About Clubroot
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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. Currently, there are no economical control measures that can remove the disease from a canola field once it has been infested. However, it is possible to curtail the spread and reduce the incidence and severity of infection. Sanitation, management of infested fields through crop rotation, 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. How the disease made its way to Alberta is not yet known. Since then, more infested fields have been identified in central Alberta each year. The pathogen (but not the disease itself) has been identified in one field in Saskatchewan. There was also one field with confirmed clubroot canola in Manitoba in 2005, but symptoms were of very low severity. No more infestations have been detected in Manitoba canola since then.
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.
Clubroot is caused by the pathogen Plasmodiophora brassicai. This agent is regarded as a protist, which means it is an organism with plant, animal, and fungal characteristics. P. brassicai 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. brassicai is shown in Figure 2.
The pathogen overwinters in the soil as very hardy resting spores. In the spring, germination of these resting spores is stimulated by secretions from the growing roots of potential host plants. If a suitable host is not present, the resting spores remain dormant in soil. Germination of resting spores produces another type of spore (zoospores) that swim in soil water until they come into contact with the root hairs of a host plant.
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 relying on random distribution methods like wind or rain. The swimming zoospore phase is short-lived.
After initial infection through root hairs or wounds, 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.
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. 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, the half-life of the resting spores is about four years, which means that after four years approximately half of the resting spores will no longer be viable if there has been no additional opportunity for clubroot to go through its entire lifecycle and add more spores. This represents a significant decline in inoculum in the field, but the more spores that start the infection, the more that will remain after four years. This is why it is important to prevent major infestations from becoming established in a field. Longer periods will be required to reduce inoculum to manageable levels.
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.
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.
A rule of thumb is that yield losses due to clubroot are about half of the percentage of infected stems. For example, if almost 100% of plants in a field are infected, expect about 50% yield loss, while infestations of 10-20% of plants should lead to 5-10% yield loss. This pattern of yield loss is very similar to sclerotinia stem rot in canola. Both clubroot and sclerotinia restrict the flow of water and nutrients from roots to aboveground plant tissues. Severe field infestations in Alberta have caused total yield loss (not worth harvesting) in a few cases.