Multi-state Fungicide Efficacy Trials to Manage Tar Spot and Improve Economic Returns in Corn in the United States and Canada

Morgan Goodnight, Purdue University; Darcy E. P. Telenko, Purdue University; Tiffanna J. Ross, Purdue University; Martin I. Chilvers, Michigan State University; Tom W. Allen, Mississippi State University; Keith Ames, University of Illinois; Adam M. Byrne, Michigan State University; Jill C. Check, Michigan State University; W. Scott Jay, University of Guelph Ridgetown Campus; Brian Mueller, University of Wisconsin-Madison; Camila Rocco da Silva, Purdue University; Emily M. Roggenkamp, Michigan State University; Sujoung Shim, Purdue University; Damon L. Smith, University of Wisconsin-Madison; Albert U. Tenuta, Ontario Ministry of Agriculture, Food and Agribusiness; and Nathanael M. Thompson, Purdue University.

Summary

Fungicides were evaluated over multiple states and provinces for efficacy against tar spot.

• All foliar fungicides that were evaluated reduced tar spot severity.

• Fungicides with two or three modes of action (MOAs) provided greater suppression than one MOA and the nontreated control.

• Yield protection from fungicides ranged from 1.5 to 7.9% under low to moderate tar spot severity and from 3.1 to 8.5% under high tar spot severity (compared to the nontreated control).

Effects of hybrid susceptibility and fungicide applications were also evaluated.

• Moderately resistant hybrids had less tar spot severity than the susceptible hybrid.

• The combination of a susceptible hybrid and a fungicide reduced tar spot severity, but fungicide application may not be necessary for reducing tar spot severity on moderately resistant hybrids.

• A fungicide application at tassel/silk (VT/R1) resulted in higher yield compared to no fungicide treatment.

• Yield varied by hybrid:

  • Only one moderately resistant hybrid had more yield than the susceptible hybrid, even though both resistant hybrids had significantly less tar spot.

  • Fungicide may still be necessary to protect yield, especially with a known susceptible hybrid or risk of other foliar/ear rot diseases.

  • More research is needed to develop high-yielding hybrids with tar spot resistance.

The economic benefits of fungicide applications were calculated for Indiana data.

• A fungicide application resulted in a yield difference ranging from 0.1 bu/A to 20.7 bu/A under high tar spot severity, and from -7.5 bu/A to 1.3 bu/A under low tar spot severity, compared to not treating with fungicide.

• Under high tar spot severity, a ground-applied fungicide resulted in a partial net-return ranging from $11.9 loss to $83.9/A gain, while under low tar spot severity, the partial net-return ranged from a $46.5 loss to a $5.8/A loss, relative to no fungicide treatment.

• Application of Trivapro (the product used in this study) at 13.7 fl oz/A between VT/R1 and dough (R4) growth stages were most effective in reducing tar spot severity and resulting in positive net return. Applications made before the VT/R1 growth stage were too early to provide lasting disease reduction and yield protection.

Introduction

Tar spot of corn, caused by the fungus Phyllachora maydis, has been a yield-threatening disease in the United States (U.S.) since 2018 (Mueller et al. 2020; Telenko et al. 2019). Tar spot has led to estimated annual yield losses in the U.S. averaging 104.8 million bushels per acre from 2018 to 2023 (Crop Protection Network 2024). Currently, tar spot has been confirmed in nineteen U.S. states and Ontario, Canada (Bergstrom 2021; Bissonnette 2020; Collins et al. 2021; Dalla Lana et al. 2019; Henrickson et al. 2023; ipmPIPE 2024; Kness 2023; Malvick et al. 2020; McCoy et al. 2018; Moura et al. 2023; Pandey et al. 2022; Tenuta 2020; Ruhl et al. 2016; Wise et al. 2023). A real-time map of active tar spot counties in the U.S. and Canada can be found here.

In the U.S. and Canada, tar spot tends to be detected later in the crop season, typically in late vegetative to early reproductive growth stages (Telenko et al. 2020). The characteristic black fungal structures (called stromata) resemble raised black drops of tar across the leaf surface and the lesion extends through the leaf. Even if the stromata are not yet visible, spores (usually ascospores) can still be present. It takes about 14 to 20 days for visible signs and symptoms to appear once the leaf is infected (Hock et al. 1995; Valle-Torres et al. 2020). Under optimal conditions, complete blighting of the plant can occur in a matter of weeks (Telenko et al. 2022). Yield impacts can occur when tar spot limits the corn plant‘s ability to reach physiological maturity, leading to a reduction in grain fill (Telenko et al. 2019).

