularly identified. Based on germination inhibition of resting spores of P. brassicae, two endophytic fungal antagonists, Trichoderma spp. ReTk1 and ReTv2 were selected to evaluate their potential for plant growth promotion and biocontrol of P. brassicae. The Trichoderma isolates were applied as a soil drench (1×107 spore/g soil) to a planting mix and field soil, in which plants were grown under non-infested and P. brassicae-infested (2×106 spore/g soil) conditions. The endophytic fungi were able to promote plant growth, significantly increasing shoot and root length, leaf diameter, and biomass production (shoots and root weight) both in the absence or presence of P. brassicae. The single and dual treatments with the endophytes were equally effective in significantly decreasing the root-hair infection, root index, and clubroot severity index. Both ReTk1 and ReTv2 inhibited the germination of resting spores of P. brassicae in root exudates. Moreover, the endophytic fungi colonized the roots of rapeseed extensively and possibly induced host resistance by up-regulated expression of defense-related genes involved in jasmonate (BnOPR2), ethylene (BnACO and BnSAM3), phenylpropanoid (BnOPCL and BnCCR), auxin (BnAAO1) and salicylic acid (BnPR2) pathways. Based on these findings, it is evident that the rapeseed root endophytes Trichoderma spp. ReTk1 and ReTv2 could suppress the gall formation on rapeseed roots via antibiosis, induced systemic resistance (ISR), and/or systemic acquired resistance (SAR). According to our knowledge, this is the first report of the endophytic Trichoderma spp. isolated from root tissues of healthy rapeseed plants (B. napus.), promoting plant growth and reducing clubroot severity.

Evaluation of genetic variation of morphological and clubroot-resistance traits of radish and metabonomic analysis of clubroot-resistant cultivar

Global commercial production of radish (Raphanus sativus) is highly threatened by Plamodiophora brassicae. Germplasm evaluation and the deep analysis of variants with clubroot-resistant trait are needed to produce quality cultivars. The genetic diversity was analyzed for 97 radish germplasms based on 20 agronomic traits and the response to clubroot. Additionally, differentially regulated metabolites (DRMs) between healthy and P. brassicae-infected resistant cultivars were measured. The genetic diversity index values of quality traits varied from 0.396 to 1.350, and that of quantitative traits varied from 1.65–2.15. The variation of radish quantitative traits ranged from 19.18% to 54.44%. The principal component analysis indicated 8 comprehensive indicators, allowing classification of germplasms into 4 classes, with 71 in class I, 20 in class II, 4 in class III, and 2 in class IV. The results of classification of germplasms by the response to clubroot infection only or by comprehensive agronomic traits including response of plants to clubroot disease infection were similar. Among all traits assessed in the present study, the clubroot severity and the depth of in-soil root were closely linked. Metabolomic analysis revealed that the metabolites changes of clubroot resistant cultivar “DHBC” pre- and post- P. brassicae infection was relatively stable. P. brassicae infection resulted in limited DRMs, significant down-regulation was identified on ginsenoside, dihydro-2(3H)-thiophenone, proline, 1-pyrroline, while apiosylglucosyl 4-hydroxybenzoate, chrycolide, glucomalcomiin, and phosphocholine were significantly up-regulated. We revealed more extensive genetic variation in quantitative traits than in quality indexes in radish and we summarized that the response of radish to clubroot disease may be a dominate indicator in evaluation of radish varieties. Finally, untargeted metabolomic study confirmed the clubroot resistance of “DHBC” and revealed a sophisticated metabolites (including some precious pharmaceutical ingredients) variation in response to clubroot infection, offers new directions in the studies of clubroot resistance mechanism of radish.

Canola with Stacked Genes Shows Moderate Resistance and Resilience against a Field Population of Plasmodiophora brassicae (Clubroot) Pathotype X.

