Spring, summer, or fall — when do you think is the best time to test for clubroot?
Visual symptoms on the plant show up later in the season, yes, but that’s not when soil spore load is highest, explains Kim Kenward of 20/20 Seed Labs, in this episode of the Canola School.
“Soil testing is the best way to track where (clubroot) is going to show up once it’s outside the plant,” she says.
That said, the best time to soil test is once you no longer have a growing crop. That’s because you need the plant to decay and release the spores into the soil. In the spring, you’ll get the spores germinating and looking for a new host, and that lowers the spore load, she says. Fall is likely the best time to look at the total risk picture, when it comes to a soil test.
Where should you soil sample if clubroot detection is the goal? If you’ve pulled plants and found symptoms, go ahead and mark that area and come back to it. As a general rule, though, high traffic areas, such as approaches and field entrances, are the most likely areas to find spores early on.
Kenward adds that mixing cores can dilute a sample too much, so aim to run as many samples as you can reasonably afford (or keep composites to a specific site).
In the absence of a host, spore loads drop quickly in the first two years, she says, but spores can persist 10 to 20 years. And just a few spores can spread the disease if there is a susceptible host present.