Clubroot is a disease caused by an obligate biotrophic pathogen that primarily affects plants belonging to the family Brassicaceae, such as cabbage, broccoli, cauliflower, etc. and crucifers.
It was first described during the nineteenth century, is worldwide in occurrence, and is responsible for severe crop losses.
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Causal Organism of Clubroot Disease
The causal organism is Plasmodiophora brassicae, which belongs to the order Plasmodiophoromycetes in the kingdom Protozoa. This organism has characteristics of both fungi and protozoa.
P. brassicae produces resting spores, primary and secondary zoospores, and plasmodial stage.
The resting spores are 3µm in size on average, spherical in shape, have thick cell walls, and can survive in soil for several years. The outer wall of spores is mainly covered with spines.
During favorable environmental conditions, the resting spores germinate into biflagellate zoospores that are motile and infect the host roots. Zoospores are spindle or pyriform in shape with two whiplash flagella of unequal length.
P. brassicae forms a multinucleate structure called plasmodium inside the host cells which helps in the absorption of nutrients from the hosts. The plasmodium is spherical with a smooth surface. Each plasmodium is enveloped by a membranous material. The vegetative plasmodia fill most of the host cell volume and provide a sponge-like appearance to the root gall.
The secondary zoospores are produced from plasmodium inside the host tissue which is further involved in secondary infection. The secondary zoospores are similar to primary zoospores and contain two whiplash flagella of unequal length.
Symptoms of Clubroot Disease
The symptoms according to affected plant parts are as follows-
Root- The typical characteristic of this disease is gall formation and swelling. It is not evident during the primary stages, but as the disease advances, it deforms the root. At later stages, the roots may become mushy and disintegrate. This causes a lack of water and nutrient supply to the plant.
Shoot- The infected plant mostly wilts during the daytime and has stunted growth. Chlorosis of leaves can also be seen due to nutrient shortage. Yield and crop quality are also decreased.
Disease Cycle of Clubroot Disease
The disease cycle of clubroot disease is as follows-
The resting spore of P. brassicae upon receiving the chemical signals from brassica root germinates into biflagellate zoospores.
These zoospores swim towards the roots of the host plant and infect the roots and root hairs. Then the zoospores eventually invade the cortex and steles of the host and form the uni-nucleate primary plasmodium.
The plasmodium eventually divides and re-divides in the cortical region of the host and produces a multi-nucleate plasmodium.
Plasmodia causes rapid proliferation of cells (Hyperplasia) and abnormal cell enlargement (hypertrophy) which eventually results in gall formation.
During the development of the pathogen inside the host, new zoospores are produced which are capable of infecting the same or different plants and thus repeating the cycle.
When the roots of the diseased plants disintegrate, the resting spores are formed that are released into the soil and serve as inoculum for further disease development on healthy susceptible hosts.
Host range
The disease mainly affects the Brassica family and some other plants of different families. The plants can be grouped as follows-
Very resistant/ immune– Plants like radishes, horseradish, peppergrass, stock, etc. are very immune to disease.
Less susceptible– Black mustard, some turnips, rape, etc. are less susceptible.
Medium susceptible– Plants like cauliflower, kohlrabi, collards, etc. are medium susceptible.
Highly susceptible– Cabbage, Chinese cabbage, Brussels sprouts, mustard, etc. are highly susceptible.
Environmental factors
Several environmental factors that affect the disease development are as follows-
Soil moisture– High soil moisture is conducive to spore germination and mobility of zoospores.
Soil pH– Acidic soil pH <7.0 favors the disease development. pH above 7.2 inhibits the spore germination.
Temperature– The optimum temperature for the activity of P. brassicae lies between 20°-25°C. If the temperature increases or decreases, it will reduce the pathogen activity.
Soil type– Types of soil such as light soil, sandy soil, clay soil, and humus-rich soil favor the infection, but the severity of the disease may vary.
Calcium and nitrogen – The presence of calcium and nitrogen in soil inhibits the growth of pathogens.
Spread mechanisms
Clubroot spreads through a variety of methods such as–
Diseased plantings- Replanting the same fields with diseased plants will directly transfer the infection to the fields that were previously free of the disease.
