All a twitter about tar spot

June 28, 2022 8:55 AM
Blog Post

Are you using the Tarspotter app this year? If you are, you may have noticed that the forecast for tarspot is high for Iowa. In addition, The Cruz Lab in Indiana tweeted they had found tar spot on V6 corn in one of their trials.  Consequently, some are thinking of pulling the trigger on a fungicide application. Before pulling the trigger, here are some additional factors to consider:

  1. The Tarspotter app recommends managing tar spot between V8 and R4.  Apart from the earlier planted corn in Iowa, most of the corn in Iowa has not reached V8 yet.  Fungicide applications prior to V8 are not recommended for tar spot (and other leaf diseases) management.
  2. For tar spot spores to germinate and infect, the leaf surface must be wet for an extended period of time. If your pants are not wet at 8 am in the morning, the corn probably hasn’t been wet long enough for infection to occur. Across Iowa, some fields were rolling last week due to a lack of moisture.  We are still pretty dry.  Having said that, observations from WI suggest tar spot can “pop up” after a rain event.
  3. For fields for which a high risk of tar spot is forecasted, scout first. Target areas of the field where leaves are likely to stay wetter, e.g., along a windbreak, low-lying areas where morning fog tends to hang around, etc. 
  4. Early signs of tar spot can be mistaken for insect frass! Tar spots (stroma) are embedded in leaf tissues and cannot be scraped off the leaf surface (Figure 1). The spots are often visible on the underside of the leaf too.
  5. Fungicides only protect the leaves to which they are applied, and they are only effective for 3 to 6 weeks depending on the product.  Thus, timing a fungicide application is important.
  6. All fungicides reduce tar spot severity. The Crop Protection Network publishes a list of available fungicide products with their relative efficacy against tar spot and other diseases (gray leaf spot, northern corn leaf blight, southern rust, etc.) to enable farmers to make informed decisions regarding what products to use
  7. Data from 2021 trials in IL, IN, MI, WI and Ontario, Canada, suggested Delaro Complete, Revytek and Veltyma were superior to other available products. Tar spot severity was greater than previous years when no differences between products were detected.
  8. Data suggest that the best time to apply a fungicide to manage tar spot is between VT and R2.  In most years, only one application may be necessary.

Early signs of tar spot on a corn leaf

Figure 1. Early signs of tar spot on a corn leaf.

For Iowa, we recommend scouting and applying a fungicide no earlier than V12, and only if tar spot is present (and increasing). If you do decide to spray before VT, remember NOT to use a non-ionic surfactant since this may cause arrested ear syndrome.

We are still learning about tar spot.  If you see tar spot, please send a photo with the county name to either myself ( or Daren Mueller ( or DM us on twitter @alisonrISU @dsmuelle.  We will be updating the map daily. 


Useful resources

  1. All you ever wanted to know about tar spot:
  2. Thoughts for 2022 from Dr Darcy Telenko, Field crops pathologist at Purdue University
  3. 2022 Fungicide data from Dr Damon Smith, Field crops pathologist at University of Wisconsin:
  4. Will a Second Fungicide Application be Worth the Cost for Tar Spot?

Alison Robertson Professor of Plant Pathology and Microbiology

Dr. Alison Robertson is a professor of plant pathology and microbiology. She provides extension education on the diagnosis and management of corn and soybean diseases. Her research interests include Pythium seedling disease of corn and soybean and Goss's wilt. Dr. Robertson received her bach...