By Carolyn King
Comparison of clubroot-resistant (left) and clubroot-susceptible (right) cultivars at flowering stage.
Photo courtesy of Sheau-Fang Hwang, ARD.
Within a few years of clubroot’s discovery in a canola field near Edmonton in 2003, the disease became a serious problem for Alberta canola production. The emergence of this devastating disease on the Prairies has triggered intensive research, including investigation of possible tools for integrated management of clubroot.
“Ideally for most diseases we want to use integrated management. That means integrating chemical controls, such as fungicides, and cultural controls, like planting practices and using resistant cultivars, and biological controls, like a microbe that kills a pathogen,” notes Dr. Sheau-Fang Hwang, a plant pathologist with Alberta Agriculture and Rural Development (AARD).
Even though a single control measure may work well in the short term, integrated management is a better approach in the long run. “For example, if you rely on a fungicide too much, you might cause issues like fungicide resistance or environmental pollution,” explains Hwang.
Clubroot is caused by Plasmodiophora brassicae, a soil-borne organism. The pathogen requires moist conditions to infect a plant, and it flourishes in warm soils around 25 C (temperatures below about 15 C inhibit its development). The pathogen attacks canola and other cruciferous plants, causing irregular swellings, or galls, to form on the roots. Those galls prevent water and nutrients from moving up into the rest of the plant, so the plant withers and dies.
Fortunately, several clubroot-resistant canola cultivars are now available. They are the foundation of any clubroot management strategy. But resistant cultivars aren’t a magic bullet. Hwang’s research has shown that a small percentage of the plants from a resistant cultivar’s seed will develop galls when exposed to the pathogen. As well, the pathogen could potentially evolve to overcome the resistance genes if a resistant cultivar is grown in the same field year after year.
To find ways to further strengthen the fight against clubroot, Hwang and her colleagues at AARD, the University of Alberta, and Agriculture and Agri-Food Canada (AAFC) conducted several trials to assess the effects of fungicides, seeding date and seedling age for suppressing clubroot.
Fungicides to control seed-borne transmission
Some of the initial questions from producers about clubroot related to seed-borne transmission of the disease. “So the objective of one of our trials was to see whether or not a seed treatment might reduce the risk of clubroot if you have contaminated canola seed,” says Hwang.
In this greenhouse trial, the researchers artificially infested seeds of a clubroot-susceptible canola cultivar, using infestation levels much higher than would occur in naturally infested seeds. Then they compared five fungicidal seed treatments: Helix Xtra, Dynasty 100 FS, Prosper FX, Nebijin 5SC, and Vitavax RS; for the control treatment, they applied water instead of a fungicide. The researchers planted the seeds in pots of clubroot-free growing materials, and allowed the seedlings to grow. Then they evaluated the disease level in the roots.
All of the fungicides were able to significantly reduce the disease levels in the seedlings, in comparison to the control. So, this trial confirmed that contaminated seed can transmit the disease and that fungicidal seed treatments will likely prevent the problem. “Given the relatively low levels of clubroot infestation found in naturally infested seeds, any of the seed treatments used in this study would likely eliminate the risk of seed-borne transmission,” says Hwang.
Fungicides for reducing disease in infested soil
In another trial, the researchers assessed the effects of fungicide seed treatments on a resistant and a susceptible canola cultivar grown in clubroot-infested soil.
The sites for this field trial were at St. Albert and Leduc, Alta. The researchers applied various seed treatments: Helix Xtra (fungicide plus insecticide), Dynasty 100 FS (fungicide), Prosper FX (fungicide plus insecticide), and Sedaxane (fungicide). They applied Cruiser 5 FS, an insecticide seed treatment, as the control treatment.
For the susceptible cultivar, none of the treatments reduced clubroot severity or improved seedling emergence or crop yield in comparison to the control. Similarly, for the resistant cultivar, none of the treatments improved emergence or yield compared to the control.
“When you treat seeds with a fungicide, the protection lasts for perhaps two to four weeks, so the treatments work well for controlling seedling blights. But clubroot spores in the soil can affect the plant at any growth stage, as long as the moisture and temperature conditions are favourable,” explains Hwang.
“None of the existing seed treatments can protect canola plants for the entire growing season. It may be a possibility for the future, but so far the chemical companies have not developed formulations that would allow the fungicide to slowly release into the soil, so it could protect against clubroot for the whole growing season.”
Adjusting seeding date and seedling age
In another field trial, the researchers evaluated the effect of seeding date on clubroot impacts. The field sites were at St. Albert and Leduc in 2008, and at Edmonton and Leduc in 2010. At each site, they planted several canola varieties at three seeding dates, ranging from mid-May to early or mid-June.
With earlier seeding, clubroot severity tended to be lower and canola yield tended to be higher.
“The results were no surprise to us,” notes Hwang. “Early seeding is always good for canola yields. In addition, early seeded crops usually are planted into cool soils, which clubroot does not like.”
In a greenhouse experiment in 2009, the researchers examined the effect of inoculating canola seedlings at different ages with clubroot. At zero, one, two, three, or four weeks after seeding, they inoculated the plant’s potting mix with clubroot resting spores. When the plants reached maturity, the researchers evaluated the seed yields and the level of disease in the roots.
For both the resistant and susceptible cultivars, the plants that were infected at the later growth stages did better – the disease was less severe and the plants had better seed yields.
This finding supports the value of early seeding as part of a clubroot management strategy. Hwang says, “If you seed early, then the soil temperature is cooler so the resting spores germinate later. So the plant has a chance to grow bigger before infection occurs and when it is bigger it has a greater ability to defend itself from pathogen.”
She adds, “In the future, perhaps we’ll have a chemical treatment or a biological agent that will delay the infection and reduce the impact on seed yield.”
The results from these studies highlight two practices that help suppress clubroot: seeding early, and using disease-free seed. “It’s always a good idea to buy certified seed to make sure the seed is not externally contaminated. If you don’t buy certified seed, then you should use fungicide-treated seed to reduce the risk of getting clubroot,” says Hwang.
However, she emphasizes the two seeding practices by themselves are not enough to manage clubroot. They must be part of an integrated strategy, with clubroot-resistant cultivars as the major weapon against the pathogen.
“Resistant cultivars are the most economically feasible and environmentally friendly approach to clubroot management. And you need to do everything you can to stop the pathogen from breaking down the plant’s resistance. That includes rotating your crops and rotating your resistant cultivars.”