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To spray or not to spray for fungal disease

You don’t have to just pencil it in and spray anyway — there are tools to help make the decision

To spray or not to spray for fungal diseases? Will the extra yield offset the cost?

In practice this decision is often made based on farmers’ comfort levels, but economics should always be considered, said Holly Derksen, field crop pathologist for Manitoba Agriculture, at this year’s Crop Diagnostic School in Carman.

“You have to understand where your break-evens are,” added Manitoba Agriculture farm management specialist Darren Bond. In canola and wheat, the break-even yield is represented by the cost of treatment divided by the price per bushel.

A fusarium and sclerotinia break-even calculator on the Manitoba Agriculture website can help producers make the decision, Bond said.

Derksen said the “big unknown” is whether the environment is right for disease and the yield potential for the crop.

While the sclerotinia fungus can remain viable in canola fields for five years or more, sclerotia — the infecting bodies — need seven to 10 days of moisture to produce mushrooms, or apothecia, which then infect canola via airborne spores. When spores land on senescing plant tissue, the fungus spreads from petals to stems, said Derksen.

Infection has less effect on the plant after 50 per cent flowering, so the spray window falls between 20 and 50 per cent flowering.

“Unless you have 10 days of surface soil moisture while the crop is still flowering, you don’t need to spray for sclerotinia,” Derksen said.

There’s a wider range of opportunity for infection when it comes to blackleg, but risk levels are calculated based on producers’ rotations — the better the rotation, the lower the risk, and the history of infection.

Unless producers have seen yield losses directly attributable to blackleg in the past, they may not need to spray, Derksen said. To diagnose blackleg infection, “get off the swather, pull stems and look for discolouration in the cross-section.”

When it comes to fusarium in wheat, spray timing falls during early anthesis. Producers don’t want to lose grade because they didn’t spray, she said, but many are pencilling in spraying before they’ve even planted the crop.

“Use flag leaf timing. Keep these green and don’t compromise them with leaf disease, because your yield comes from these,” she said. “But if it’s dry and you’re not having any dews, you’re making the right call not spraying,” Derksen said.

Demonstrations

Derksen’s sclerotinia demonstration at the site showed three treatments: no fungicide application, a fungicide application at 20 to 30 per cent bloom, and an application at the two- to four-leaf stage and at 20 to 30 per cent bloom.

While it was too early to see major differences, Derksen said she expected to see very little sclerotinia even in the untreated plot given that the weather had been so dry.

A demonstration comparing fungicide treatments and timing for fusarium applications in wheat yielded different results.

“In the wheat I was seeing differences in plots with different fungicide application timing, and I could see more infection in the untreated versus treated. Whether that will translate into yield or quality differences, it’s hard to say,” Derksen said in an interview after the event.

Whereas for sclerotinia and blackleg, environmental conditions and cultural factors make a big difference in risk assessments, fusarium risk can spike even in a single day.

“Quality loss due to fusarium can be such a big hit to your price point. Even on a low-yielding year you can make up that difference by spraying,” she said.

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