Corn School: Tackling tar spot in Ontario


Tar spot has moved through the U.S. Midwest, into Michigan and Wisconsin, and now officially calls Ontario home.

Ontario Ministry of Agriculture, Food and Rural Affairs’ plant pathologist Albert Tenuta has seen the evidence in a field near Ridgetown, Ont., and there are also reported sightings of the corn leaf disease in Lambton and Essex Counties.

Characterized by tar-like speckling on the upper surface of corn leaves, the 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.

On this episode of RealAgriculture’s Corn School, we catch up with Tenuta at the location of the first confirmed case of tar spot. He offers tips on how to identify the disease and discusses how environmental conditions play a significant role in determining its severity from year to year.

Although tar spot does have the ability to over-winter on corn residue, Tenuta says the potential impact of the disease in 2021 will be determined by environmental conditions during the growing season — it loves cooler, wet, humid conditions, which promotes higher spore load and earlier tar spot development. These conditions helped fuel big yield hits in 2015 and 2018 for U.S. crops. (Story continues after the video.)

If these conditions dominate the U.S. midwest’s weather in 2021, the potential of the disease migrating north via weather systems increases the risk for Ontario corn fields. “But we still have to remember that every year is different,” notes Tenuta. “There’s no guarantee that if you had tar spot in 2020 that you will have it in 2021.”

When it comes to managing the disease, genetics is the first line of defence. Whether resistant hybrids exist has yet to be determined but research has indicated that some hybrids are more tolerant than others.

Tenuta adds that growers already have many fungicides in their management toolbox that are effective against tar spot, including products currently used to fight common rust, northern corn leaf blight, and grey leaf spot. However, more research is required to determine the optimum application timing to defend corn against the disease.

In the hard-hit 2015 and 2018 U.S. crops, early-applied fungicide applications at tassle delivered the best protection, but most years tar spot tends to arrive at the R3 stage or later. Tenuta says he’ll be working with his U.S. colleagues to evaluate late fungicide application to determine efficacy and return on investment.

For now, growers need to put tar spot on their radar as they scout cornfields prior to harvest. Note any leaves or areas of the field that are infected. In these situations, growers may need to manage residue to reduce over-wintering of the disease. Disease spores may again blow into the field but effective residue management would reduce in-field risk.

If growers believe they have tar spot in their fields they should call Tenuta or their CCA, crop consultant or local seed dealer. They can then determine if immediate measures are required to stop the disease from gaining a foothold in the infected fields.

Click here for more Corn School episodes.

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