Locations where tar spot has been confirmed as of 2020.

Locations where tar spot has been confirmed as of 2020.  

Editor’s note: The following was written by Nathan Kleczewski, University of Illinois Extension field crop pathologist, for the Illinois Field Crop Disease Hub blog.


It’s early in the season, but before we know it, corn will be chest high and we will be thinking about if in-season management is needed.

Tar spot is endemic to the region and it isn’t going anywhere any time soon. What sorts of things should you keep in mind this year when management is concerned?

Resistance

Tiffany Jamann’s lab from UIUC conducted a multi-state trial assessing several hundred exotic sources of resistance, screened smaller subsets of diverse materials across several states and identified several promising sources of resistance. These results are confirmed, and the genomes mapped. Hopefully by fall we will have more information and tools to help screen materials for tar spot resistance.

That being said, remember that currently no materials that are available are resistant to tar spot in the U.S. Some hybrids are more tolerant than others.

Residue management

Tar spot is residue borne, but also disperses on air currents. Although we do not know exactly how far the pathogen can spread from a source location, observations indicate that the disease likely travels at least several miles from a source.

This limits the usefulness of residue management. The bottom line is that if you are practicing conservation tillage/no till, there isn’t a reason to return to conventional tillage systems.

Similarly crop rotation is unlikely to have much of an effect on tar spot.

Fungicides

There are many fungicides labeled for tar spot suppression. Fungicides can reduce tar spot. However, this disease ramps up reproduction and symptom/sign development rapidly. This means that timing of application is critical.

Should you be making two sprays? Maybe under limited circumstances, but the likelihood of recovering your application and product costs under current commodity prices is low. A single application of a fungicide with mixed mode of action can be very effective. Shoot for the R1-R3 window, and remember, revenge sprays after the fact are not going to help you manage tar spot, although they may make you feel better in the short term.

Other items

Spacing and planting rate potentially may impact canopy closure and humidity in the canopy. In Mexico they also suggest to avoid excessive fertilization.

The last thing to remember is that this disease is heavily impacted by weather. Just because you had severe tar spot one season does not mean you are going to have it the following year. However, if you start to see the forecast calling for persistent rains heading into VT and you had it in previous seasons, you might need to consider a fungicide application.