Canola and clubroot disease
Canola (Brassica napus L.) is an important oilseed crop in the world. End uses of canola plants include cooking oil, processed food, biofuel, animal feed, etc.
Canola production in Canada is distributed in the provinces of Alberta, Manitoba, Saskatchewan, Ontario, British Columbia and Quebec (Rempel et al. 2014). Based on the Canola Council of Canada, the canola industry in Canada is estimated to an annual average value of $30 billion to the Canadian economy per year. Moreover, the canola industry provides more than 207 thousand jobs in Canada. However, in recent years, the occurrence of clubroot is increasingly severe, and in countries like Canada, which is considered an imminent risk to the canola industry (Dixon 2009; 2014; Javed et al. 2023). Clubroot disease was first found in western Canada in 2003, Alberta (Strelkov and Hwang 2014).
Clubroot disease
Clubroot disease is caused by Plasmodiphora brassicae (P.brassicae), a protist affecting cruciferous vegetables and canola worldwide. P. brassicae is a soil-borne obligate parasite belonging to the Rhizaria group (Javed et al. 2023; Ludwig-Müller 2022). The typical symptoms of clubroot disease are swelling of the roots and the formation of club-shaped galls, inhibiting the absorption of water and nutrients by the roots, and subsequent death of the plant (Li et al. 2018).
The life cycle of clubroot disease
The clubroot pathogen is a biotrophic obligate plant parasite that requires the plant host to complete its life cycle (Kageyama and Asano 2009). To this day, the clubroot pathogen has not been cultured in axenic media, despite multiple tries, although it has been possible to see different P. brassicae developmental stages when callus was generated from infected Brassicas roots (Tu et al., 2019). Although the work with this pathogen is very challenging, in the last ten years, significant progress has been made clarifying several controversial points in P. brassicae’s life cycle, i.e. where primary and secondary infection takes place in the roots and if a sexual life stage exists (Auer & Ludwing-Müller, 2015; Liu et al., 2020).
P. brassicae life cycle is divided into two main phases: the primary infection and secondary infection of the cortex tissue (Kageyama & Asano, 2009). The primary infection was considered to occur exclusively in the root hairs, but recently, it has been shown that primary infection can also occur in epidermal cells in the root elongation zone, the zone where the gall initiates (Tu et al., 2019; Liu et al., 2020a). After resting spores’ germination, primary zoospores are released, and primary infection is established in root hairs or epidermal cells passing from an uninucleate primary plasmodium to a diploid uninucleate zygote (Liu et al., 2020).
Clubroot management
In general, there are several strategies to avoid clubroot disease, including biological control, chemical control, and agronomical practices. For example, several synthetic fungicides, including fluazinam, pentachloronitrobenzene, metalaxyl-mancozeb, azoxystrobin, difenoconazole, and carbendazim, have been tested. However, there is no consensus on these effects has been reached due to variable levels of control depending on the crop, geographical location, and application strategies (Liao et al. 2022; Peng et al. 2014). Additionally, some microorganisms such as Trichoderma, Bacillus and endophytes have been extensively used to control clubroot disease (Javed et al. 2023). It is reported that inoculation of Trichoderma harzianum can reduce clubroot disease in Chinese cabbage by regulating the rhizosphere microbial community (Li et al. 2020).
Overall, the use of clubroot-resistant (CR) cultivars is considered the best management practice and environmentally friendly to avoid clubroot disease in canola plants (Javed et al. 2023; Marino et al. 2023), and several clubroot resistant B. rapa, B. oleracea and B. napus cultivars have been developed (Diederichsen et al. 2009). Clubroot disease resistance has been introgressed into canola following different approaches. For example, “Mendel” is a heterogeneous F1 hybrid clubroot resistant cultivar, in which clubroot resistance is from a B. napus line (B. rapa ECD-04 x B. oleracea ECD-15) (Hasan et al. 2021a; Rahman et al. 2011). The resynthesized Mendel cultivar is likely to carry a major dominant resistance gene from turnip type B. rapa ECD-04 (Diederichsen et al. 2009). Although CR cultivars of some Brassica crops have been bred and developed (Cai et al. 2023), the major challenges of CR breeding remain the identification of the resistance (R) genes. Moreover, all these cultivars are based on a single R gene, and that can be vulnerable because the P. brassicae population in the field is diverse and the clubroot resistance could be overcome.
Genetic analysis and fine mapping studies have identified major loci, quantitative trait loci (QTLs) and genes from different Brassica species involved in P. brassicae resistance (Hasan et al. 2021b; Lv et al. 2020). Several current sources of clubroot-resistant (CR) canola germplasm are limited and conserved among seed companies: (i) resistance prevenient from the winter canola cultivar ‘Mendel’ (CRM), (ii) resistance prevenient from the Chinese cabbage ‘Flower Nabana’ (CR1), and (iii) resistance prevenient from rutabaga ‘Brookfield (CRB) (Rahman et al. 2014).