Infected implements- Farming tools and machinery in contact with infected soil leads to the transfer of spores from one area to another.
Water spread- Spores are transferred through water across the fields and into the irrigation sources.
Clubroot Disease Management
Cultural practices
Crop rotation between host plants and non-host plants especially legumes could reduce the spores of pathogens in the soil. Repeated rotation through several crop cycles may reduce the influence of pathogens.
Soil pH adjustments should be done by applying lime to raise the pH above 7.2 to reduce clubroot severity.
Properly drained fields would reduce chances of waterlogging, and zoospore transfer.
Destruction of diseased plants at the end of each cropping season should be performed to reduce the pathogen concentration in the soil.
Chemical control
Some fungicides can be applied as soil drenches with fluazinam inhibiting activity by P. brassicae. Overreliance on chemicals can lead to resistant varieties and ecological problems.
Soil amendments can be done by adding wood ash, and calcium cyanamide, which provides unfavorable conditions for pathogens.
Controlling brassica weeds in fields can also be done to prevent further increase in spores.
Biological control
Organisms such as Trichoderma spp., and Bacillus subtilis can be used to suppress the activities of P.brassicae. These organisms inhibit the growth of pathogens by competing for resources or producing antimicrobial compounds.
Resistant varieties
Plant breeders have developed crops with resistance to different strains of P.brassicae. Varieties such as Shangpin, a disease-resistant variety of cabbage, Crispus, a resistant variety of Brussels, Emerald Jewel, a resistant variety of broccoli, etc. are developed.
Besides, regular monitoring of fields should be performed. Early detection through soil testing helps in identifying the disease before it causes severe outbreaks. Farmers should be educated on disease identification, prevention, and best management by organizing several awareness programs and workshops should be done.
References
- ClubRoot. (n.d.). UMN Extension. https://extension.umn.edu/plant-diseases/clubroot
- ClubRoot | Symptoms, Prevention, & Control | Britannica. (n.d.). Encyclopedia Britannica. https://www.britannica.com/science/clubroot
- Bhattacharya, I., & Dixon, G. (2010). MANAGEMENT OF CLUBROOT DISEASE (PLASMODIOPHORA BRASSICAE) OF BRASSICAS USING TRAP CROPPING TECHNIQUES. Acta Horticulturae, 867, 157–164. https://doi.org/10.17660/actahortic.2010.867.20
- Canola Council of Canada. (2024, August 28). ClubRoot Disease | Canola Encyclopedia. https://www.canolacouncil.org/canola-encyclopedia/diseases/clubroot/
- Agriculture | Province of Manitoba. (n.d.). Province of Manitoba – Agriculture. https://www.gov.mb.ca/agriculture/crops/plant-diseases/clubroot-brassica.html
- Anonymous. (2023, March 6). Brassicas, Clubroot. Center for Agriculture, Food, and the Environment. https://ag.umass.edu/vegetable/fact-sheets/brassicas-clubroot
- Akahoshi, K., Wages, S. a. M., Shintaku, M., & Hamasaki, R. (2019). Clubroot disease of crucifer crops. ResearchGate. https://www.researchgate.net/publication/342501460_Clubroot_Disease_of_Crucifer_Crops
- Gahatraj, S., Shrestha, S., Devkota, T., & Rai, H. (2019). A review on clubroot of crucifers: symptoms, life-cycle of the pathogen, factors affecting severity, and management strategies. Archives of Agriculture and Environmental Science, 4(3), 342–349. https://doi.org/10.26832/24566632.2019.0403012
- Clubroot of cabbage. (2019, April 13). [Slide show]. SlideShare. https://www.slideshare.net/slideshow/clubroot-of-cabbage/140678857
- ClubRoot / Cole Crops / Agriculture: Pest Management Guidelines / UC Statewide IPM Program (UC IPM). (n.d.). https://ipm.ucanr.edu/agriculture/cole-crops/clubroot/
- ClubRoot of Brassicas | NC State Extension Publications. (n.d.). https://content.ces.ncsu.edu/clubroot-of-brassicas
