Clubroot Disease of Canola and Mustard

Clubroot is a serious soil-borne disease of cruciferous crops (canola and cabbage family) worldwide and was first identified in Europe in the thirteenth century. This disease is a major problem in cole crops (cruciferous vegetables) in some areas of British Columbia, Quebec, Ontario and the Atlantic provinces.

There have been two previous reports of clubroot in cole crops in Alberta. So, clubroot is not a new disease in Canada or Alberta. However, in 2003, clubroot was confirmed in several canola fields near Edmonton, Alberta, which was the first report on canola in western Canada.

Clubroot has continued to spread in the Edmonton area, mainly in the counties of Sturgeon, Parkland, Leduc and Strathcona.

Click here to see where clubroot has been found in Alberta.

The disease can affect broccoli, Brussels sprouts, cabbage, cauliflower, Chinese cabbage, kale, kohlrabi, radish, rutabaga and turnip. Canola/rapeseed and mustard are also susceptible to this disease. There are several weak, non-cruciferous hosts, but their contribution to disease development and carryover of the clubroot pathogen is not well known.

Clubroot was added as a declared pest to Alberta’s Agricultural Pests Act (APA) in April 2007. The APA is the legislative authority for enforcement of control measures for declared pests in Alberta. The Minister of Alberta Agriculture and Food is responsible for this Act.

However, enforcement of pest control measures is the responsibility of the municipal authority, and Agricultural Fieldmen are responsible for enforcing pest control measures in their municipalities. Pest inspectors have the power to enter land at a reasonable hour, without permission, to inspect for clubroot and collect samples. The owner or occupant of the land has the responsibility for taking measures to prevent the establishment and spread of clubroot.

This factsheet contains current information about clubroot in canola and describes options for Canadian canola growers to prevent this disease from being introduced and becoming well established in their fields.

The Disease Cycle

The causal agent of clubroot is Plasmodiophora brassicae Woronin. In the past, this agent has been classified as a slime mould fungus (myxomycete), but more recently, it is regarded as a protist (an organism with plant, animal and fungal characteristics).

There are normally several different races or pathotypes in established infestations. Plasmodiophora brassicae is an obligate parasite, which means the pathogen cannot grow and multiply without a living host. The life cycle of P. brassicae is shown in Figure 1.


Figure 1. Life cycle of Plasmodiophora brassicae, the pathogen that causes clubroot (source: Ohio State University).

Resting spores germinate in the spring, producing zoospores that swim in soil water to root hairs. These resting spores are extremely long lived, with a half-life of about 4 years, but they can survive in soil for up to 20 years. For example, Swedish research in clubroot-infested spring rapeseed fields found that 17 years were needed to reduce the infestation to non-detectable limits.

The longevity of the resting spores is a key factor contributing to the seriousness of the disease. Resting spore germination is stimulated by exudates from the roots of host plants.

After the initial infection through root hairs or wounds, the pathogen forms an amoeba-like cell. This abnormal cell multiplies and then joins with others to form a plasmodium, which is 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 Figures 2, 3 and 4).

Figure 2. Very severe clubroot on canola. Photo courtesy of Kelly Turkington		Figure 3. Severe clubroot galls or “clubs” on canola root. Photo courtesy of Kelly Turkington
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Figure 4. Moderately infected canola root. Photo courtesy of Valeria Sowiak

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Some amoeba-like cells are able to move up and down roots in vascular tissue. After the secondary plasmodia mature, they divide into many resting spores. The galls are quickly decayed by soil microbes, leaving millions of resting spores in the soil.

Although there are no airborne spores released by this pathogen, the resting spores are capable of moving with infested soil transported by wind or water erosion and field machinery.

Warm soil (20-24°C), high soil moisture and acid soil (pH less than 6.5) are environmental factors that favour infection and severe disease development. Unfortunately, these conditions exist in a significant portion of the traditional canola growing areas of Alberta.

High soil moisture areas of the field typically have the most severe infestations. These wet areas are found in depressions, spots with higher clay content or with subsoil horizons that cause poor water infiltration (such as Gray Wooded or solonetzic soils).

Clubroot Symptoms on Canola and Mustard

Clubroot galls are a nutrient sink, so they tie up nutrients, and severely infected roots of canola cannot transport sufficient water and nutrients for aboveground plant parts. Symptoms will vary depending on the growth stage of the crop when infection occurs. Early infection at the seedling stage can result in wilting, stunting and yellowing of canola plants in the late rosette to early podding stage.