Research Goals

• Evaluate foliar fungicide efficacy on tar spot of corn in different disease pressure situations in multiple Midwest states and Ontario, Canada

• Assess integrated management strategies on tar spot severity and yield

• Evaluate economic returns for fungicide trials controlling tar spot in Indiana

The Research

Multi-state fungicide efficacy for tar spot (2019 and 2020)

In 2019 and 2020, a total of eight small-plot trials were performed in Freeport, IL (2019); Monmouth, IL (2020); Wanatah, IN (2019 and 2020); Fennville, MI (2019); Decatur, MI (2020); and Arlington, WI (2019 and 2020). Hybrids were chosen based on maturity relative for each location and considered susceptible to tar spot. In these trials, 13 fungicide products (Table 1) were used in determining efficacy for reducing tar spot severity and yield impacts. All products were applied at the corn tassel/silk (VT/R1) growth stage, except for Illinois in 2020, which was applied at the milk (R3) growth stage due to delayed tar spot onset. Tar spot severity was assessed at the dent (R5) growth stage. Tar spot severity ranged from 1.6 to 23.3% across all environments and yield ranged from 182.5 to 275.9 bu/A. All fungicides reduced tar spot severity compared to the nontreated control across all eight environments (Figure 1). Aproach Prima, Delaro, Headline SC, Revytek, and Veltyma reduced tar spot severity compared to the nontreated control, although these fungicides were not different from Headline AMP, Miravis Neo, and Topguard EQ. Delaro, Revytek, and Veltyma protected yield compared to the nontreated control, but these were not different from all other treatments except for Aproach Prima and Tilt (Figure 2). When grouping products based on the number of fungicide classes products with two or three classes reduced tar spot severity more over products with only one fungicide class (Figure 3).

Table 1. Fungicide product, FRAC code, active ingredients, and application rates of fungicide products used in the trials conducted in 2019, 2020, and 2021. All products were applied at tassel/silk (VT/R1) corn growth stage.

Multi-state fungicide efficacy for tar spot during a severe epidemic (2021)

An additional set of trials took place during the severe tar spot epidemic of 2021. Five small-plot field trials were established in Urbana, IL; Wanatah, IN; Decatur, MI; Arlington, WI; and Rodney, Ontario Canada. Hybrid selections were based on what was appropriate for each location and deemed susceptible based on the researchers’ experience (Telenko et al. 2019). Nine fungicide products (Table 1) were applied at the tassel/silk (VT/R1) growth stage. Tar spot severity were assessed at the dent (R5) growth stage. All fungicide applications significantly reduced tar severity compared to the nontreated control (Figure 4). Delaro Complete, Revytek, and Veltyma fungicides reduced disease severity the most, but were not significantly different from Headline SC, Headline AMP, and Aproach Prima. Delaro Complete, Revytek, and Veltyma fungicides protected yield the most (Figure 5), but were not different from the other fungicide treatments.

Integrated management of tar spot with hybrid by fungicide (2019 to 2021 Indiana)

A multiyear field study was conducted in Wanatah, IN from 2019 to 2021 to evaluate the impacts of hybrid resistance and fungicide on tar spot severity. Three corn hybrids were selected, two moderately resistant and one susceptible, and the foliar fungicide Trivapro was applied VT/R1. Tar spot severity was significantly lower in the partially resistant hybrids compared to the susceptible hybrid when no fungicide was applied, but there was no notable difference between the resistant hybrids (Figure 6). The single application of Trivapro at VT/R1 did significantly reduce tar spot in the susceptible hybrid, but showed no difference from the nontreated for the moderately resistant hybrids. Only one of the partially resistant hybrids had greater yield than the susceptible hybrid, but there was no significant yield difference between the two partially resistant hybrids (Figure 7). Overall, a fungicide application increased yield in all hybrids when compared to the nontreated control.

Integrated management of tar spot with hybrid by fungicide (2020 IN, MI, WI)

A multistate field study was performed in 2020 in Wanatah, IN; Decatur, MI; and Arlington, WI to assess hybrid resistance and fungicide efficacy on tar spot. Two hybrids (susceptible and partially resistant) were tested and one fungicide (Trivapro) was used with an application at tassel/silk (VT/R1). There were no significant differences in tar spot severity at the R4 growth stage between hybrids or fungicide application. However at the the R5 and late R5/R6 growth stages, both the resistant hybrid and a fungicide application at VT/R1 significantly reduced tar spot severity compared to the susceptible hybrid and no fungicide application, respectively (Figures 8 and 9). There were no significant differences in yield for hybrid susceptibility or fungicide application (Figures 8 and 9).

Economic return analysis for fungicide application timing for tar spot management in Indiana.

