Vexed by Clubroot? Tips from an Expert

Posted: 5/10/2019

By Luis Del Rio Mendoza, Ph.D.

What is clubroot?
Clubroot is a disease that affects the roots of plants from the botanical family of crucifer crops. This family includes broccoli, cabbage, canola, cauliflower, kale, radish, mustard, among others. It can also hit plants considered weeds, like Shepard’s purse and wild mustard. The causal organism requires a living plant to reproduce and complete its lifecycle; however, in the absence of a host, it can survive in soil as a cyst for many years. Cysts are resting spores that can germinate in the proximity of roots of host plants. Upon germination, “zoospores” emerging from cysts will swim towards root hairs and penetrate. 

Clubroot development is influenced by physical characteristics of soil like temperature, moisture content, and pH (acidity) as well as by the type of microorganisms living in it. Optimum soil temperatures for clubroot development are 68-77° F (20-25° C). Little infection occurs in soils with temperatures below 59 ° F (15° C). In general, soil pH from 6 to 6.5 is considered optimum for clubroot development. However, the disease can develop and become severe under alkaline conditions (7.5+ pH). 

How do I know if my field has clubroot?
Growers should inspect their fields for areas where plants are dying prematurely. These areas will be more evident during and after flowering. Pay special attention to low areas and edges of fields.. The inspection should entail using a shovel to loosen soil around a plant and gently pulling it to inspect its roots. Diseased plants will have deformed, stubby roots with galls that are creamy in color when fresh and brittle and brown when old. Materials can be sent to specialized laboratories for a more definitive diagnosis. 

 My field is infested with clubroot; what now?
Once clubroot is confirmed in canola, growers should work infested fields last and clean their equipment afterwards. This will reduce the possibility of transferring clubroot to other fields. While it is almost impossible to completely stop disease movement, the most common way to do it is on chunks of soil stuck on equipment.

Moving forward, develop a crop rotation that includes planting canola every three to four years with clubroot-resistant hybrids. Also, otate these hybrids. Planting the same clubroot-resistant cultivar more than twice, especially in tight rotations (every one or two years) can stimulate development of clubroot strains capable of infecting the resistant cultivars (this recently happened in Alberta, Canada). Consult your local extension specialist for information on clubroot-resistant hybrids.

Chemical control of clubroot is not economical. While a few fungicides have proven to be effective, their application is impractical due to the elevated doses and costs required. Instead, focus on containing and phasing out clubroot with diligent cleaning and rotations.

Luis Del Rio Mendoza, Ph.D., is professor of plant pathology at North Dakota State University in Fargo, N.D.

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