Genetic resistance is a cornerstone for managing clubroot (Plasmodiophora brassicae). However, when used repeatedly, a clubroot resistance (CR) gene can be broken rapidly. In this study, canola inbred/hybrid lines carrying one or two CR genes (Rcr1/CRaM and Crr1rutb) were assessed against P. brassicae pathotype X by repeated exposure to the same inoculum source under a controlled environment. Lines carrying two CR genes, either Rcr1 + Crr1rutb or CRaM + Crr1rutb, showed partial resistance. Selected lines were inoculated with a field pathotype X population (L-G3) at 5 × 106 resting spores/g soil, and all clubs were returned to the soil they came from six weeks after inoculation. The planting was repeated for five cycles, with diseased roots being returned to the soil after each cycle. The soil inoculum was quantified using qPCR before each planting cycle. All lines with a single CR gene were consistently susceptible, maintaining high soil inoculum levels over time. The lines carrying two CR genes showed much lower clubroot severity, resulting in a 10-fold decline in soil inoculum. These results showed that the CR-gene stacking provided moderate resistance against P. brassicae pathotype X, which may also help reduce the pathogen inoculum buildup in soil.

Soil fumigation with Vapam (metam sodium) to control clubroot (Plasmodiophora brassicae) of canola (Brassica napus)

Clubroot, a damaging disease of canola ( Brassica napus L.) caused by the soilborne parasite Plasmodiophora brassicae Woronin, is spreading across Alberta and other provinces of western Canada. The movement of infested soil on field machinery is the main mechanism of dispersal, with clubroot generally occurring first as localized patches near field entrances. In this study, the soil fumigant Vapam (metam sodium) was evaluated as a management option for foci of P. brassicae infestation. Replicated experiments at two field sites in central Alberta showed reductions in clubroot severity ranging from 9% to 51% following treatment with varying rates of Vapam. Decreases in clubroot severity of up to 28% were observed in the year following Vapam treatment, indicating some potential residual effects and (or) a reduction in the amount of inoculum returned to the soil in the previous year. While Vapam shows some promise as a clubroot management tool, an integrated approach will be required for the sustainable management of this disease on canola.

The Pursuit in Nepal of Native Trichoderma spp. for Plant Disease Biocontrol

The characteristics of 41 native Trichoderma isolates collected from diverse microclimatic domains in Nepal and two Ohio isolates and their efficacy in biocontrol of plant diseases were assessed. Species assignment was based on a maximum likelihood phylogenetic analysis constructed on a concatenated dataset using the internal transcribed spacer region ( ITS), translocation elongation factor ( tef1), and RNA polymerase II subunit ( rpb2). Most of the Nepal isolates ( n = 37) were assigned to the Viride clade and identified as T. asperellum or T. asperelloides, whereas a minority ( n = 4) were assigned to the Harzianum clade and identified as T. lixii or T. rugulosum. One of the Ohio isolates belonged to the Hamatum clade and was closely related to the T. hamatum reference strain, whereas the other Ohio strain was closely related to T. ghanense (Longibrachiatum clade). Confrontation assays conducted to evaluate mycelial growth reduction of Rhizoctonia solani indicated that mechanisms of action included competition, mycoparasitism, and antibiosis. Trichoderma asperellum isolates NT22, NT8, NT24, and NT25 and T. asperelloides isolates NT1 and NT30 reduced Rhizoctonia root rot severity in greenhouse assays by more than 50% compared with the nontreated controls. Trichoderma asperellum isolates NT4, NT25, NT28, and NT17 and T. asperelloides NT8 suppressed clubroot severity in greenhouse assays by more than 75% compared with the nontreated controls. A significant negative correlation was observed between the percentage of mycelial growth reduction in vitro (confrontation assay) of R. solani and root rot severity. This study provides a basis for additional investigations of Trichoderma diversity and biocontrol potential in Nepal. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Evaluation of Host Resistance, Hydrated Lime, and Weed Control to Manage Clubroot in Canola

Plasmodiophora brassicae Wor. is a soilborne parasite causing clubroot of canola (Brassica napus L.), a serious disease managed mostly by planting clubroot-resistant (CR) cultivars. Recently, new pathotypes of P. brassicae have emerged that overcome resistance, highlighting the need for a greater understanding of resistance stewardship and an integrated approach to clubroot management. Replicated field experiments were conducted in Edmonton, AB, in 2018 and 2019, to evaluate the effect of lime application and weed management on clubroot severity, crop yield and various growth parameters in clubroot-susceptible (CS) and CR canola cultivars. P. brassicae resting spore densities were also monitored by quantitative PCR for each treatment. When hydrated lime was applied to increase the soil pH from initial values of 5.2–5.5 to 7.2, clubroot severity decreased by 34–36% in the CS canola cultivar, while seed yield increased by 70–98%. The application of hydrated lime also resulted in resting spore densities that were 48–80% lower, relative to untreated controls, in plots where the CS cultivar was grown. Lime application or management of weeds did not significantly affect spore densities in plots with the CR cultivar. The results from the field trial suggest that the application of hydrated lime may be a useful strategy to manage clubroot, when used in combination with genetic resistance to reduce disease pressure.

