Tar spot is starting to progress in some Missouri corn fields. The most common question I have received is should I make a fungicide application to my early-planted corn, which is at dent (R5 growth stage)? There is consensus in the academic community that if disease begins progressing when the corn is at dent, a fungicide application is not needed. This consensus is based on academic research trials. Table 1 is a summary of fungicide application recommendations for tar spot control. The table was developed by State Extension Crop Pathologists across the corn belt and recently published on the Crop Protection Network website.

For late-planted corn that is just now near or at tasseling, it is worth scouting to determine whether a fungicide application is warranted. There is a window of opportunity here so you have some time to scout and monitor tar spot progression. Disease pressure restricted to the lower canopy is not as concerning as disease beginning to move onto the ear leaf and upper canopy. Take into consideration average air temperatures as well. Northeast Missouri has had average daily air temperatures hovering around the 73°F mark over the last couple of weeks, and this is within the optimal temperature range of 64 to 73°F for tar spot progression.

The most consistent applications for tar spot management have been the VT/R1 through R3 applications. By waiting, you can minimize the chances of needing a second application for tar spot. Products with multiple modes of action have been most consistent with regards to yield protection across trials.

Table 1 Recommendations for Fungicide Application to Manage Tar Spot. (published on the Crop Protection Network in 2024)