Clubroot is a disease that affects most members of the Brassicacaeae family, particularly the Brassica genus. Key agricultural crops impacted by clubroot include broccoli, cabbage, rapeseed (canola) cauliflower, radishes, turnips, Brussels sprouts, rutabaga, and Chinese cabbage, as well as many members of the mustard family. Caused by the single-celled parasite Plasmodiophora brassicae, plants infected with clubroot grow galls within their root systems, which cause wilting, stunting, and death.
Clubroot’s key symptom is the club-like galls that form in the root systems of infected plants. Secondary symptoms such as yellowing and wilting of the plant may appear much later.
Clubroot is well-named, given the swollen, clubbed galls that grow in the root systems of infected plants—sometimes resembling the damage caused by root-knot nematodes.
In previous centuries, the finger-like growths were sometimes referred to as ‘finger and toe.’ The severity of symptoms can vary widely. Mild infections may produce a couple of small growths, while at the other end of the spectrum, the entire root systems of plants may be almost unrecognizable knots of swollen, knobby tissue.
One of the challenges posed by this disease when it comes to identifying and remedying it is the fact that the initial symptoms of the disease are out of sight. The disease may be extremely advanced in the course of its development before symptoms become evident above-ground.
As the infection spreads through the roots, they lose their ability to absorb water and nutrients. Eventually this causes leaves and stems to yellow, wilt, or grow stunted. Due to the lack of moisture, plants may wilt during sunny days and then recover somewhat during cooler nighttime hours.
The disease often doesn’t cause death on its own. Instead, the knobbed root systems become vulnerable to secondary pathogens, which cause the death of the roots, and eventually the entire plant. As the roots rot, they release inoculum into the soil, where it can spread to other plants.
Clubroot is caused by the pathogen Plasmodiophora brassicae, a eukaryotic single-celled organism.
Plasmodiophora brassicae was once considered to be a slime mold, but it has been identified as a type of single-celled organism known as a eukaryote (sometimes also referred to as a ‘protist’), which is not an animal, plant, fungus, or virus. P. brassicae is an obligate parasite, which means that it needs to infect a living plant in order to live and reproduce.
The spores of P. brassicae are found in soil, and can survive for 5 to 20 years while waiting to be brought into contact with a suitable host. The spores can be transported long distances by contaminated water, plants, or the feet of grazing animals. The spores will remain dormant until they detect desirable soil conditions—P. brassicae prefers moist, acidic soil conditions, with temperatures between 64 to 77 degrees Fahrenheit—in junction with exudates produced by the roots of suitable plant hosts.
When conditions are right, the resting spores will germinate, producing zoospores which can move through the soil using flagella. The zoospores will navigate to nearby root hairs or damaged roots, and can use either of these as an entrance into the roots.
Once inside the root, the zoospores have a new mission: to increase their numbers. The zoospores will reproduce rapidly, and then join together to form a plasmodium, and then a zoosporangia. The latter is used to produce many more zoospores. This second generation of zoospores then swims away, infecting other areas of the root, or navigating through the soil to adjacent roots, where the reproduction process is repeated.
The formation of one plasmodium after another alters the hormones of the infected plant, causing the clubbed galls for which the disease is named. Resting spores are produced and deposited within the galls, ready to be released into the soil when the infected root dies the gall decays.
The galls do double damage to infected plants, as (1) the galls disrupt the vascular system of the roots, leaving them unable to effectively transport the water and nutrients the plant needs to thrive, and (2) the growth of the galls requires the dedication of significant amounts of energy. Affected roots are quite vulnerable to secondary infections—especially from bacteria such as P. fluorescens and S. marcescens, as well as several species of fungi—frequently succumbing to the onslaught and decaying, releasing newly spawned inoculum into the soil. Consequently, infected plants are underdeveloped and sickly, and quite prone to wilting and secondary infections.
Diagnosis of the disease can be challenging due to the disparity in symptoms depending on when infection occurs. Plants that are infected at the seedling stage will be significantly stunted, and may die before maturation. But mature plants that are infected with clubroot may not show any signs of yellowing, wilting, or stunting. However, these plants often ripen prematurely, and produce shriveled, nonviable seeds.
