Three Years Later, Tar Spot Questions Outweigh What’s Known

What we know:

  • We’ve had Tar Spot in the U.S. since 2015---first identified in Indiana. This year, it’s been reported in Illinois, Wisconsin, Michigan and Indiana.
  • It’s a foliar disease. Requires a living host. It’s similar to a rust.
  • When identifying, it’s going to produce black, lumpy (almost warty) structures. Can be pinheaded or extended.
  • And it can look like aged rust pustules, but rust pustules will come off if you scrape them with your fingertip.


What we don’t know:

  • The impact is unsure. If you had it this year, you’re likely to see it every year, but the extent of the disease you’ll see is unknown. The epidemiology and biology are unknown–the weather conditions, its overwintering, management tips, timing and frequency all need to be answered.
  • Timing of infection is not known—which will direct fungicide applications.

I talked about tar spot with AgriTalk host Chip Flory: 


What farmers can do:

  • Hybrid ratings. Breeders are becoming more aware. And when selecting seed for 2019, it could be helpful to ask about how a hybrid performs against tar spot.
  • Ask your extension for their latest information.


Here’s an update from University of Illinois’s Nathan Kleczewski:


Here’s an update from University of Wisconsin’s Damon Smith:


Here’s a map from last week of Tar Spot testing results in the first state the disease was identified in—Indiana: