The presence of corn tar spot has been increasing over the years. learn what you can do to manage it and how adjuvants help.

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Corn Tar Spot 2021 and Beyond

Steve Doench, Head of Sustainable Agronomy
Steve Doench, Head of Sustainable Agronomy

This crop season, corn tar spot has developed into an important issue for midwestern corn farmers. Especially in eastern corn belt areas where abundant rainfall mid-summer helped increase disease establishment and spread.  Heavy tar spot infections have caused corn to mature early. As a result, the corn did not complete grain fill, leading to smaller and lighter kernels which reduced yield. In addition, the above-normal temperatures since late August enhanced this, pushing late-season grain fill faster than normal.

Tar Spot Origins

Tar spot was first confirmed in the US in 2015 in Illinois and Indiana.  Tar spot (Phyllacohra maydis) originated in Central and South America, particularly Mexico.

Corn Tar Spot. Image credit: Darcy Telenko and Gail Ruhl, Perdue Botany & Plant Pathology. https://www.extension.purdue.edu/extmedia/BP/BP-90-W.pdf
Corn Tar Spot. Image credit: Darcy Telenko and Gail Ruhl, Perdue Botany & Plant Pathology. https://www.extension.purdue.edu/extmedia/BP/BP-90-W.pdf

Historical Tar Spot Presence vs. 2021

The images below show the presence of corn tar spot over the years including the detections for 2021.

Data from Corn IMP pipe. IMP stands for Integrated Pest Management  https://corn.ipmpipe.org/tarspot/

After tar spot infections occurred this summer and started to kill corn plants prematurely, the higher than normal August temperatures at the same time pushed corn to mature faster.  The net result was corn that matured too fast and could not fill corn kernels or create maximum kernel weights. As a result, the tar spot combined with a hot August led to less grain fill and kernel size, which caused yield reductions in infected fields.

The weather data is from The High Plains Regional Climate Center (HPRCC). https://hprcc.unl.edu/maps.php?map=ACISClimateMaps
The weather data is from The High Plains Regional Climate Center (HPRCC). https://hprcc.unl.edu/maps.php?map=ACISClimateMaps

What’s next?

Now that harvest has arrived, there are some things farmers can do to help reduce problems harvesting and get ready for next season. First, fields that had a heavy amount of tar spot are likely to have less stalk integrity and should be prioritized for harvest to avoid harvest issues with downed corn. Also, the hybrids in those fields should be noted for their susceptibility to tar spot and actual performance this season with current heavy disease pressure to help plan future hybrid selection.

For next season, growers east of the Mississippi River have an established inoculum base that will survive over winter. This may produce issues again if there are wet conditions next summer. For growers to the west, tar spot has been recorded as far west as the Missouri River in Iowa. So, it is moving west, and with heavy use of irrigation in Nebraska, it can become a seasonal issue there as well.

Like other crop diseases, tar spot presence can be reduced using cultural practices, like crop rotation to non-host crops, degrading corn crop residue after harvest via tillage, or rotating away from corn until the residue has decomposed.

As seed companies develop and understand more about hybrid tolerance and resistance to tar spot, planting corn hybrids that are not highly susceptible is essential.

In-season control

Fungicides are a crucial in-season tool to help prevent yield losses from tar spot.  As tar spot is a newer disease, not all fungicides are labeled to control corn tar spot, so it is crucial to know which fungicides are labeled for control.  We recommend that you work with your local agronomist or trusted advisor to understand what products, rates, and application timing will work best. Also, ask about which adjuvants would enhance the performance of your fungicide application. 

Interested to read more about fungicides for corn tar spot?
Check out this article from Purdue University: Fungicide Efficacy Purdue University

Adjuvants Role in Increasing Fungicide Efficacy

When applying fungicides to control fungal pathogens such as corn tar spot, it is crucial to increase their effectiveness as much as possible. To get the most out of a fungicide application, it is essential to get more active ingredients to stay on the leaves and spread out across leaf surfaces. The use of spreader sticker adjuvants helps achieve that. 

Corn tar spot infects plants from the lower leaves upward, so initial fungicide applications benefit from deposition aids that help the lower corn canopy receive active ingredients. Treating the corn plants before the upper leaves, which are capturing the most sunlight, are affected will be most beneficial.

Fungicides can be applied by crop dusters

Optimizing fungicide applications

Fungal infections cause most foliar diseases with a regional variation in the type of diseases present. While prevention is best, unfortunately it is not always possible to avoid infections. What can be done to optimize a fungicide application to get the desired results and minimize fungicide resistance?

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