Are you ‘always wrong’ at camp sclerotinia?

Your decision to spray essentially comes down to one simple equation

Ah, all the joys of summer camp… first loves, campfires and archery supremacy. And don’t forget wood ticks, swimmer’s itch and the stench of a cabin on weiners and beans night.

In fact, it probably isn’t too big a stretch to say that the way we remember summer camp is probably as diverse as the way growers look at sclerotinia stem rot, the most widespread and costly canola disease across the Prairies. This article divides growers into four cabins based on their approach to the fungicide spray decision. Two cabins agonize over the decision. Two cabins don’t.

Which cabin are you in?

sclerotinia stem rot

A classic example of sclerotinia stem rot.
photo: Canola Council of Canada

Cabin 1: Seeking a sure thing

These growers are well read, they follow Canola Watch, they attend crop tours, and they scout their fields. They have heard all the reasons to spray for sclerotinia stem rot, but they have run the numbers on cost and they’re not seeing the clear benefit they think they should be seeing.

The cost of the fungicide, plus the application cost, plus crop trampling losses, plus the opportunity cost of scouting add up to $40 per acre, based on these campers’ assessment of the latter two costs, so they want to see at least a five bu./ac. benefit from spraying — preferably more.

In their experience, losses have not been enough to make the decision a lock. If it’s really wet during flowering, they may spray, but most years they won’t bother. Yet in the years when they don’t spray, they worry they’ve left profit on the table. There is a gap in knowledge and forecasting ability that makes the decision agonizing for them.

Cabin 2: Always wrong

Growers in this cabin fear they’re always making the wrong spray decision. At harvest one year, they see fairly serious levels of disease and realize they should have sprayed. They react by spraying the following year no matter what, even though it turns out conditions were not favourable for disease and yield losses were very low. Now their mind is in turmoil because they feel they miss the mark every year. They beat themselves up over this disease and are frustrated with their decision-making.

Cabin 3: They just spray

These growers spray every year because they know they’re in a high-yield high-moisture area and the risk, to some degree, is going to be there every year. So they just accept the fact that spraying fungicide is part of their risk management program — like buying crop insurance. Agonizing? Heck no.

Cabin 4: July is for chillin’

This camp doesn’t agonize because they’re not spraying for sclerotinia stem rot. Period. When herbicide spraying is over, this group stops thinking about their canola crop until bertha armyworms show up. They won’t change this approach until the year they’re out looking for berthas and find that 25 per cent of stems are shredded and falling over.

In a way, it’s hard to find fault with this laissez-faire approach if it suits their personal approach to risk management. And sclerotinia stem rot — unlike clubroot — is a disease that’s already everywhere and doesn’t really require advance planning to prevent it. This “yield losses are always the same as the cost of control” approach to sclerotinia doesn’t threaten the farm like it would for clubroot. Besides, it also doesn’t hurt neighbours, and it saves the grower from having to wrestle with all the other variables that come with a fungicide spray decision, including timing, product, and whether to spray twice.

Help for Cabins 1 and 2

Cabins 3 and 4 may not need much help, but it would be good to make the decision a little less ulcer-inducing for Cabins 1 and 2.

“We like Camp 1’s lead-in approach. We like to see growers become familiar with the factors that increase the risk, and scout in the days leading up to flowering,” says Curtis Rempel, the Canola Council of Canada’s vice-president for crop production and innovation. “The grower’s frustration at not being able to accurately predict the loss and decide whether fungicide will pay may be a matter of mindset more than anything.”

Three things need to happen for sclerotinia stem rot disease to develop. Canola plants must be at the vulnerable or susceptible stage, which is flowering. The fungus must be present at the same time. And there has to be moisture in the form of rainfall or dew. This is the disease triangle. “If one of these is missing or out of sync, you will not likely have much sclerotinia in your crop,” Rempel says. “If these three factors all line up, you will have the disease and it will likely be a yield robber.”

The most uncertain item in this triangle is the moisture. Lots of smart people all around the world are trying to improve weather forecasting. Yet forecasting remains hit and miss once we go beyond a couple of days. No one can predict with any degree of certainty whether rains during flowering will turn off and stay away for three weeks. So, until long-term accurate forecasts are possible or until a curative fungicide hits the market, the decision to spray a pre-emptive protective fungicide depends on moisture conditions leading up to petal drop.

“With rain, dew or high humidity before and during flowering, and yield potential of at least 30 to 35 bu./ac. — which generally means a denser more humid canopy and more potential for yield loss — the scenario is set for a likely return on the fungicide investment,” Rempel says.

Another factor making the fungicide spray decision agonizing for Cabins 1 and 2 is a complete lack of confidence in their ability to estimate yield loss. Visually, it can be difficult to tell how many plants have lost yield, even if growers are seeing high incidence and severity of infection. It could be two bu./ac., or five or 30.

“The rule of thumb to estimate yield loss is to take incidence and divide by two,” says Faye Dokken-Bouchard, provincial plant disease specialist with the Saskatchewan Ministry of Agriculture. Incidence is the percentage of plants in a field that have visible symptoms of the disease. If 20 per cent have whitened brittle stems typical of sclerotinia, then estimated yield loss would be 10 per cent.

“If farmers don’t have their own records, they can use provincial disease survey data to see the potential for infection,” Dokken-Bouchard says.

Northeast Saskatchewan, according to the survey, has had five years in the past 15 where the overall average incidence rate was 20 per cent or more. The highest recorded average was 31 per cent in 1999. Using the rule of thumb to estimate yield loss, one year in three had a region-wide yield loss of 10 per cent or more due directly to sclerotinia stem rot. “Given that the northern region also tends to be the highest yielding area of Saskatchewan, the economics are there to justify a fungicide spray when moisture conditions are favourable for disease,” Dokken-Bouchard says.

In dry years, there has been virtually no sclerotinia stem rot, Dokken-Bouchard adds.

Growers looking for their own data could leave a few untreated check strips in each field they spray. Set up an accurate and fair comparison and take the strips to yield. (The Canola Council of Canada’s Ultimate Canola Challenge is showcasing on-farm research this year, and has protocols growers can use to produce good data.) One year is not enough to provide a clear pattern, but over five years, growers can see the typical yield loss from sclerotinia stem rot and note the factors that contribute to higher yield loss. If a few neighbours collect data and compare notes, that’s even better.

“With this information, you get a better picture of the risk in your area,” Rempel says. “If fungicide provides a return on investment one year in five, perhaps you’ll be a little less likely to spray. If two or three years in five pay off, and one of those years pays off really well, you may find yourself creeping into Cabin 3.”

Another factor is a grower’s own tolerance for risk. “If you know going in that you’re willing to accept a three bu./ac. yield loss, a five bu./ac. yield loss or a 10 bu./ac. yield loss before taking action, this makes the decision easier,” Rempel says.

With a few on-farm trials, familiarity with the situations that lead to economic loss, and an assessment of one’s personal appetite for yield loss, the decision to spray comes down to a simple mathematical equation. If potential yield loss exceeds your risk tolerance, you spray. If it doesn’t, you don’t. That’s the best a canola grower can do. Everything else is up to Mother Nature. And as farmers in all camps know, you can’t beat yourself up over that.

Jay Whetter is communications manager with the Canola Council of Canada. For more on the Ultimate Canola Challenge, search for articles at canolawatch.org. While there, sign up for the free and timely agronomy newsletter.

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Jay Whetter is communications manager for the Canola Council of Canada.

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