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Prevent Tar Spot Before it Infects Your Corn

March 15, 2019
This agronomic image shows tar spot.

Tar spot produces black tar-like spots on the leaf surface.

Because tar spot is a relatively new corn disease in the U.S., you may not recognize it as a threat. However, due to the significant yield damage it is capable of causing, it’s definitely a disease you’ll want to keep an eye on this season.

What to look for:

You’ll know tar spot by its dark spots on the leaf surface, but it infects the plant well before these symptoms are visible. It has a long latent period, meaning that corn can be infected 14 days or more before spots are visible, so the disease could be causing costly damage to your plants before you’re able to see it.

Why should you be on the lookout for tar spot?

You may feel that because tar spot wasn’t in your field last year, you really shouldn’t be concerned. However, we’d warn you to be on your guard this year if tar spot was confirmed in neighboring states or counties. It grows best in the Midwest climate due to the cooler temperatures and high relative humidity, and has the ability to overwinter. So, in areas where tar spot was found last year, there is increased risk of it showing up and spreading further in 2019.

Potential yield loss:

Although only 20 to 25% of overwintering spores will survive, tar spot is capable of producing millions of spores when conditions are right, so even just a few overwintering spores can cause significant damage.

Iowa State University research says with every 10% increase in disease severity, you could lose 10 to 15 bu/A. Tar spot can easily cause yield losses of 30 to 40 bu/A, and yield loss can be even greater when corn is infected early in the season.

How to manage it:

Saving on input costs can be attractive, but the potential damage and yield loss that can occur from tar spot isn’t worth it. Trials show that preventive fungicide applications are significantly more effective than curative applications against tar spot.

For best control, Syngenta offers Trivapro® fungicide. In a 2018 Arlington, WI field trial, corn treated with Trivapro applied at VT/R1 showed 2.8% tar spot severity compared to untreated corn with 11.3%.

If you had tar spot in your field in the past or are concerned about infection early in the season, we recommended a 2 spray of Trivapro at V4-V8 timing and VT/R1. If your risk is lower, we recommend 1spray at the VT/R1 timing to combat tar spot and many of the other yield-reducing corn foliar diseases, like Northern corn leaf blight and gray leaf spot.

To learn more about Trivapro performance on a broad spectrum of corn diseases, visit NotAfraidToWork.com or talk to your local Syngenta sales representative.

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All photos are either the property of Syngenta or are used with permission.

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