Such symptoms may be wrongly attributed to heat stress during periods with high temperatures or to other diseases such as blackleg or fusarium wilt. In such cases, proper diagnosis includes digging up wilted plants to check for gall formation on roots (see Figures 2, 3 and 4).

Infection that occurs at later crop stages may not show plant wilting, stunting or yellowing. However, infected plants will ripen prematurely and seeds will shrivel. Thus, yield and quality (oil content) are reduced.

Swedish researchers found that infestations nearing 100 per cent affected plants caused about 50 per cent yield loss, while infestations of 10 to 20 per cent led to 5 to 10 per cent yield loss. This result is similar to sclerotinia stem rot infection in canola in which, as a general rule of thumb, the yield loss estimate is half of the percentage of infected stems. This comparison is a reasonable one since both diseases restrict the flow of water and nutrients to developing seeds.

Patches of prematurely ripening canola due to clubroot infection (Figure 5) could be confused with other diseases such as sclerotinia, blackleg and fusarium wilt. In such cases, proper diagnosis should include digging up affected plants to check for gall formation on roots.


Figure 5. Patchy, premature ripening.
Photo courtesy of Stephen Strelkov

If the suspected plants are not sampled until after swathing, the root galls may have decayed already, and typical whitish galls will no longer be present (see Figure 6). Instead, the decayed galls cause roots to have a brown peaty appearance rather than a healthy white colour, which should be a signal to carefully dig up more roots for closer inspection.


Figure 6. Decayed clubroot galls and whitish stem appearance.
Photo courtesy of Kelly Turkington

Hybridization nodules on canola roots (see Figure 7), although rare, could be confused with clubroot galls, but they appear as small, round nodules located at root nodes. The interior texture of a clubroot gall is spongy or marbled while hybridization nodules are uniformly dense inside, like healthy roots. Furthermore, hybridization nodules will not decay rapidly to a peaty appearance like clubroot galls do.


Figure 7. Hybridization nodules on canola root.
Photo courtesy of Rob Dunn

For confirmation of suspected canola root galls, send samples to:

Dr. Stephen Strelkov
410 Agriculture/Forestry Centre
Department of Agricultural, Food and Nutritional Science
University of Alberta
Edmonton, Alberta, T6G 2P5

Prevention and Management of Clubroot in Canola and Mustard

Since clubroot infestations are still not widely distributed in Alberta, various precautionary measures should be taken by producers to curb the spread of this disease outside the known infested areas. Growers should be vigilant to prevent the introduction of clubroot to clean fields because few economic options currently exist to control infestations in canola.

Recommended preventative measures include the following:

• Use long rotations: canola every four years or more. Although this practice will not prevent the introduction of clubroot to clean fields, it will restrict this and other disease development within the field and probably avert a severe infestation.
• Practice good sanitation to restrict the movement of possibly contaminated material (this approach will help reduce the spread of other diseases, weeds and insects too). The resting spores are most likely to spread via contaminated soil and infected canola plant parts. Thus, producers should follow the practice of cleaning soil and crop debris from field equipment before entering or leaving all fields. The equipment cleaning procedure involves knocking or scraping off soil lumps and sweeping off loose soil.
  
  For risk-averse producers, the following additional cleaning steps may provide some extra benefit, but they involve considerably more work and expense:
  - After removing soil lumps, wash the equipment off with a power washer, preferably with hot water or steam.
  - Finish by misting equipment with a weak disinfectant (1-2% household bleach solution).
• Use direct seeding and other soil conservation practices to reduce erosion. Resting spores move readily in soil transported by wind or water erosion and by overland flow.
• Scout canola fields regularly and carefully. Identify causes of wilting, stunting, yellowing and premature ripening – do not assume anything!
• Avoid the use of straw bales and manure from infested or suspicious areas. Clubroot spores are reported to survive through the digestive tracts of livestock.
• Avoid common untreated seed (including canola, cereals and pulses). Earth-tag on seed from infested fields could introduce resting spores to clean fields. Certain seed treatment fungicide may control spores on contaminated seed, but this observation needs further research to confirm its validity. Note: the risk of spreading clubroot by contaminated seed or straw is much less than by the transportation of contaminated field equipment.