Information adapted from Agri-Facts, Clubroot Disease of Canola and Mustard, Alberta Agriculture and Rural Development, March 2011 Revision.
Clubroot is a serious soil-borne disease of cruciferous crops. In canola, it causes swellings or galls to form on the roots, which ultimately causes premature death of the plant. It is caused by a fungus-like protist called
Plasmodiophora brassicae. Currently, there are no economical control measures that can remove this pathogen from a field once it has become infested. However, it is possible to curtail the spread of the pathogen and reduce the incidence and severity of the disease. Management of infested fields through the combination of scouting and record keeping, sanitation, crop rotation, soil amendments and the cropping of resistant varieties are the most effective methods of controlling this disease.
Clubroot had been seen before in cole crops across Canada; it has been recognized as a problem in
Brassica vegetable crops (e.g. broccoli, cabbage, radish, rutabaga) in Ontario, Quebec, British Columbia, and Atlantic Canada for several years. However, the first report of this disease in a commercial canola field in western Canada occurred near Edmonton, AB in 2003. Since then, thousands more infested fields have been identified in Alberta.
Since clubroot has continued to spread in Alberta, particularly in the Edmonton area, it has become a key disease for the canola industry. Preventing the spread of clubroot spores through contaminated soil movement is critical to managing this disease.
Plasmodiophora brassicae is regarded as a protist, which means it is an organism with plant, animal and fungal characteristics. P. brassicae is an obligate parasite, which means the pathogen can't grow and multiply without a living host such as canola or other susceptible crops and weeds. The lifecycle of P. brassicae is shown in Figure 2.
The pathogen overwinters in the soil as very hardy resting spores. In the spring, the frequency of resting spore germination is stimulated by secretions from the roots of various plants. Resting spores then germinate and transform into zoospores, capable of swimming short distances in water or water-films in the sil, seeking root hairs to infect.
The mobility of these zoospores makes clubroot a very different disease from others in canola. This mobility actually allows the zoospore to 'seek out' potential host plants instead of solely relying on random distribution methods like wind or rin (sclerotinia, blackleg, etc.). The swimming zoospore phase is short-lived.
After initial infection through root hairs or woulnds, the zoospore forms an amoeba-like cell. This abnormal cell multiplies and joins with others to form a plasmodium (a naked mass of protoplasm with many nuclei). The plasmodium eventually divides to form many secondary zoospores that are released into the soil after the host root begins to decompose.
These second generation zoospores re-infect the roots of the initial host or nearby plants and are able to invade the cortex (interior) of the root. This is where genetic resistance operates, preventing the invasion from secondary zoospores.
Once in the cortex, the amoeba-like cells multiply or join with others to form a secondary plasmodium. As this plasmodium develops, plant hormones are altered, which causes the infected cortical cells to swell. Clusters of these enlarged cells form clubs or galls (see Figure 3). Some amoeba-like cells are able to move up and down roots in vascular tissue, which is plant tissue that transports nutrients and water throughout a plant. After secondary plasmodia mature, they divide into many resting spores. When the galls are rapidly decayed by soil microbes, millions of long-lived resting spores are left in the soil.
This longevity of the resting spores is a key reason why clubroot is known as a serious disease. The spores can survive in the soil for up to 20 years. However, many of the resting spores appear to become inactive or non-viable after a 2 year break from a host crop, indicating the importance of crop rotation, weed control and sanitation to help reduce spore loads in the soil. This represents a significant decline in inoculum in the field, but the more spores that start the infection, the more that will remain over time. This is why it is important to prevent major infestations from becoming established in a field using a crop rotation of 3 to 4 years out of a susceptible crop. Under severe infestations, which can occur quickly when a susceptible variety is grown on infested land, much longer periods will be required to reduce inoculum to manageable levels via natural population decline.
Warm soil (20-24°C), high soil moisture and acid soil (pH less than 6.5) all favour infection and the severity of disease development. Soils with a pH over 7.2 tend to inhibit spore germination and disease development, although the disease can still develop. High pH soils will not prevent
P. brassicae from arriving in a field, nor prevent the disease from infecting susceptible plants, and nor will it prevent yield loss from occurring. Liming fields to a pH>7 will be most successful under low levels of infestation and become less effective under high spore loads in the soil. So liming proactively or when soil infestations are discovered early will help reduce the severity and impact of the disease. This is another beneficial practice which relies on intensive scouting and early detection of clubroot.
Areas of a field with more soil moisture typically see the most severe infestations. These wet areas are found in depressions, spots with higher clay content, or with subsoil horizons that result in poor water infiltration (such as Gray Wooded or solonetzic soils).
Symptoms in the field will be dependent on timing – see
Identify Clubroot for more information.
Yield loss is dependent on many factors including time of infection, soil moisture and temperature, spore load, soil pH, soil texture, host genotype, pathogen pathotype, etc. An early infection with favorable conditions and moderate to high spore loads can lead to 100% loss, while low spore loads with less favorable conditions may result in little or no yield loss.
Management using clubroot-resistant canola
Genetic resistance to clubroot is available in an number of canola cultivars. The resistance is very effective and can reduce disease loss to zero. However, the resistance is not durable when used in short rotation situations in heavily infested areas. The pathogen has many naturally-occurring pathotypes in each soil population, and will commonly have at least one pathotype capable of causing disease on the resistant canola plants. These rare pathotypes start out at very low, almost non-detectable, levels in the soil population, but quickly multiply when resistant canola is grown repeatedly. This rapid multiplication is due to the fact that they are the only pathotype(s) that can complete their lifecycle, so they have a selective advantage, and also because they produce billions of new resting spores in each root they infect. The loss of disease control can be detected after two or three exposure to the resistant cultivar. In order to prevent or postpone loss of control, the following principles should be used:
Scout for clubroot, even in resistant canola cultivars
Avoid growing canola more than once every 3-4 years in the same field
Do not plant canola in heavily infested fields