TAR SPOT & FUNGICIDE - featured image

Tar Spot and Fungicide

We’ve mentioned tar spot in the previous newsletter, but this was the first growing season in which we actually saw the foliar disease in our local geography. In fact, I’ve seen it with my own eyes. I took the photo that accompanies this article, of a corn leaf infected with tar spot.

Tar spot is a fungus that got its name from the raised black spots appearing on the leaves of an infected plant. The easiest way to check whether dark spots on corn leaves are tar spot is to rub the leaf vigorously. 

If the dark spots do not come off, then it is probably tar spot.

Tar spot interferes with photosynthesis in corn and has the potential to lower yields by 20-60 bushels if the disease sets in early. [The University of Minnesota estimates tar spot losses from 0-50 bushels per acre.]

This newly arrived disease will overwinter on crop residue, so we’ll have to be diligent in scouting our fields in 2023. Prime conditions for the development of tar spot are a cool environment that stays wet for several hours. Of course, that kind of environment exists often in south central Minnesota and north central Iowa.

Make plans now. If the disease appears early, apply a fungicide with multiple modes of action. One of the newest fungicides – Mirvas Neo by Syngenta—offers three modes of action and up to three weeks of residual. This fungicide is rated very good against tar spot. The more modes of action a fungicide offers, the better off you will be in your fight against tar spot, and oh, you may need two applications if the disease is severe.

Fungicide is only a temporary fix, however. There is no silver bullet, and there are, at this time, no corn genetics that will resist the disease.


Fungicides could be tight this year, so place your orders early. Also, please let your NuWay-K&H Agronomy account manager know whether you plan to apply your fungicide by ground or by air. If we have advance notice how you want it applied, we can have the equipment and product in place for you.

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