Prairie grain producers always keep close tabs on the weather during the growing season to see if conditions turn favourable for disease development in their crops.
If that happens, there may not be a simple answer to the question of whether to spray or not. And much of that has to do with timing.
“The first question growers usually ask is should I spray?” says Randy Kutcher, a plant scientist at the University of Saskatchewan. “The second question is, ‘I’ve decided to spray — what’s the best timing?’ Those are the two main questions you get with any fungicide application in field crops.”
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Kutcher has studied the optimal timing for fungicide applications in wheat, specifically to manage fusarium head blight (FHB) or leaf diseases like tan spot and septoria leaf spot, as has Sheri Strydhorst, an agronomy consultant in Neerlandia, Alta.
Their research suggests fungicide treatments timed to coincide with herbicide or plant growth regulator (PGR) applications earlier in the season likely do very little against FHB or leaf diseases, since a wheat plant’s development isn’t far enough along to provide effective control.
The best timing for fungicide applications to control leaf diseases is generally viewed to be the flag leaf stage in wheat. The later anthesis or wheat head flowering stage, on the other hand, is considered the best time to spray for FHB.
But what if you want to only spray once? The studies by Kutcher and Strydhorst suggest if you hold off a few days and do your fungicide application at the fusarium head blight timing rather than at the flag leaf stage, you’ll be better off.
“I think if I had to pick one, I would pick the FHB timing because you get that tool for controlling FHB and you’re going to get a yield advantage that’s equal to or better than the flag leaf timing,” says Strydhorst.
Don’t waste your money
Strydhorst was working as an agronomy research specialist for Alberta Agriculture and Rural Development when she published a 2021 study called Evaluation of disease, yield and economics associated with fungicide timing in Canadian Western Red Spring wheat.
The report was based on two years of research in 2018 and 2019. Fungicide treatments containing different combinations of propiconazole, benzovindiflupyr, azoxystrobin, prothioconazole and tebuconazole were evaluated at different sites across Alberta to determine the optimal application timings for two of the most commonly grown CWRS cultivars, AAC Brandon and AAC Viewfield.
Strydhorst’s research, which was funded by the Saskatchewan Wheat Development Commission, Alberta Innovates and the Alberta Wheat Commission, suggested earlier season fungicide applications at the herbicide or PGR timings were essentially a waste of money.
“Spraying at herbicide timing or at PGR timing, we had no statistical improvement in yield versus the control,” says Strydhorst. “There was no yield advantage and you didn’t get the disease control, so there wasn’t any economic benefit.”
The research showed fungicide applications at the flag leaf stage generally provided the best level of leaf disease control. Strydhorst says spraying at the anthesis stage when wheat is most susceptible to FHB infection resulted in slightly less effective leaf disease control but still produced the highest yields in the study.
Kutcher’s study examined whether fungicide applications at the anthesis stage for FHB management would also adequately suppress leaf spot disease. Released in 2018, the research was carried out in three Alberta and three Saskatchewan locations using two fungicides containing tebuconazole along with a biological product (active ingredient Bacillus subtilis).
The research results showed applying foliar fungicide applications at anthesis produced equivalent yields and similar grain quality compared with flag leaf applications, even though the leaf spot severity was slightly higher with anthesis timing.
The study also concluded two fungicide applications, one at flag leaf and another at anthesis, were not economical given the commodity prices and fungicide costs at the time the research was carried out, since this only provided a small incremental benefit compared with a single application at anthesis or FHB timing.
Rotate fungicides
Kutcher and Strydhorst both agree if you decide to do separate spray applications at the flag leaf and anthesis stages in wheat, it’s really important to rotate your fungicides. Using active ingredients from two different groups will help prevent fungicide resistance.
“If we overuse fungicides or just use them without thinking about it, they may not be effective in the future,” says Kutcher. “You don’t really want to spray if you don’t need to, just from an economic point of view, but also because you don’t want to encourage fungicide resistance.”
Strydhorst says a main reason why she conducted her study was to help bring attention to proper fungicide stewardship.
“Farmers are certainly aware of herbicide-resistant weeds, but fungicide resistance is perhaps a newer concept,” Strydhorst says, adding that by reducing fungicide applications, farmers can help “delay resistance from coming to our doorstep.”
