How to Identify, Control, and Prevent Clubroot

Clubroot strikes against cruciferous plants, reducing their ability to absorb water from the soil. If you fear you may have this rare problem in your garden, follow this guide by Huan Song on how to control and prevent clubroot.

Written by Huan Song Horticulture review by Sarah Jay Last updated: August 8, 2024 | 3 min read

A plant with vivid green leaves infected with clubroot, with rows of crops showing in the background

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Fans of Harry Potter are no doubt familiar with the mandrake. This magical plant has screaming roots and a cry that can be fatal for anyone who hears it. In our muggle world, some plants can develop a condition called clubroot. This disease gives the roots of the plants a similarly engorged appearance but is thankfully only fatal to the host plant.

Clubroot is a devastating crop disease that impacts many cruciferous vegetables such as cabbage and Brussels sprouts. Around 300 brassica species are particularly susceptible to this disease. In addition to the crops that we consume, processed plants (such as canola crops used to make oil) are also threatened.

Unfortunately, clubroot is a persistent soil pathogen. If it is found in your garden or growing site, it is extremely difficult to eradicate. Its resting spores will likely remain indefinitely, poised to strike suitable future hosts. Crop rotation, selecting resistant cultivars, and good soil management are key to reducing the spread of this disease.

What Is Clubroot?

Plant infected by fungal disease held up to see it clearly, with may trees and the clear blue sky in the background on a sunny day — It is not common but causes significant damage when it emerges.

Clubroot is a plant disease found in brassica crops as a result of the eukaryotic protist pathogen Plasmodiophora brassicae. This type of soil-borne disease is distinct from fungal, bacterial, and viral plant diseases and is relatively rare. However, there are several notable protist plant pathogens in addition to P. brassicae that impact other economically important crops such as potatoes, beets, and grains.

P. brassicae specifically grows within the root cells of infected plants. It causes the roots to form distinctive galls or club-shaped deformations. This parasite reprograms the cellular functions of its host cells to divert nutrients away from the plant in order to help with its own spore formation and reproduction.

Eventually, the roots of the infected plants will die, disintegrate, and release these spores into the soil. The resting spores can stay in the soil for up to 20 years! P. brassicae is an obligate parasite that must feed on living plant tissues and cannot live without a host.

Life Cycle

A person wearing pink work clothes holding an infected cabbage with both hands while other crops appear in the background — Its life cycle is more complex than similar pathogens.

Plasmodiophorids have a complex life cycle that’s not entirely understood by scientists. In fact, the genome of P. brassicae was only recently sequenced. What researchers do know about its disease cycle is that it involves three processes: zoosporic stages, formation of plasmodia inside host cells, and resting spore formation.

The cycle starts with resting spores releasing primary zoospores that infect the root hairs of host plants. These zoospores penetrate the cell walls of root hair cells and form the primary plasmodia, or a mass of cells.

The primary plasmodia produce zoospores and then release these secondary zoospores. When the primary or secondary zoospores reach the root cellular cortex, they develop into a secondary plasmodium which triggers swollen roots in diseased plants.

After further growth, the secondary plasmodium eventually divides into multinuclear plasmodia and finally becomes resting spores which get released into the soil during plant decay.

Symptoms

Someone holding an infected green crop with one hand, having clumps of brown soil attached to one end and other vegetables in the background — It impacts the plant’s leaves and yield.

Above-ground symptoms of clubroot disease are similar to many other diseases. Affected plants develop yellowing leaves, wilting, and reduced production. Eventually, you’ll see stunted growth and even premature plant death.

P. brassicae disrupts the root system of plants. The roots are not able to effectively take up water and nutrients to support the rest of the plant. Infected plant roots develop galls or swollen, club-like structures. During the early stages, the root galls may be small but will expand to the entire root system.

Infected plants later in the season may have very little root system remaining as the roots become brittle. P. brassicae zoospores are then released into the soil.

If you suspect your plants may have clubroot from contaminated soil, farm machinery, or infected transplants, carefully dig up the plant to make sure the root hairs and root system stay intact for close inspection.

Control

Someone using green rake to clean up dry leaves from ground appearing dark brown under the sun with green grass growing for soil — Clean the area and collect infected plants for proper disposal, including parts usually used for composting.

Do not compost diseased plants in the regular compost pile. Remove any susceptible plants or cabbage family weeds, like wild mustard, as they might also be host plants.

Infected soil should not be used to grow any plants in the cabbage family for at least five to seven years. However, it is still suitable to grow nonhost crops.

To prevent contaminated equipment or tools from spreading the pathogen, sanitize with a 1:9 solution of bleach and water. While clubroot is typically transmitted through farm equipment, spores can also be transported by strong winds and soil movement through water erosion.

Prevention

A pH meter appears as a handheld device with a digital display and a probe attached by a wire, used for measuring the acidity or alkalinity of a solution. — Test the soil to check if it is ideal for this disease to start and spread.

Prevention is absolutely key for clubroot because it is difficult to eradicate from infested soil. Long-term control and mitigation are needed for this disease.

High soil moisture, low soil pH, and soil temperatures between 68-77°F (20-25°C) can all exacerbate the impact of clubroot. One way to remedy acidic soils and help prevent clubroot is to add lime to raise the soil pH above 7, out of the acid soil range. Another method is to improve the overall soil draining. Plants in soil with poor drainage are more susceptible to root damage in general.

Using good garden sanitation and IPM measures to control root-feeding pests is always a good preventative measure. There are also clubroot-resilient hybrids of broccoli, cauliflower, Chinese cabbage, and rutabaga that are currently available. Finally, if using transplants, grow them in sanitized soil or a soilless medium to prevent transmission of soil diseases including clubroot.

Frequently Asked Questions

Clubroot (Plasmodiophora brassicae) is nearly impossible to eradicate once you have infested soil due to the longevity of the resting spores. Crop rotation with nonhost plants is a must and can help to decrease the P. brassicae inoculation of the soil over time.

No, clubroot, a disease caused by Plasmodiophora brassicae, is a protist plant pathogen. Protists are unicellular organisms whereas fungi are multicellular. Protists and fungi are two separate kingdoms of organisms. P. brassicae has more amoeba-like cells than fungal cells.

Clubroot is a parasitic disease that affects the plant roots of susceptible hosts in the cabbage family which includes canola. Clubroot is a serious threat to canola production around the world because it destroys the roots of the plants and may lead to plant death.