Clubroot is yet another disease of Brassica family crops that can cause economic loss and is nearly impossible to fully eradicate. The causal organism, Plasmodiophora brassicae, remains present in the soil in a resting spore state. When soil conditions are right, and susceptible brassica roots are present, these spores will germinate into a motile zoospore, which will then infect the root hairs of Brassica seedlings.
The lifecycle of Plasmodiophora brassicae, a lower fungus that does not have true mycelium, involves this primary infection of zoospores, but also a necessary secondary infection where the plasmodium actually infect the roots themselves, causing rapid cell division and the formation of root galls. The resulting clubbed roots are diagnostic of the disease. Root galls also cause increased plant susceptibility to other secondary infections.
When the Plasmodiophora brassicae infected roots disintegrate, resting spores are released back into the soil, where they remain for many years. Although the actual lifespan is not known, six to eight years is considered to be the key amount of time needed to reduce resting spore populations to any significant degree.
Not many growers are able to maintain a 6- to 8-year crop rotation schedule for all Brassica crops. One mistake often made is not considering salad greens – arugula, turnip or mustard greens – as a part of the Brassica rotation, Alex Stone, Oregon State University, Department of Horticulture, said in a recent webinar on the disease.
“The rotation length for clubroot is something that people continually discuss,” Stone said. If you are relying heavily on rotation for management, she said, seven years as a minimum is recommended and includes all brassicas.
But a more integrated approach will provide the best results in eliminating clubroot losses. Although the disease can cause complete crop loss, it can also be present in tolerable amounts. If conditions for germination aren’t optimal, the disease will have lesser impact.
Clubroot requires a soil pH of below 7.0, enough soil moisture to allow the zoospores to move, and soil temperatures above 55 degrees Fahrenheit. Brassicas planted in the cooler months can often yield well despite the presence of the resting spores in the soil, Stone said, due to the cooler soil temperatures.
Enhancing soil physical properties by reducing tillage and increasing organic matter as well as via alleviating compaction will result in less water retention.
Soils that have a high organic matter are better equipped to drain excess water away from plant roots, decreasing the ability of the spores to infect plants. Managing irrigation to prevent saturated soils is also prudent. Enhancing soil pH above 7.0 is recommended in fields with high clubroot pressure.
Infected roots cannot absorb water or nutrients properly. The above-ground symptoms of clubroot include daytime wilting of plants – despite adequate soil moisture – which may then “recover” at night, stunting of growth and yellowing of leaves. But the definitive diagnostic symptom is the presence of those root galls, so pulling up plants strategically throughout the field is the proper way to scout.
There is no other pathogen that causes clubbed roots to develop, so clubroot is “an easy disease for farmers to diagnose by themselves,” Stone said.
Knowing whether clubroot spores are present in a field is an imperative first step to preventing future crop loss and exacerbation of the disease, even if symptoms aren’t currently present above ground. Spore pressure may be low, or soil conditions may be keeping spores in check in fields without symptoms.
“Scout all fields, not just those with the above-ground symptoms,” Stone said.
When scouting, wet areas take precedence, as the disease needs soil moisture to germinate. Water moving from one field to another, such as during a flood event, can readily transport clubroot spores.
The spores can also travel on tires, shovels, boots and other equipment. Scraping dirt off, sampling for spores, and using a bleach solution to disinfect equipment and gear coming out of infected fields is prudent.
Aaron Heinrich, also of the Oregon State University Department of Horticulture, co-presented the webinar with Stone. He discussed the proper use of lime as a clubroot preventative.
A soil pH of above 7.0 prevents clubroot spores from germinating. And, if infection is already in the root hairs of Brassica crops, increasing the pH will inhibit that important secondary infection, which leads to the formation of the root galls.
But liming for clubroot is much more precise and extensive than liming for crop production.
In order to be effective, the soil pH must uniformly be 7.0 or above, requiring very consistent mixing to prevent pockets of low and high pH. Soils must be tested at the depth of tillage, with a pH and pH buffer test needed.
The type of liming agent used is important. Dolocite, calcitic and prilled limestone will all have the same long-term effects on soil pH if mixed properly and applied at the needed rates. But the short-term reactions differ, Heinrich said.
The cost, the speed of the reaction, the availability and the equipment needed to spread and incorporate the products are factors to consider.
Another concern is the non-linear relationship between lime application rates and soil ph response. Above pH levels of 6.4, it will take larger amounts of lime to continue to raise the pH than at lower pH levels.
The pH of the soil must already be greater than 7.0 at the time of germination or transplanting to prevent clubroot infection. Lime needs to be wet in order to be activated; it is not mobile, so proper mixing and direct soil/lime contact is required. As the pH climbs above 7.0, the infection rate is reduced, as long as the lime has been well-incorporated, Heinrich said.
Trials with high soil pH after liming, but where mixing of lime was inconsistent, didn’t yield good control of clubroot.
But those where lime was incorporated via multiple tillage events showed better control even when the pH was not as high as in unevenly incorporated soils.
Fall application of lime is recommended, and lime should be applied at least 30 days prior to planting.
“Resistant cultivars are probably the best strategy,” Heinrich said. “Resistant plants can cause spore germination, but they don’t create new spores to increase the spore population in the future.”
This is because the presence of Brassica roots will cause the first zoospores to germinate and migrate to the root hairs, but the secondary infection that actually causes the clubroot symptoms won’t be able to infect the plant.
Without this vital part of the Plasmodiophora brassicae life cycle, new resting spores are not generated. Using resistant plants reduces the population of clubroot spores in the field.
But resistant cultivars aren’t numerous, and they are generally only resistant to certain pathotypes of the clubroot disease. There is also concern that resistance can be diminished due to pathogen adaptation.
Chemical control of clubroot is available, but not necessarily practical, with some common soil fumigants able to reduce clubroot spore populations. Chlorination of water sources may also help to control spore populations.