Corn School: Timing tar spot fungicide applications

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Fungicides are a key tool in the tar spot management toolbox. But what’s the best time for growers to apply a fungicide to get optimal control of the disease and the best return on their crop protection investment?

On this episode of the RealAgriculture Corn School we catch up with Ontario Ministry of Agriculture, Food and Rural Affairs plant pathologist Albert Tenuta at his tar spot research nursery in Rodney, Ont.

Characterized by tar-like speckling on the upper surface of corn leaves, the tar spot fungal pathogen has been delivering yield hits ranging from 20 to 60 bushels per acre (in highly infected fields) since it was first identified in Indiana and Illinois in 2015. To combat the disease, Tenuta and other pathologists from across North America have been working to evaluate tar spot fungicides, and application timings.

In the video, Tenuta looks at how one of the leading fungicides performs when applied at different stages throughout the growing season to a hybrid with strong tar spot tolerance. In the untreated check, Tenuta notes the disease is visible from the bottom leaves all the way to the top with an over 25 percent tar spot infection. He then evaluates a plot where the fungicide was applied at pre-tassel at the 8 to 10-leaf stage. Here there is slightly less tar spot but he’s doesn’t expect the fungicide to deliver a significant yield benefit. (Story continues after the video).

Next up is a plot where the fungicide was applied at the more traditional VT/RI stage. Tenuta says this application timing has delivered the most consistent control of tar spot, gibberella ear rot and other foliar diseases. True to form, this application strategy is delivering better performance in this plot, compared to the untreated check and the pre-tassel application. “We see less tar spot here. The fungicide is working,” he notes.

Tenuta’s final test plot looks at a double application where the fungicide was applied at the VT/RI stage and again three weeks later at R3. “This one has the least amount of visible tar spot, not only down low, but above as well,” he notes. But will it pay for growers to double apply fungicide to control tar spot? Does is deliver higher yields and a return on investment?

Tenuta and his colleagues in other tar spot hot spots ran similar trials in 2022. They’ll be comparing yields and sharing information and insights with growers at winter meetings.

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