17 Jul Canola Clubroot: What You Need To Know This Season
Last week on Growing Possibilities we discussed the yield-limiting canola disease Sclerotinia Stem Rot. This week, we are continuing with this theme of canola disease and discussing what is likely the most destructive disease North American canola growers have to worry about: Clubroot.
Clubroot is caused by the soil-borne pathogen Plasmodiophora brassicae, which colonizes canola roots and prevents uptake of water and nutrients by their host plants. This infection begins in the root hairs before eventually invading root tissues and causing large growths or galls to form on roots (1). These galls proliferate, producing new spores that deposit into the surrounding soil that can be detrimental to canola crops for years afterward. Clubroot has become an issue in all canola growing areas of Canada, first appearing in the early 2000s, and as of 2013 has been diagnosed in canola fields in North Dakota (2). This year, several rural municipalities in Manitoba have had fields with a high spore distribution, with several of these areas having canola crops that are symptomatic for Clubroot (Click Here for a full map of Clubroot distribution in Manitoba).
So what can be done about Clubroot?
Genes for P. brassicae resistance have been isolated in canola, and have led to the production of numerous commercial seed varieties offering Clubroot resistance (Click Here for a list of available varieties), but there are currently no economical control measures for plants/fields in which Clubroot has already established (2). Crop rotation has been shown to limit Clubroot incidences in canola, but the underground threat of P. brassicae spores means that contaminated soil can still lead to infestations. Movement of spore-containing soil is the main means of spreading Clubroot infestations from field-to-field, with soil from machinery and implements having the potential to spread spores even in non-canola years. Because of this, field entrances (where contact with machinery is the highest) are often the most at-risk areas of a field for establishment of P. brassicae infestations (1)
Scouting for Clubroot and soil testing are the main measures for identifying this threat in your fields. Soil testing can be done earlier in the season than scouting, but relies on a large number of samples to get accurate information, something that is not practical for most growers. Scouting for galls on your plant roots must be done later in the season, as galls take 6-8 weeks to form after infection begins. Scouting a couple weeks before swathing ensures galls are large enough to be noticed (1). Above-ground signs of disease include yellowing and wilting of plants, stunted growth, and premature death, but these symptoms occur only after galls have developed, and are sometimes only noticeable in severe cases (2).
Recent research has shown the potential for a biocontrol agent against P. brassicae using bacteria species. Two Bacillus strains were shown to limit Clubroot disease severity by more than 40% and were shown to inhibit early root hair infections and prevent P. brassicae spores from forming secondary zoosporangia, stopping their life cycle (3). This research gives hope for an effective and eco-friendly treatment against Clubroot, which would be a huge breakthrough in treating this disease.
2) Hwang, S. F., Strelkov, S. E., Feng, J., Gossen, B. D., & Howard, R. J. 2012. Plasmodiophora brassicae: a review of an emerging pathogen of the Canadian canola (Brassica napus) crop. Molecular plant pathology 13(2): 105–113. https://doi.org/10.1111/j.1364-3703.2011.00729.x
3) Zhu, M., He, Y., Li, Y., Ren, T., Liu, H., Hunag, J., Jiang, D., Hsiang, T., Zheng, L. 2020. Two New Biocontrol Agents Against Clubroot Caused by Plasmodiophora brassicae. Front Microbiol 10:3099. doi:10.3389/fmicb.2019.03099