Tar spot of corn has been confirmed in northeast Kansas, east central Iowa, and northern Indiana recently. We have scouted northwest, north central, and northeast corn fields and although we have heard reports, we have yet to confirm tar spot in Missouri in 2024. That is probably not for long.

Fungicide applications are an important tool for managing tar spot during seasons with high outbreak. And while waiting is hard, current research data recommends waiting until closer to the VT/R1 growth stage for fungicide applications to manage tar spot¹.

A few points to consider from some of the most recent academic research on fungicide applications for tar spot:

1. Environment will influence how quickly the disease progresses².
2. Commonly-used fungicides are likely to reduce tar spot symptoms^1,3-4.
3. However, reduced symptoms do not always correspond to corn yield protection^1,3-4.
4. The most consistent results have been a well-timed one pass fungicide application between VT and R3¹.
5. A yield increase does not always equate to a positive return on investment¹.

The fungal pathogen that causes tar spot, Phyllacora maydis, favors cooler temperatures for disease development (64 to 73°F)². Air temperatures are going to be in the 80's to 90's most of this week and that may slow disease progression down. In 2023, tar spot was confirmed the week of June 23^rd in Missouri. However, to the best of my knowledge, disease progression remained slow until mid-August when irrigation and scattered storms provided rainfall over the northern half of the state.

For 2024, we should be scouting for tar spot, especially in those regions where we know the pathogen is present as it can survive Missouri winters. Scouting or monitoring fields allows us to be prepared for a fungicide application as the corn nears reproductive stages.

Most data generated to date on fungicide efficacy for tar spot management is based on fungicide applications at the VT/R1 corn growth stage. Studies have been conducted in the corn belt of the US and Ontario, Canada^3-4 and across seasons with high and low tar spot severity. In some of the trials, tar spot lesions were observed a month prior to the VT/R1 fungicide application, which may be similar to 2024.

Figure 1 QR Code that leads to the Crop Protection Network Corn Fungicide Efficacy Chart (https://cropprotectionnetwork.org/publications/fungicide-efficacy-for-control-of-corn-diseases).

Collectively, the results demonstrate that an application at VT/R1 can effectively reduce tar spot symptoms when compared to non-treated corn^3-4. Many products evaluated over the course of these research trials have offered yield protection to some extent during seasons with high tar spot severity. Veltyma, Revytek, and Delaro Complete have been the most consistent across environments with regards to reduced disease severity and yield protection^1,3-4. The Crop Protection Network publishes a corn fungicide efficacy guide that is updated annually and includes efficacy information on tar spot (Figure 1).

What is the optimal growth stage for the fungicide application?

Obviously this is a big question and the research is ongoing. Dr. Telenko's group at Purdue has been in the "hot spot" for this disease over the last six years. Her group has done a great job of initiating research on this question, and they recently published their initial findings³. The team applied Trivapro at 9 distinct corn growth stages, which are listed in Table 1. This table summarizes their results during seasons with high tar spot severity. Applications made from VT/R1 through the R3 (Milk) stage reduced tar spot symptoms compared to corn with no fungicide. A two-pass application at V6/V8 followed by VT/R1 had a similar effect on visible symptoms. When tar spot severity was low (<5%) no yield differences were observed between any of the application timings and the non-treated corn (data not shown)³. Yield was protected during high tar spot severity conditions when applications were made between V10/V11 through R4 (Dough) stages and with the two-pass application. However, in this study, the cost of the second application, did not compensate for the increased yield and did not provide a positive return on investment (ROI). Applications made at VT/R1 through R4 resulted in positive returns on investment (ROI) that were distinct from the non-treated corn when cost of product and application and corn prices were considered (Table 1).

Table 1 Summary of findings from Ross et al. 2024¹ on fungicide applications at different corn growth stages during a high tar spot severity (>5%) year.

^*No differences in symptoms or yield were observed among treatments in seasons with low tar spot severity.
^**N = Symptoms were not different than the non-treated; Y=yes, symptoms were reduced.
^***Y=Yes, yield was increased compared to the non-treated; N=no, yield was not increased compared to the non-treated. Differences ranged from 12 to 22 bu/a.
^****Return on investment of the fungicide application with cost of aerial application + product; corn prices estimated at $4.32 per bushel. ROI ranged from $66 to 82 bu/a.

Research on optimal timing and 1-pass vs 2-pass applications continues across the US corn belt, including Missouri. Also in 2024, MU is conducting on-farm strip trials to evaluate 1-pass vs 2-pass fungicide applications on tar spot and/or southern rust. The project is supported by a North Central-SARE partnership grant and space is limited. If you have interest in participating or learning more, please visit the MU Extension Strip Trial Options website or contact Dr. Bish at bishm@missouri.edu. To confirm tar spot, please send an image to Dr. Bish or the MU Plant Diagnostic Clinic at plantclinic@missouri.edu.

References

¹Ross T, Allen T, Shim S, Thompson N, Telenko D (2024) Investigations into Economic Returns Resulting from Foliar Fungicides and Application Timing on Management of Tar Spot in Indiana Hybrid Corn. Plant Disease 108:461-472

²Webster R, Nicolli C, Allen T, Bish M, Bissonnette K, Check J, Chilvers M, Duffeck M, Kleczewski N, Luis J, Mueller B, Paul P, Price P, Roberston A, Ross T, Schmidt C, Schmidt R, Schmidt T, Shim S, Telenko D, Wise K, Smith D (2023) Uncovering the environmental conditions required for Phyllacora maydis infection and tar spot development on corn in the United States for use as predictive models for future epidemics. Scientific Reports 13: https://doi.org/10.1038/s41598-023-44338-6

³Telenko D, Chilvers M, Byrne A, Check J, Da Silva C, Kleczewski N, Roggenkamp E, Ross T, Smith D (2022) Fungicide Efficacy on Tar Spot and Yield of Corn in the Midwestern United States. Plant Health Progress 23:281-287

⁴Telenko D, Chilvers M, Ames K, Byrne A, Check J, Da Silva C, Jay W, Mueller B, Ross T, Smith D, Tenuta A (2022) Fungicide Efficacy during a Severe Epidemic of Tar Spot on Corn in the United States and Canada in 2021. Plant Health Progress 23: 342-344