Answers are from the Indiana certified crop adviser panel: Betsy Bower, Purdue Extension senior research associate, West Lafayette; Abby Horlacher, Nickel Plate Consulting, Frankfort; Brian Mitchem, Farmer 1st Agronomy Consulting Services, Decatur; and Dan Quinn, Purdue Extension corn specialist.

 

I recently spotted tar spot in one of my fields. Is it worth it to spray fungicide? Is one pass enough? Should I spray the neighboring cornfield to be safe?

 

Bower: What is corn growth stage? What is infection level? How susceptible is the hybrid? If corn is denting, you can temporarily stop disease, but you may not see a good return. If before tassel or early into grain fill, it is perfect timing for fungicide to save bushels. Infection levels below 5% are where you want to target control. Once infection level reaches 5%, you cannot get ahead.

 

No hybrid is truly resistant. But there are differences in susceptibility. Even moderately resistant hybrids consistently outperform susceptible ones when tar spot is present without fungicide control. The earlier a susceptible hybrid is infected, the more likely you’ll need a second pass.

 

Fungicides work very well protecting corn even after a few lesions have been found. If you find tar spot before tassel on a susceptible hybrid and environmental conditions support new infections, you may need a second application.

 

Check the neighboring field for tar spot, understand if it is a susceptible or moderately susceptible hybrid, and evaluate growth stage before deciding to spray.

 

Horlacher: If you find tar spot before reproductive stages, spray fungicide. If your application will not cover most of the reproductive stage, spray a second application. If tar spot showed up during reproductive stages, evaluate weather patterns and location of infection. You may be able to get away without spraying with dry conditions.

 

If disease is above the ear leaf, you need to protect those leaves. If you have experienced tar spot in neighboring fields, consider applying fungicide to those fields as well, especially if rain is coming to activate spores.

 

Mitchem: Where on the plant did you find disease? At what stage? If on the ear leaf and higher, the disease has likely spread. It will be difficult to protect yield as much as if you saw the disease below the ear leaf.

 

We have seen positive returns with fungicides when applied up to dough stage with heavy infestation and susceptible hybrids. However, an application with tassel emergence has been most consistent. Protecting the plant for the first three weeks around tassel emergence results in most return.

 

If the hybrid in the neighboring field has similar tar spot tolerance, spray if conditions are similar. Pending the environment from the first application and the yield potential, you may make a second pass of fungicide about three weeks after the first. The more tolerant the hybrid is, the less likely a two-pass program will pay.

 

Quinn: If tar spot first appears in vegetative and early grain fill stages, fungicide application is likely warranted. However, if the tar spot is first found at R5 or later, fungicide will not provide benefit.

 

Also, growth stage, environmental conditions and severity of disease determine if one or two fungicide passes are required. When tar spot infects the plant earlier and environmental conditions are conducive, multiple fungicide applications may be required. However, in most cases, only one fungicide application at R1 or R2 provides the most consistent response.

 

Lastly, inspect the neighboring field similarly to make an informed decision on fungicides.