Tar Spot Appeared in Mid-June

As anticipated, tar spot was confirmed in June again this year (Figure 1). The fungus that causes tar spot (Phyllachora maydis) prefers cooler 24-hour temperatures (64 to 73°F), like those we observed in May and early June, which likely contributed to its early appearance. To date, all confirmed incidences have been at low severity. However, these confirmations mean it is worth scouting closely as the season progresses, paying special attention to the ear leaf.

Figure 1 The 2025 Tar Spot map updated on June 25, 2025. Yellow indicates counties with confirmations in 2025. Gray indicates counties where tar spot was found in previous years. Map at https://corn.ipmpipe.org/tar-spot/

The most consistent return on investment for a fungicide application to manage tar spot is an application between VT and R3, when the disease reaches the ear leaf and begins to spread. We want to make the application before tar spot lesions cover 5% or more of the ear leaf.

Remember tar spot develops best under cooler 24-hour temperatures, and I anticipate the persistent hot temperatures we are experiencing now will slow the spread. Scouting and monitoring are important. If the disease has not reached the ear leaf or is not spreading, then you may want to delay the fungicide application until conditions become more favorable for the disease to spread.

Initial symptoms can be difficult to distinguish from insect frass (insect poop), which is shiny and can usually (but not always) be scratched off the leaf surface. For more information on tar spot identification and its look-alikes, see the new MU Extension fact sheet, g4156: Tar Spot of Corn (https://extension.missouri.edu/publications/g4156), published by Dr. Tian and myself.

Southern Rust on the Move

The fungus that causes southern rust, Puccinia polysora, cannot survive Missouri winters and the fungal spores must be transported from the south annually. (This is unlike the tar spot pathogen, which can survive winter and is present in Missouri annually.) Also unlike tar spot, southern rust favors warmer temperatures (77 to 88°F). This disease has been confirmed in very southern Arkansas and Mississippi in 2025 (Figure 2).

Figure 2. The 2025 Southern Rust of corn map updated on June 27, 2025. Red indicates counties with confirmations in 2025. Yellow indicates probable and green is when fields were scouted, but no disease was found. Map at https://corn.ipmpipe.org/southerncornrust/

Southern rust consistently shows up in the Missouri River bottom; however, last year, it appeared in areas where it is not routinely observed and at higher severity levels. The disease closely resembles common rust, making proper identification critical as most corn hybrids are susceptible to southern rust.

Distinguishing between the two can be challenging. Southern rust produces pustules that are typically more orange and circular (Figure 3) and primarily develop on the upper leaf surface with only yellowing (chlorosis) observed on the lower leaf. Common rust produces pustules that are darker, brick-red in color, and more elongated (Figure 4), and these pustules can be observed on the upper and lower leaf surfaces. Both types of rusts can occur on the same plant, and in some cases, microscopic examination of the spores are needed for definitive identification. If you have identification questions or suspect southern rust, please consider submitting samples to the MU Plant Diagnostic Clinic.

Figure 3 Early onset of southern rust with pustules that are more orange and circular compared to common rust and usually only found on the upper leaf surface.

Figure 4 Common rust produces pustules that are typically more dark to brick red in color and elongated when compared to southern rust. Pustules can be found on upper and lower leaf surfaces.

What You Need to Do

Stay informed and scout frequently. The best approach right now is to monitor fields closely and stay updated. Although the early presence of these diseases can be concerning, we do not want to make hasty fungicide decisions. Making a fungicide application too early can be costly and ineffective if disease pressure doesn't justify it. Table 1 summarizes Crop Protection Network recommendations for fungicide applications to tar spot and southern rust. Additionally, the most recent fungicide efficacy table (https://cropprotectionnetwork.s3.amazonaws.com/corn-foliar-efficacy-2025.pdf) produced by the Corn Disease Working Group (CDWG) can be found on the Crop Protection Network and contains observations and data from 31 universities, including the University of Missouri.

Table 1 Recommended Application Timings when Southern Rust or Tar Spot are Detected (summarized from the Crop Protection Network)^*

^*Visit the Crop Protection Network for the Full Table and More Details.
Southern Rust: https://cropprotectionnetwork.org/maps/southern-corn-rust
Tar Spot: https://cropprotectionnetwork.org/maps/tar-spot-of-corn

There are tools available to help with monitoring and decision making. Here are a few:

• IPM Pipe maps for corn to monitor where tar spot and southern rust have been confirmed
• Crop Risk Tool — disease forecasting tool for corn and soybean that includes tar spot
• Corn Fungicide ROI Calculator — Crop Protection Network tool to help calculate potential return on investment using university data from across corn growing states