Severity of clubroot in kale related to management practices and soil attributes

ABSTRACT: Clubroot disease, caused by Plasmodiophora brassicae, limits the production of Brassica spp. worldwide. Little is known about the factors related to the development of the disease in kale (Brassica oleracea var. acephala) plants and in crops in mountainous areas under tropical conditions. This study examined the severity of clubroot in kale crops as well as identify potential flaws in management and the soil and relief factors related to its occurrence. The study was conducted in 24 kale fields in the mountainous region of Rio de Janeiro (Brazil). Soil and kale growth management practices adopted in the region were identified and samples of soil and plants were collected. Subsequently, soil and relief attributes, disease severity, biomass and nutrient and Al contents and accumulation in the plants were determined. There was a high spread of the pathogen in the areas. Inappropriate and recurrent practices in the region were detected, e.g., sequential cultivation of host species, low adoption of soil fertility analysis and liming and conservation practices, and community use of agricultural machinery and implements without prior cleaning. The disease was associated with more acidic soils, subject to greater water accumulation and with high levels of Al3+ as well as with higher Al contents and accumulation in the roots. Management practices must be adopted in the region to reduce the potential inoculum of P. brassicae and to increase soil fertility.

Effect of Pathogen Virulence on Pathogenicity, Host Range, and Reproduction of Plasmodiophora brassicae, the Causal Agent of Clubroot Disease.

A series of greenhouse experiments was conducted to evaluate the effect of Plasmodiophora brassicae virulence on clubroot development and propagation of resting spores in 86 plant species from 19 botanical families. Plants were artificially inoculated with two isolates of P. brassicae, which were virulent on clubroot-resistant oilseed rape cultivar Mendel [pathotype 1; P1 (+)] or avirulent on this cultivar (P1). Clubroot severity and the number of resting spores inside the roots were assessed 35 days post inoculation. Typical clubroot symptoms were observed only in the Brassicaceae family. P1 (+)-inoculated species exhibited more severe symptoms (two- to 10-fold more severe), bigger galls (1.1- to 5.8-fold heavier), and greater numbers of resting spores than the P1-inoculated plants. Among all Brassica species, Bunias orientalis, Coronopus squamatus, and Raphanus sativus were fully resistant against both isolates, whereas Camelina sativa, Capsella bursa-pastoris, Coincya monensis, Descurainia sophia, Diplotaxis muralis, Erucastrum gallicum, Neslia paniculata, Sinapis alba, Sinapis arvensis, Sisymbrium altissimum, Sisymbrium loeselii, and Thlaspi arvense were highly susceptible. Conringia orientalis, Diplotaxis tenuifolia, Hirschfeldia incana, Iberis amara, Lepidium campestre, and N. paniculata were completely or partially resistant to P1 isolate but highly susceptible to P1 (+). These results suggest that the basis for resistance in these species may be similar to that found in some commercial cultivars, and that these species could contribute to the buildup of inoculum of virulent pathotypes. Furthermore, the pathogen DNA was detected in Alopecurus myosuroides, Phacelia tanacetifolia, Papaver rhoeas, and Pisum sativum. It can be concluded that the number and diversity of hosts for P. brassicae are greater than previously reported.

Variation of Glucosinolate Contents in Clubroot-Resistant and -Susceptible Brassica napus Cultivars in Response to Virulence of Plasmodiophora brassicae.

The present study investigated the changes in total and individual glucosinolates (GSLs) in roots and leaves of different clubroot-resistant and -susceptible oilseed rape cultivars following artificial inoculation with Plasmodiophora brassicae isolates with different virulence. The results showed significant differences in clubroot incidence and severity as well as in the amount of total and individual glucosinolates between oilseed rape cultivars in response to virulence of the pathogen. Single among with total aliphatic and total indolic glucosinolate contents were significantly lower in leaves of susceptible cultivars compared to resistant ones due to the infection. Similarly, single and total aliphatic as well as indolic glucosinolate contents in roots were lower in susceptible cultivars compared to resistant cultivars analyzed. The different isolates of P. brassicae seem to differ in their ability to reduce gluconasturtiin contents in the host. The more aggressive isolate P1 (+) might be able to suppress gluconasturtiin synthesis of the host in a more pronounced manner compared to the isolate P1. A possible interaction of breakdown products of glucobrassicin with the auxin receptor transport inhibitor response 1 (TIR1) is hypothesized and its possible effects on auxin signaling in roots and leaves of resistant and susceptible cultivars is discussed. A potential interplay between aliphatic and indolic glucosinolates that might be involved in water homeostasis in resistant cultivars is explained.

