Clubroot in Canola
Clubroot in Canola
The causal agent of clubroot is Plasmodiophora brassicae Woronin. There are normally several different races or pathotypes present in established infestations. Plasmodiophora brassicae is an obligate parasite, which means the pathogen cannot grow and multiply without a living host. 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.
Appearance: Clubroot galls are a nutrient sink, so they tie up nutrients, and severely infected roots of canola cannot transport sufficient water and nutrients for above ground 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. Infection that occurs at later 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. Patches of prematurely ripening canola due to clubroot infection 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. Swathing is an excellent opportunity to spot clubroot infestations.
Disease Cycle: Resting spores germinate in the spring, producing zoospores that swim very short distances 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. 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 unusual 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 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. 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 within the gall tissue. The galls are quickly decayed by soil microbes, leaving millions of resting spores in the soil.
Disease Control: Clubroot is very hard to control. The primary step for management and long-term control is exclusion of the disease. Good sanitation practices are important with regard to the use of tools and machinery in order to prevent the introduction of the pathogen to a disease-free field. Although it is difficult to eradicate the pathogen once it is introduced to a field, there are several methods for its control. Keeping the soil at a slightly basic pH of 7.1 – 7.2 by the addition of agricultural lime as well as the integration of crop rotation will reduce the occurrence of clubroot in already infected fields. Fumigation using metam-sodium in a field containing diseased cabbages is yet another way to decrease the buildup of the pathogen.