A new study shows that even after a plant infected with clubroot has been killed with a herbicide and frozen to ensure all plant cells are dead, the pathogen survives and continues to mature.
The study led by Mary Ruth McDonald, a professor of plant agriculture at the University of Guelph, and Agriculture Canada scientist Bruce Gossen, has major implications for canola producers.
It means infected volunteer canola, or any other clubroot-susceptible weeds such as shepherd’s purse and pennycress, must be pulled within three weeks of germination to prevent pathogen buildup in the soil. If infected plants are mowed or sprayed, even after only four to five weeks following germination, the clubroot pathogen Plasmodiophora brassicae can finish its life cycle and produce resting spores that persist in the soil for up to two decades.
In the study, the plants were sprayed with glyphosate. In one treatment, plant stems were pulled from the field, leaving clubs in the ground. In the others, the plants were pulled, and the clubroot-infested roots harvested and frozen.
McDonald says the organism that causes clubroot is an “obligate parasite” that needs a living plant to infect, grow and reproduce.
“With a lot of those organisms, if you kill the plant, (the pathogen) stops dead. But in this case, we showed that once the clubroot organism infects and develops to a certain stage, even if the plant cells were all dead, the organism continued to mature. The resting spores were still perfectly alive and could go back into the soil and cause disease.”
Freezing the plant actually sped up cell maturation in the trial. One hypothesis was that by freezing and then thawing host plants a few days later, the pathogen can harvest a “burst of nutrients” to use for its final stage of maturity. McDonald says another hypothesis is that plant hosts naturally “push back” at pathogens and attempt to limit their growth, but if a host is killed the pathogen has no obstacles in the way of obtaining nutrients and finishing its life cycle.
Only one other study, a 2018 European study on winter oilseed rape, has found a similar result, McDonald says. The findings help explain why so many producers continue to find clubroot in their fields even after the recommended two-year rotation away from canola.
Rotation is absolutely key to clubroot control. In Alberta, soils can have between 10 and 100 million resting spores per gram. A two-year rotation away from susceptible crops will typically reduce the pathogen load by up to about 90 per cent, which can still leave more than a million hardy spores that can persist in soils for 10 to 20 years.
McDonald says the 90 per cent reduction is essential, but from the standpoint of clubroot control there’s little advantage to lengthening rotations beyond two years. Importantly, though, for those two years to be effective they need to be “real rotations” without volunteer canola and other susceptible weeds.
A single infected plant can have a billion resting spores. If producers spot these weeds in the field after they’ve been growing for a month or two, killing them does nothing to stop pathogen buildup in the soil.
“Killing the plant doesn’t reduce the disease at all, and could in fact speed its development,” McDonald says.
If clubroot-infected weeds cannot be sprayed, mown or disced without worsening the problem, how can producers get rid of them?
The only answer is to pull up the infected plants, McDonald says. Then, the question is how to destroy them: left in a pile, clubs will decay but spores will continue to mature and eventually make their way back into the field. They can be buried with a backhoe, but this method is likely too labour-intensive in season.
The most practical solution is to destroy smaller batches of pulled plants in a burn barrel, although the most environment-friendly option would be to destroy them in a biodigester, McDonald says.
The study’s results highlight the importance of scouting frequently. Luckily new research is always cropping up to make this easier.
It helps that there are clear patterns to clubroot development and its effects on a field.
Research done by University of Alberta professor Stephen Strelkov a few years ago shows, for example, that clubroot is typically first found at the field entrance and to the right, says McDonald. Clubroot travels on clumps of soil which are carried by tractors. An equipment operator will almost invariably pull to the right, stop the tractor and then open it up, dislodging clumps into the field.
“As you go further into the field there’s less clubroot because most clumps of soil have fallen off the equipment already and you end up with parts of the field with no clubroot, and year after year the clubroot might move a little bit as seeders and other equipment move through the field,” she says.
Because the organism moves through soil moisture, clubroot often shows up in low spots or areas with soil compaction. Infection often delays flowering, making infected green plants easy to spot in the sea of yellow blooms. Later, infected plants die prematurely.
Clubroot is easier to detect with a drone than other diseases due to patterns like these. This summer, McDonald and Gossen are collaborating with David Halstead, a researcher at Saskatchewan Polytechnic, on a two-year drone scouting project funded by SaskCanola through the Agriculture Development Fund and the Saskatchewan Ministry of Agriculture. The project will assess drones’ ability to spot clubroot infection from the air.