From 2019 to 2021, field experiments were established that evaluated fungicide application timings based on corn growth stages. Locations were in northwestern IN (Wanatah) and west-central IN (West Lafayette). Wanatah had high tar spot severity (>5%, 6.7 to 100.0 % severity) and was designated the high disease location and West Lafayette had low tar spot severity (<5%, 0.0 to 0.3 % severity) and was designated the low disease location. Trivapro at 13.7 fl oz/A was used as the fungicide in the application timing experiments and partial economic return analysis (see Table 1 for additional fungicide information). Nine timings were chosen based on corn growth stages at the time of application and a nontreated control was also included. Tar spot severity was rated weekly from first detection to R5 or R6. Significant reductions in stromata in the lower canopy were observed following an application between VT/R1 and R3. Compared to the nontreated control, the greatest reduction in tar spot severity in the lower canopy (50% reduction) occurred with the application made at R2. An application at R5 resulted in the smallest reduction in tar spot severity (13% reduction compared to nontreated). Similar observations occurred for ratings taken in the mid-and upper-canopies. However, for the R2 timing, there was a more substantial reduction in severity on the ear leaf, with more than a 50% decrease, compared to a 9% increase following an R5 application compared to the nontreated (Figure 10). An application of Trivapro did not result in significant yield protection, regardless of growth stage, in low tar spot severity situations. However, when severity was high, grain yield preservation was significantly greater with applications made at all growth stages except at the V6/V7, V8/V9, and R5, compared to the nontreated control (Figure 11). Although the R5 application was not significantly different from all other application timings, the greatest yield response was observed following an application made at R2, with general reductions in yield preservation declining from that growth stage onward. In situations of low disease severity, no significant differences were observed in partial returns regardless of timing, and in fact, they resulted in negative partial returns. However, in high severity situations, significant differences were noted. Specifically, significant increases in partial returns were only observed for applications made at the VT/R1, R2, R3, and R4 growth stages (Figure 10).

Conclusion

Many fungicides were effective at reducing tar spot severity and protecting yield, with some products demonstrating greater efficacy than others. In 2019 and 2020, Revytek, Delaro, Headline, Aproach Prima, and Veltyma were the most effective at reducing tar spot severity compared to the nontreated control. Applications of Delaro, Revytek, or Veltyma also had higher yield compared to the nontreated control. In 2021, Delaro Complete, Revytek, and Veltyma were most effective in reducing tar spot severity, and these same fungicides provided the highest yield protection compared to the nontreated control.

Although applying a fungicide can substantially reduce tar spot severity, it may not be necessary for hybrids with some level of disease resistance. However, for susceptible hybrids, a fungicide will be needed to limit tar spot impacts and protect yield in high severity situations.

The results from these studies improve our understanding of the effectiveness of different fungicides and hybrids under varying levels of tar spot severity. An integrated disease management program, including hybrid selection, disease prediction tools, and optimally timed fungicides, is vital for reducing disease and protecting yield. Corn farmers should regularly scout their fields and consider field history, as knowing past levels of tar spot is important for assessing potential residue buildup and its impact on tar spot progression under conducive environmental conditions.

This research update is based on the work described in the following peer-reviewed research articles:

Telenko, D. E. P., Chilvers, M. I., Ames, K., Byrne, A. M., Check, J. C., Rocco Da Silva, C., Jay, W. S., Mueller, B., Ross, T. J., Smith, D. L., and Tenuta, A. U. 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. Article / Google Scholar

Telenko, D. E. P., Chilvers, M. I., Byrne, A. M., Check, J. C., Rocco Da Silva, C., Kleczewski, N. M., Roggenkamp, E. E., Ross, T. J., and Smith, D. L. 2022. Fungicide efficacy on tar spot and yield of corn in the Midwestern United States. Plant Health Progress 23:281-287. Article / Google Scholar

Ross, T. J., Chilvers, M. I., Byrne, A. M., Smith, D. L., Mueller, B., Shim, S., and Telenko, D. E. P. 2023. Effects of disease resistance and fungicide application on management of tar spot on corn in the North Central United States. Plant Health Progress 24:439-444. Article / Google Scholar

Ross, T. J., Allen, T. W., Shim, S., Thompson, N. M., and Telenko, D. E. P. 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. Article / Google Scholar

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Acknowledgements

Authors

Morgan Goodnight, Purdue University; Darcy E. P. Telenko, Purdue University; Tiffanna J. Ross, Purdue University; Martin I. Chilvers, Michigan State University; Tom W. Allen, Mississippi State University; Keith Ames, University of Illinois; Adam M. Byrne, Michigan State University; Jill C. Check, Michigan State University; W. Scott Jay, University of Guelph Ridgetown Campus; Brian Mueller, University of Wisconsin-Madison; Camila Rocco da Silva, Purdue University; Emily M. Roggenkamp, Michigan State University; Sujoung Shim, Purdue University; Damon L. Smith, University of Wisconsin-Madison; Albert U. Tenuta, Ontario Ministry of Agriculture, Food and Agribusiness; and Nathanael M. Thompson, Purdue University.

Reviewers

Andrew Friskop, North Dakota State University; Daren Mueller, Iowa State University; and Kiersten Wise, University of Kentucky.