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Resistant to resistance?

If you don’t have clubroot, should you be planting clubroot-resistant canola?

Should a farmer who regularly practises a one-in-four-year rotation of canola in an area without confirmed clubroot be growing clubroot-resistant (CR) cultivars of canola? This was a question I asked at a canola agronomy meeting last November.

At that time, I was describing my own farm and I thought the answer would be straightforward, but I received conflicting responses. One canola company representative said growers in an area without confirmed clubroot and practising a one-in-three or one-in-four rotation should not grow canola with clubroot resistance because it will cause the resistance to break down prematurely. Another agronomist said that growing a CR hybrid is a good choice in almost any situation because it will keep a low spore load from proliferating into an unmanageable establishment of the disease.

The confusing response left me scratching my head, but I wasn’t too worried until January when my county confirmed the first occurrence of clubroot within its borders. Now, I don’t know where this occurrence happened. It could be 50 km from my farm, but the fact that Kneehill County turned from green to yellow on the clubroot map increased the stakes and made me that much keener to find an answer to my question.

I first wanted to understand if I was doing harm by growing a CR hybrid if I didn’t truly need it. Like all farmers, I’ve absorbed many lectures about herbicide and fungicide resistance. I understand how weeds and diseases can adapt over time so the chemicals we spray no longer control them. Could the same thing happen with clubroot spores that encountered a resistant cultivar of canola? Would they adapt more quickly, rendering the resistant genes useless? This seemed to be one of the messages I received at the November meeting.

I reached out to Dr. Stephen Strelkov, professor of plant pathology at the University of Alberta, who explained that for selection to resistance to occur the disease actually has to be present in the field. In fields with very low or no spore load, there will be no selection pressure.

“If you’re growing a clubroot variety but there’s no actual resting spores present in the field because there’s no clubroot in the field then that concern to me is no longer an issue because there’s no pathogen population for the resistant variety to act against. The resistant genes aren’t exerting any selection pressure on anything because there’s no pathogen population,” explained Strelkov.

To put it another way, just like you can’t select for resistance in wild oats if there are no wild oats in your field, you can’t select for pathogens that are resistant to CR cultivars if there are no clubroot spores in the field.

Growing a CR hyrbrid when you have a low chance of having clubroot is “at worst… harmless,” said Strelkov, adding, “But in another scenario it can be helpful because if for some reason some spores are introduced… and if the first host that they encounter are resistant hosts, the chance that the pathogen will become established is much smaller.”

That’s a lot to digest, but as of now, the science says it’s safe to grow a CR canola even if you do not believe you have clubroot in your field.

Risk assessment

That said, BASF technical marketing manager Jared Veness said his company encourages farmers to start with a risk assessment on their own farm before deciding to grow a cultivar with a “C” in the name.

Risk factors for clubroot include: whether clubroot is present in the community in which you farm; the amount of outside traffic (custom operators or oilfield) that comes on your land; your soil type; your own scouting practices; rotation; tillage practices; and equipment sanitation.

The reality, for BASF, is that not every hybrid in its current Invigor canola lineup (formerly owned by Bayer) has CR genetics. Its concern is making sure farmers in areas where clubroot is established or at the most risk of becoming established have access to CR genetics and in particular the second generation of resistant genes. Because the pathotypes of clubroot found in core clubroot geo-graphies in Alberta are shifting.

“If you don’t think you have an issue on your land but based on your risk assessment you want to protect against clubroot, we recommend going in with that first generation of clubroot resistance and then after a couple of cycles perhaps move into the second generation,” said Veness. BASF’s first generation of resistant hybrids includes L135C, L241C, L255PC and L258HPC. Some more virulent pathotypes of clubroot have been found that can overcome the resistant genes in the first-generation hybrids and now new hybrids are being brought to market that are resistant to some of these strains — for now.

“We’re trying to maintain the lifespan of the resistance genes we have. We do know in certain hotspots that first generation is already starting to break down. So we want to provide these growers a tool to move to. We’re recommending L234PC, the second-generation clubroot package, for those who have grown a couple of cycles of that first generation,” explained Veness.

Proactive planning

There are more CR options in the Roundup Ready canola systems, but I like to grow Liberty Link canola in order to rotate through a different herbicide group than the one I often use for pre-seeding spray.

For 2019, I decided to plant two-thirds of my Invigor canola acres to a first-generation CR hybrid — L255PC. My decision was the result of several agronomic factors. First the L255PC seed wasn’t any more expensive than the non-CR cultivars. Second, according to both AFSC’s Yield Alberta report and in the canola performance trials published in the Alberta Seed Guide it has performed well in my area. Third, as Veness pointed out, “A proactive approach to getting ahead of the disease is the optimal way to manage it,” he said. “Almost act as if you have it today.” Especially considering the positive identification of clubroot in my county.

Growing CR canola is probably the easiest part of being proactive, but cannot be the only step to mitigating the chances of clubroot coming to my farm. A two- or three-year break from canola is not that difficult, since that’s what I am doing already. Stepping up my scouting game will be challenging since it needs to be done during the busy growing season. However, it seems that across the Prairies, fewer and fewer farmers will have the option of ignoring the clubroot issue.

“Every year there’s so many cases that appear in areas where they say clubroot isn’t and that’s the issue,” explained Dan Orchard, agronomy specialist with the Canola Council of Canada. “We don’t know where it isn’t, we know where it is. To define where it isn’t is impossible and the risk of finding it too late is far too risky right now.”

Orchard pointed to areas in Saskatchewan and south of Calgary where clubroot was found in 2018, surprising growers and agronomists who were confident that they didn’t have clubroot.

“There were fields there that were heavily infested that should’ve deployed resistance prior to this finding,” said Orchard, adding, “Rather than following the leading edge of this disease, we need to get ahead of its establishment.”

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