Canola Diseases: Clubroot

The pathogen causing clubroot, Plasmodiophora brassicae, is a soil-borne organism in the taxonomic kingdom Protista. Compared with other pathogen groups (such as fungi, bacteria and viruses), Protista pathogens are relatively rare, and their biology and the management strategies to manage them are somewhat unique.

P. brassicae affects cruciferous plants including crops such as broccoli, Brussels sprouts, cabbage, camelina, canola, cauliflower, mustards, radishes and kale, and weeds such as shepherd’s purse, pepperweed, Western wallflower, tumble mustard, stinkweed and volunteer canola. The pathogen is genetically variable and can develop different pathotypes (similar to races or strains) that overcome resistance genes. Consequently, robust weed control and hybrid selection are very important for management.

Symptoms of clubroot can include wilting, stunting, reduced seed production, a thin stand and premature plant death (Figures 1-3). However, above-ground symptoms do not always occur and are not diagnostic of the disease. Additionally, above-ground symptoms of clubroot resemble many other ailments, including moisture stress (flooding or drought) and infection with other diseases (blackleg, sclerotinia stem rot, etc.).

Figure 1. Area of field with clubroot resulting in a stunting, wilting and thin stand. (Venkata Chapara, NDSU)

P. brassicae colonizes root systems, compromising the moisture and nutrient uptake of the plant. When scouting, whole root masses should be dug up and carefully examined, rather than pulled at the base of the stem. Infected roots will appear deformed with swollen areas of tissue called galls. Galls will be small as they begin to develop but may be up to 1 inch in diameter by the time of maturity. Infected roots may have just one or a few galls in discrete areas of the root tissue (resembling clubs) or numerous galls that impact nearly the entire root system (Figures 4 and 5). Galls commonly lose integrity as the season progresses, and by plant maturity are brittle and dusty when broken. 

The pathogen survives as robust resting spores, which can remain viable for more than a decade in the soil. In the spring, resting spores germinate to produce motile zoospores that swim through water in the soil.

Zoospores encyst on root hairs of susceptible plants and form a cell that (under a microscope) resembles an amoeba, called a plasmodium. The plasmodium invades cells and divides to become more zoospores that re-infect additional root tissue or are released into the soil and can infect neighboring plant roots.

Once infection occurs in the cortical tissue (central cell layer), the pathogen produces secondary plasmodia that cause plant cells to divide and swell, resulting in gall formation on the roots. The plasmodia eventually divide into millions of resting spores that are released back into the soil when galls rapidly disintegrate and are decayed by soil microbes.

High soil moisture, acidic soil (low pH) and temperatures around 68 to 77 F favor infection and disease severity. Poorly drained areas or parts of fields commonly flooded are more prone to infection. Although infection (and yield loss) can occur in soils at any pH, those with pH above 7.2 may have less damage.

P. brassicae is most commonly spread during its resting spore stage through infested soil that is stuck on farm equipment or other vehicles that enter infested fields. However, strong wind gusts and flooding also have been identified as capable of transferring spores. Consequently, new clubroot infestations are commonly observed first in field entrances (from soil moved on equipment), in low-lying or flood-prone areas and/or along shelter belts.

The best approach to managing clubroot is a combination of rotation of genetics for resistance, longer crop rotations, sanitation and excellent weed control. We recommend a six-year plan with two years of nonhost crops followed by a clubroot-resistant canola hybrid, then another two years of nonhost crops followed by a clubroot-susceptible canola hybrid or a clubroot-resistant hybrid that contains different resistant genes than the hybrid planted earlier (Figure 6).