Tar Spot Update

Summary

Last week Dr. Damon Smith, with the University of Wisconsin, gave an update on Tar Spot and I thought his findings were extremely valuable and the most relevant information I have seen to date.

Tiny black spots against a brown lesion are a symptom of the tar spot complex in corn.

Keynotes

- Tar Spot can overwinter and has been in WI for 3 years. It is also in Eastern IA. The first two years Tar Spot was in Wisconsin, it did not infect plants until late August. This year it arrived Mid-June.

- There hasn’t been a single plant found with the Monographella version (the really bad type only found in Mexico so far)

- Tar spot is causing yield loss in the absence of any another disease, such as grey leaf spot.

- Hybrid tolerance incredibly variable. Some can handle it, some take a huge yield hit with this disease.

- Early hybrids take less of a hit. Research is showing that at 10% of the leaf area covered with Tar Spot yields are reduced by 8 bu/ac. Longer maturity (103-113 day) hybrids lost 15 bu/ac when 10% of the leaf area was infected.

- University plant pathologist are creating a phone app (the TarCaster) that will hopefully be able to predict the arrival of the disease based on the weather. They already have a similar program for predicting white mold. They expect that to be out for testing this upcoming year.

- Yield losses appear to be dependent on when the plants become infected with Tar Spot. For example, this year infection started between V8 and VT is some regions but in previous years infection did not start until after milk stage.  There is barely a hit on yield if it arrives during the Milk stage.

-Fungicide does help if timed properly, and at least Headline Amp and Delaro are labeled for Tar Spot.

-University plant pathologist plan on releasing a fungicide update around the end of December to show when the optimum time will be for applying fungicides to control/suppress Tar Spot.