Greenhouse Evaluation of Clubroot Resistant-Brassica napus cv. Mendel and Its Efficacy Concerning Virulence and Soil Inoculum Levels of Plasmodiophora brassicae.

Clubroot resistance of oilseed rape (OSR) cultivars frequently relies on a major resistance gene originating from cv. Mendel. The efficacy of this resistance was studied in greenhouse experiments using two Plasmodiophora brassicae isolates, which were either virulent (P1(+)) or avirulent (P1) on Mendel. Seeds of clubroot-susceptible cultivar Visby and clubroot-resistant cultivar Mendel were sown in soil mixtures inoculated with different concentrations of resting spores (101, 103, 105, and 107 resting spores/g soil). Clubroot severity, plant height, shoot and root weight as well as resting spore propagation were assessed for each isolate and cultivar separately at four dates after sowing. The OSR cultivars behaved significantly different in the measured parameters. The threshold of inoculum density to cause disease depended strongly on the virulence of the pathogen and susceptibility of the host plant. In Visby grown in soil infested with P1, clubroot symptoms and increases in root weight and the number of propagated resting spores occurred at inoculum levels of 101 resting spores and higher, whereas Mendel was not affected in soils under the three lowest inoculum densities. In contrast, the P1(+) isolate led to earlier and more severe symptoms, heavier galls, and a significantly higher number of new resting spores in both cultivars.

Exogenous inoculation of endophytic bacterium Bacillus cereus suppresses clubroot (Plasmodiophora brassicae) occurrence in pak choi (Brassica campestris sp. chinensis L.).

The presence of Bacillus cereus plays a key role in clubroot suppression and improves plant biomass in pak choi. B. cereus is reported for the first time as a novel biocontrol agent against clubroot. Plasmodiophora brassicae Woronin causes a devastating infectious disease known as clubroot that is damaging to cruciferous vegetables. This study aimed to isolate beneficial bacteria from the rhizosphere soil of pak choi (Brassica campestris sp. chinensis) and to evaluate the ability of the isolate to reduce the severity of clubroot. Strains obtained from the rhizosphere of symptomless pak choi were first selected on the basis of their germination inhibition rate and effects on the viability of P. brassicae resting spores. Eight bacterial isolates had inhibitory effects against the resting spores of clubroot causing pathogen. However, MZ-12 showed the highest inhibitory effect at 73.4%. Inoculation with MZ-12 enhanced the plant biomass relative to plants grown without MZ-12 as well as P. brassicae infected plants. Furthermore, enhanced antioxidant enzymatic activities were observed in clubroot-infected plants during bacterial association. Co-inoculation of the plant with both P. brassicae and MZ-12 resulted in a 64% reduction of gall formation in comparison to plants inoculated with P. brassicae only. Three applications of MZ-12 to plants infected with P. brassicae at 7, 14 and 21days after seeding (DAS) were more effective than one application and repressed root hair infection. According to 16S rDNA sequence analysis, strain MZ-12 was identified as had a 100% sequence similarity with type strain Bacillus cereus. The findings of the present study will facilitate further investigation into biological mechanisms of cruciferous clubroot control.

Seed treatment of canola (Brassica napus) with the endomycorrhizal fungus Piriformospora indica does not reduce clubroot

A Basidiomycota endomycorrhizal fungus, Piriformospora indica Verma, colonizes and promotes the growth of canola (Brassica napus L.) and other Brassica crops and can reduce diseases of other crops. Clubroot is an important disease of Brassica crops caused by the obligate, soil-borne pathogen Plasmodiophora brassicae Woronin. The effect of P. indica on clubroot severity in canola was assessed in replicated growth room studies. Seed was treated with P. indica using a proprietary process. Microscopic observation confirmed that canola roots grown from treated seed were colonized by P. indica; however, P. indica did not consistently reduce clubroot severity and did not promote the growth of canola.