Losses from clubroot can be quite significant. Some growers go by a rule of thumb: yield losses from clubroot are equal to roughly half the infection rate. Thus, a field where half the plants are infected will have yield losses of about 25%. But this isn’t a hard and fast rule, as 100% losses have been seen with extreme infestations.
Clubroot is readily misidentified as anything from heat stress to Fusarium wilt. But a preliminary diagnosis of clubroot can be easily made by digging up symptomatic plants and looking for the telltale galls.
Clubroot is not curable. Thus, treatment relies on implementing preventative regimes to control and contain the spread of the disease.
When an outbreak of clubroot has occurred, it is critical to control the spread of the disease by removing all infected plants, stumps, roots, dropped litter, and any other infected debris. Good sanitation is critical to preventing the spread of clubroot.
Analysis of an extensive outbreak of clubroot in Alberta, Canada in the early 2000s yielded an interesting observation: levels of the causal P. brassicae pathogen were highest at the entrances to fields, and dropped as you moved into the field, away from the entrance. The conclusion was that farm equipment played a significant role in the spread of the pathogen. Thus, when using farm equipment to remove infected plants or till fields where latent spores exist, all equipment should be thoroughly cleaned before being used elsewhere.
Alteration if your irrigation practices may also be necessary. Do not allow runoff from infected fields to pass into non-infected fields.
Because of the fondness of P. brassicae for acidic soils, liming has long been an effective means of controlling the disease. Applying lime annually, or any time the soil pH is below 7.2, can help make conditions more inhospitable to the pathogen. However, liming is most effective when inoculum levels are quite low. It is iineffective at stemming the advance of disease mid-outbreak. Also, high soil pH can result in boron deficiency, making occasional applications of boron necessary.
One of the best means of reducing soil inoculum levels is by rotating in plants not susceptible to clubroot infections. While the resting spores can survive for quite long, their half-life is about 3.6 years. Thus, avoiding all brassicas for a period of 5 years would allow inoculum levels to drop by more than 60%, giving newly planted brassicas a better chance at thriving when combined with liming and other preventative efforts.
As a reference, here is a simple chart of how much viable P. brassicae inoculum would be expected to remain in the soil after an outbreak and subsequent removal of all brassica crops:
- 1 Year: 82%
- 2 Years: 68%
- 3 Years: 56%
- 5 Years: 38%
- 7 Years: 26%
- 10 Years: 15%
- 15 Years: 6%
But growers using crop rotation to control the disease must be extremely careful to remove all weeds which can harbor the disease, such as mustard, wild radish, and arugula. Failing to do so can undermine the effectiveness of crop rotation. When long-term crop rotation is difficult, soil solarizing can expedite the lowering inoculum levels.
Lastly, as with most pathogens, P. brassicae is most at home in environments where there is little competition, and host plants are compromised due to poor nutrition. By stimulating both soil microbial activity and the health of the plants themselves, it becomes much more difficult for P. brassicae to colonize and spread through crops.
In clubroot outbreaks in California over the last few years, we have seen some success with a combined soil and foliar nutritional regime. This is not a formal recommendation or prescription for the treatment of clubroot, but simply a program that has had some success in the field, in terms of controlling clubroot, and increasing crop yields and quality.
The following should be applied as soon as possible. Fill a mix tank with at least 20 gallons of water, and gradually add the ingredients below in the order listed.
- Fusion 360 Soil – 30 gallons
- Integrity Z-422 – 2 quarts
- Integrity FG-Micros – 2 quarts
- Iota 0-0-1 – 2 quarts
- 10% Boron – 6 ounces
- Silicone surfactant – 1/3 ounce
Take a spray tank and fill it three-quarters full of water and start agitation before adding the materials listed below. Apply at a volume that delivers a fine mist. Do not drench plant tissues. The first 3 sprays should be applied at 5 to 7-day intervals. After that, apply at 7 to 10-day intervals.