Received several calls about tar spot yesterday and today. As of right now, it’s been confirmed by UNL at LOW incidence (1-2 lesions per leaf) in Saunders, Pierce, and Clay (on 6/16/25), Polk, and Seward Counties (on 6/17/25). I really appreciate Craig Anderson and Mike Byers bringing me leaf samples to confirm. I also appreciate those who were calling to hear of any confirmations from leaf samples.

There’s a lot of fear surrounding this disease, and still some unknowns. We haven’t seen tar spot in Nebraska this early. It would be helpful if consultants/agronomists would confirm samples to Dr. Tamra Jackson-Ziems, the UNL Plant and Pest Diagnostic Lab (tar spot testing is free), or a local Extension Educator so that we have the most accurate information to provide. Tamra will update the tar spot map at: https://cropprotectionnetwork.org/maps/tar-spot-of-corn.

We are NOT recommending fungicide application in fields where tar spot is confirmed in these early vegetative stages.

Reasons for not applying fungicide now:
1-Economics: Corn economics are already a struggle. When tar spot appeared in the early vegetative stages, research from Dr. Darcy Telenko’s lab at Purdue University showed it wasn’t economical to apply at V6-V7 as it didn’t suppress disease enough. It was economical when the corn reached tassel stage or beyond. I show the research data below and you will also see the chart on the website link I shared above. In this post, Dr. Telenko shares 7 years of experience dealing with tar spot when it occurs early in the season and how to make fungicide decisions.
*Some Quick Tips & Tools for Preparing for Tar Spot in Corn-Dr. Darcy Telenko
*Tar Spot: What to look for in corn and making an informed fungicide application-Dr. Darcy Telenko
*Crop Disease Forecasting Tool for Tar Spot

2-Applications: Dr. Tamra Jackson-Ziems shared that some products say “no more than 2 applications per year”. Using those active ingredients now would mean you could use them again around tassel but no later when you may need the fungicide to finish the season. (See Dr. Telenko’s posts above).

3-Resistance management and integrated pest management. The photo below shows the economic threshold is 5-7% leaf severity for tar spot before it’s economical to spray. We’re a long way from that threshold on leaves confirmed for tar spot thus far. Avoiding unnecessary fungicide applications and using two modes of action when fungicides are applied may help in delaying resistance.

What to do now:
1-Continue to scout fields and wait till a 5-7% threshold on leaves before applying fungicide (see photo below)
2-Observe fields as to which hybrids have more tolerance to tar spot
3-When irrigating, consider less frequent and deeper irrigations
4-Consider plant nutrition? Manganese, Zinc, Copper, Boron, Sulfur all have a role in defending plants against pathogens. Will share more thoughts in another blog post.

Slide courtesy of Dr. Tamra Jackson-Ziems, UNL and Dr. Telenko, Purdue University. Fungicide application at V6-V7 resulted in negative economic return and similar disease pressure as the non-treated areas. Best fungicide timing for disease suppression occurred from tassel to milk and for economic return from tassel to dough.