Mythbusting Tar Spot: Separating Fact from Fiction

Tar spot, a fungal disease caused by Phyllachora maydis, has caused periodic yield losses in corn since 2018 in the U.S. and Canada, drawing significant attention from farmers and others in the agricultural industry (Figure 1). As tar spot continues to spread into corn-producing regions where it has previously not been detected, so do myths and misconceptions about the disease. Distinguishing science-based facts from unproven claims about the fungus and disease development is critical to better understanding how to manage the problem. In this article, we address a few of the common myths about tar spot and provide research-backed guidance to support effective disease management.

Myths about tar spot center around claims that the tar spot fungus is influenced by micronutrients such as manganese or other chemical elements. Similar claims about the influence of micronutrients and/or common herbicides on plant diseases are not new in agriculture and have previously been addressed in multiple articles, including “Glyphosate-Manganese Interactions and Impacts on Crop Production: The Controversy” by Purdue University. However, new iterations of these myths have emerged and are addressed below.

Myth: The black spots on corn leaves with tar spot are manganese oxide buildup, not fungal spores.

Facts: No research-based evidence supports the claim that the black spots associated with tar spot on corn leaves are caused by a buildup of manganese oxide (MnO₂). While manganese can play a role in plant health and disease severity, the black spots are indeed fungal material. Studies using electron microscopy (a specialized, high-powered microscope) by researchers at Purdue University confirm that the black spots on symptomatic leaves are fungal structures, not mineral deposits. These structures, known as stromata, are fungal tissue structures that contain both sexual and asexual spores, providing clear evidence that tar spot is caused by a fungus. The abstract, which describes this research, is “Uncovering the Infection Strategy of Phyllachora maydis during Maize Colonization: A Comprehensive Analysis” by Caldwell et al. Readers interested in additional close-up images of fungal structures can refer to this article. Misidentifying tar spot as a mineral deposit can lead to ineffective management strategies.

Myth: The tar spot fungus “mines” manganese, causing the plant to become manganese deficient.

Facts: Manganese (Mn) is an essential plant micronutrient involved in various key physiological processes, including resistance to some diseases. While Mn's role in tar spot development is not fully understood, its importance in overall disease resistance in plants is well-documented. For example, Mn affects lignin production, strengthening cell walls and hindering pathogen invasion.

It is critical to understand that environmental conditions can influence both nutrient availability and plant health. In the case of Mn, plants take up the reduced form of Mn which occurs in moist soil conditions, and not the oxidized form of Mn which is found in dry soil conditions. During periods of insufficient rainfall, plants can exhibit Mn tissue deficiencies (both visual and analytical), which dissipate once rainfall is adequate. Plants already stressed from dry soil conditions may be more susceptible to both pathogen infection and Mn deficiencies. Dry soil conditions can limit the diffusive movement of Mn to plant roots, affecting uptake patterns. Soil physical and chemical properties can also influence Mn tissue concentrations at R1 (silking). Soil pH and imbalances with other cations are two additional factors that may lead to Mn deficiencies.

Recent research from Purdue University observed a poor correlation (R² = 0.10) between Mn concentrations at R1 (silking) and tar spot severity at R5 (dent) on the ear leaf of corn (Figure 2). Typically, healthy corn plants showed Mn tissue concentrations between 40 and 100 ppm with concentrations declining with plant growth and growing season progression. Note that some of the highest levels of tar spot were observed from plants testing in the optimal range for Mn. Without the context of growing environments, tissue testing may be misinterpreted. This means that there are a lot of factors associated with tar spot severity and that Mn concentrations are not very useful for understanding tar spot severity.

Myth: Feeding your plants helps the plants stay healthier and defend against tar spot during the season.

• Sugar and molasses. While it is true that healthy plants are better able to defend against diseases, there is no evidence that applying sugar to corn has any impact on reducing corn diseases like tar spot or improving plant health. This topic has been addressed by many University Extension Specialists over the years, and a 2024 summary on this topic by the University of Wisconsin-Madison indicates that sugar and molasses, in particular, are not likely to help with crop health.

• Potassium (K). There are over 200 peer-reviewed publications related to the impact of K and its effect on plant diseases. While many of these studies suggest that applying K reduced disease severity, there are also studies where K application increased disease severity. It is important to consider both soil and plant tissue concentrations, as well as specific fertilizer sources because the application of one nutrient can influence the efficiency of another (e.g., nitrogen), making plants more susceptible to disease through changes in plant moisture or cell wall thickness. Moreover, many of these studies did not include plant tissue concentration or environmental data, which could affect results. To date, no published studies have documented the effect of K on tar spot.   

• Nitrogen (N). Research trials conducted at Michigan State University demonstrated no relationship between N application rate and tar spot severity. A summary of these trials can be found in the article How Do Agronomic Choices Affect Tar Spot Severity.

Recent research from Purdue University observed poor correlation between ear leaf K concentrations at R1 (silking) and tar spot severity at R5 (dent) of corn ear leaves (R² = 0.15; Figure 3). This means that there are a lot of factors associated with tar spot severity and that ear leaf K concentrations are not very useful for understanding tar spot severity.

Myth: Glyphosate and glufosinate stimulate tar spot development.

Facts: There is no research-based evidence that indicates these herbicides affect tar spot development or the pathogen itself.

Conclusions

Many misconceptions about tar spot persist, including those related to hybrid susceptibility or resistance, fungicide efficacy and timing, and additional management practices. Some of these misconceptions arise from field observations where an adequate control or comparison is not present. While it is crucial to address these misconceptions, it is equally important to recognize that regional variability and environmental conditions play a significant role in the effectiveness of specific management strategies. The myths outlined above are factually inaccurate regardless of region, hybrid, etc. If you have concerns about the validity of claims regarding tar spot management, consult your state extension specialist for the latest research-based recommendations tailored to your region. Making management decisions based on reliable, unbiased, rigorously tested data is the most effective way to protect corn yield from the impact of tar spot.

Current science-based tar spot management recommendations include employing an integrated management program: selecting hybrids with moderate resistance, scouting regularly, and using disease prediction tools to help determine optimal fungicide application timing that will reduce disease and protect yield. Many fungicides that are effective in reducing tar spot have shown the greatest efficacy when applied between tassel (VT) and milk (R3) growth stages. See the Corn Fungicide Efficacy Tool for more information on selecting a fungicide and the corn tar spot map for in-season updates.  

Acknowledgments

Authors

Martin Chilvers, Michigan State University; Kurt Steinke, Michigan State University; Dan Quinn, Purdue University; Darcy Telenko, Purdue University; Daren Mueller, Iowa State University; Alison Robertson, Iowa State University; Damon Smith, University of Wisconsin-Madison; and Kiersten Wise, University of Kentucky 

Reviewers

Daisy Ahumada, North Carolina State University; Tom Allen, Mississippi State University; Mandy Bish, University of Missouri; Boris X. Camiletti, University of Illinois Urbana-Champaign; Nicholas Dufault, University of Florida; Maira Duffeck, Oklahoma State University; Travis Faske, University of Arkansas; Tamra Jackson-Ziems, University of Nebraska-Lincoln; Shelly Pate Kerns, Louisiana State University AgCenter; Madalyn Shires, South Dakota State University; and Yuan Zeng, Virginia Tech.