Will a Second Fungicide be Worth the Cost for Tar Spot Management?

When is the best time to apply a fungicide for tar spot management?
Like most of the other diseases of corn, the timing of fungicide application to hedge your bets against tar spot generally is at tasseling (VT) to the silking (R1) growth stage. Recent regional research has demonstrated that while there might be little yield benefit with an application at the V6 growth stage, a single application of fungicide at VT-R1 on average can result in as much as 7 bushels or more yield compared to not treating. This is compared to just 2-3 bushels at the V6 timing, and suggests that farmers are more likely to recover their fungicide costs if applying just one application at VT-R1. In the absence of tar spot and southern rust, spraying at V6 AND VT-R1 also has not resulted in economically positive returns. This practice, on average only results in an additional 1 bushel of yield compared to one application at VT-R1. There is no considerable return on investment (ROI) with two-pass fungicide programs for many corn diseases. But what about the tar spot situation this season? What do the data say about a second fungicide application to manage tar spot if I have already sprayed at VT-R1 and the disease is continuing to increase?

Should I apply a second or late fungicide application for tar spot management?
Possibly. It is important to realize that fungicides should be applied to reduce disease development. Remember that many fungicides only effectively manage disease for about 14 to 21 days after application. Therefore, if a disease continues to increase, a second application might be beneficial, but there are a few things to consider first.

Applying a fungicide late in the season will not ‘cure’ an already diseased plant. Dry matter accumulation in the kernel continues until physiological maturity, which is defined as maximum kernel weight. At R5, kernel dry matter accumulation is approximately 45% of total dry weight, leaving half to be accumulated during this developmental stage. At ½ milk line, approximately 90% of total dry matter exists. Stresses during this period results in a reduction in kernel weight. The degree of yield protection will depend on the level of disease currently in the crop, susceptibility of that hybrid to tar spot (and other diseases), and upcoming weather conditions.

If curious how the weather in your area might impact tar spot development, there is an app for that, called “Tarspotter.” Tarspotter is a new smartphone tool that can be used to determine if weather has been conducive for tar spot at a specific field location using GPS coordinates. This tool should be used in conjunction with scouting to confirm the pathogen is present. If these conditions are met and the crop is still accumulating dry matter, then a fungicide application could be warranted, even if one was already made at VT-R1.

For example, in Michigan, an industry colleague applied a second fungicide application on August 20 in 2019, and saw a 20 bu/A benefit over a single fungicide application at silking. Tar spot was detected in mid-August in that field and continued to develop with overhead irrigation. However, when those trials were repeated in 2020, there was no yield response from the second fungicide application even in the presence of disease.

If a second fungicide is applied, leave non-fungicide-treated check strips to determine the ROI and learn from the process. Also, be sure to read the label and be aware of preharvest intervals. A fungicide efficacy table is available at the Crop Protection Network.

Severe tar spot development on corn foliar tissue.
Image: Darcy Telenko

What else should I be aware of?
Be mindful of lodging potential in fields with heavy tar spot disease. Don’t forget about other diseases. This season we have also seen a greater than normal amount of southern rust.

What about corn cut for silage?
Tar spot is capable of rapidly drying corn plants. Thus, if you are considering making silage with your crop, watch moisture levels closely, as the crop can quickly become too dry, impacting the bunker-packing process and negatively influencing fermentation. Quality parameters such as digestibility and energy content can also be impacted. The tar spot fungus does not produce any known mycotoxins.

What has been the distribution of tar spot this season?
Tar spot developed relatively early this season, with confirmations in some areas of July 1. Heavy and prolonged rainfall at the end of June at least in the central and eastern part of the corn belt led to conditions that were favorable for the disease to initiate.

We again would like to document tar spot disease distribution and severity across the U.S. and Canada. Our research trials are in full swing, and optimistically we will have additional results about management after this season. However, we still need your help collecting samples and documenting this disease. It is not only essential to understand tar spot distribution nationwide, but it is extremely important to know if this disease is present in your fields for future risk assessments and to implement disease management tools if necessary. If you observe tar spot in a county that has not reported this season, then please get in touch with your local extension office or the University plant diagnostic clinic for that state.

Additional resources

• University of Wisconsin Fungicide Trials
• Purdue University Tar Spot Trials
• Tar Spot Web Book from the Crop Protection Network
• Fungicide Efficacy for Control of Corn Diseases from the Crop Protection Network