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New strains of clubroot in oilseed rape in Canada

A ProMED-mail post
http://www.promedmail.org
ProMED-mail is a program of the
International Society for Infectious Diseases
http://www.isid.org

Date: Tue 5 Mar 2024
Source: Farmtario [summ. Mod.DHA, edited]
https://farmtario.com/crops/clubroot-pathotypes-overcoming-resistance-research-finds/

University of Guelph scientists report that the number of clubroot pathotypes is expanding. New pathotypes are evolving to overcome first-generation canola host resistance, the initial resistance to clubroot bred into canola varieties. This will require new management and monitoring techniques to keep disease spread in check.

Clubroot requires a multi-faceted approach due to its ability of survival in soil. Using clubroot-resistant canola varieties helps keep the resting spore populations low. It also selects rare pathotypes capable of overcoming genetic resistances in those varieties, so overall disease increases.

Many 2020 samples found pathotypes capable of overcoming first-generation resistance. Fortunately, second-generation resistance canola is available. To tell which kind of pathotype is in the field has been slow so far. A rapid molecular method to detect the pathotypes that can overcome first-generation clubroot resistance has now been developed. Further developments aim to identify pathotypes that can overcome second-generation resistance.

Different and rare pathotypes can exist in one place but not another. There are currently no suitable methods to assess a field's overall resting spore load. Limited data from mostly Western Canada indicate that wheat in rotation with canola can reduce resting spores faster.

[Byline: Diana Martin]

--
Communicated by:
ProMED

[Clubroot of Brassicaceae is caused by the fungus-like organism _Plasmodiophora brassicae_. It is a destructive, soil-borne disease affecting nearly all cultivated members of the family, including oilseed rape (_Brassica napus_; 'canola' designates a specific group of cultivars), cabbages, turnips, as well as many wild species which may serve as pathogen reservoirs. It is found worldwide and is most damaging in temperate regions and tropical highlands.

The pathogen enters through root hairs and wounded roots. It multiplies rapidly, causing abnormal enlargement of the underground stem, taproot or secondary roots ("clubs"). Affected roots often decay before the crop has matured. Depending on the timing of infection in the crop cycle, symptoms may include wilting, stunting and yellowing of plants; or premature ripening resulting in shriveled seeds. Due to the distortion of the roots, plants may wilt in dry weather and then recover at night.

Decaying roots release many resting spores, which can survive in the soil for a decade or more in the absence of a susceptible host plant. Thus, crop rotation of just a few years will not control clubroot. The disease can be spread with soil (e.g., on agricultural machinery), farming activities and infected plant debris. Use of clean planting material and phytosanitary measures to prevent spread between fields is essential. Disease management is difficult due to the longevity of the spores and the inaccessibility of underground plant parts to fungicides. Raising soil pH by addition of lime and fumigation treatments have been used with varying degrees of success (e.g., ProMED post 20120521.1139354).

The species is composed of numerous pathotypes which makes breeding crop cultivars with durable resistance difficult. It is therefore important to use multiple approaches to disease management simultaneously to counteract pathogen adaptation and extend the useful duration of both host resistances and effectiveness of fungicides.

Pictures
Clubroot symptoms on roots of oilseed rape:
https://www.realagriculture.com/wp-content/uploads/2012/08/club-root.jpg and
https://www.agric.wa.gov.au/sites/gateway/files/styles/original/public/Club%20root.JPG
Clubroot-affected oilseed rape plants:
https://www.canolacouncil.org/wp-content/uploads/2020/10/Clubroot-rotation-effect-cont.-canola.jpg (resistant vs. sensitive cultivars)
Clubroot symptoms on other brassica crops:
https://pnwhandbooks.org/sites/pnwhandbooks/files/plant/images/broccoli-brassica-oleracea-clubroot/broccolitransplantclubroot.jpg,
https://c8.alamy.com/comp/BG75EM/clubroot-plasmodiophora-brassica-distorted-root-on-a-cabbage-plant-BG75EM.jpg and
https://www.omafra.gov.on.ca/IPM/images/brassica/diseases/clubroot_brussels_zoom.jpg (affected field)

Links
Information on clubroot of oilseed rape:
https://www.canolacouncil.org/canola-encyclopedia/diseases/clubroot/,
https://www.alberta.ca/clubroot-disease-of-canola-and-mustard,
https://www.canolacouncil.org/canola-watch/2013/04/04/rotations-role-in-clubroot-management/,
https://doi.org/10.1080/07060661.2013.860398 and
https://doi.org/10.1111/ppa.12949
Information on clubroot on crucifer crops:
https://www.agric.wa.gov.au/broccoli/managing-clubroot-vegetable-brassica-crops and
https://cropscience.bayer.co.uk/threats/diseases/field-brassica-diseases/clubroot-brassicas/
Clubroot disease cycle:
https://www.gov.mb.ca/agriculture/crops/plant-diseases/images/fac63s00a_sm.jpg
_P. brassicae_ taxonomy:
http://www.indexfungorum.org/Names/NamesRecord.asp?RecordID=214750
- Mod.DHA

ProMED map:
Canada: https://promedmail.org/promed-post?place=8715369,12]