An Overview of Tar Spot

Updated in 2026, this version replaces the 2020 An Overview of Tar Spot publication.

Tar spot is a foliar disease of corn that commonly occurs throughout the northern and southeastern U.S., Canada, Caribbean, Central America, Mexico, and South America. Since its first confirmation in the United States in 2015, the pathogen that causes the disease has become established across much of the U.S. Midwest and northeast, and Canada, and is annually yield limiting in many of these regions (Figure 1). This publication summarizes the current knowledge of tar spot biology, diagnosis, and management, incorporating advances from recent research.

Figure 1. Map of where tar spot has been confirmed in the United States and Canada since 2015. 

Symptoms and Signs

In the United States and Canada, tar spot of corn is caused by the fungus Phyllachora maydis. The pathogen produces small (1/16 to 1/4 inch; 0.16 cm to 0.63 cm), round to irregular diamond-shaped, slightly raised black structures called stromata. These structures develop on both the upper and lower surfaces of corn leaves and are embedded in the tissue, giving the leaves a rough or bumpy feel (Figure 2). In severe cases, stromata may also be observed on leaf sheaths, husks, and tassels.

Disease severity can increase rapidly under favorable conditions. On susceptible hybrids, severity on ear leaves can exceed 50 percent by the R5 (dent) growth stage. Premature leaf death may occur once severity reaches approximately 30 percent, reducing photosynthesis capacity during grain fill.

Tan-to-brown lesions with darker borders may occasionally develop around stroma, producing the characteristic “fisheye” appearance (Figure 3). Although fisheye lesions are common in parts of Mexico and Central America and have been associated with another fungus (Monographella maydis), this association has not been confirmed in the U.S. or Canada.  

The causes of fisheye lesions are unclear in the U.S. and Canada, but may reflect interactions among corn hybrid genetics, the genetics of the tar spot fungus, environmental conditions, or other microbes.

Disease Cycle

The tar spot fungus is an obligate pathogen that requires living corn tissue to grow and reproduce. Although other species of Phyllachora infect several grass hosts, P. maydis is currently known to infect only corn. 

In the U.S., P. maydis overwinters in infested corn residue, where stromata persist and serve as a primary source of inoculum the following growing season. Research confirmed stromata can overwinter for one season and produce viable spores, although spore viability varies depending on environmental conditions and residue degradation. Under favorable conditions, particularly with rainfall and high humidity, stromata release spores (ascospores and conidia; Figure 4). These spores are dispersed primarily by rain splash and short-distance wind movement, making local field-level inoculum an important factor in early infections. Longer distance spore dispersal between fields also appears to occur.

Following infection, new stromata can develop within approximately 12 to 21 days, depending on environmental conditions and host susceptibility. These stromata then produce additional spores, enabling multiple infection cycles within a single growing season when conditions remain favorable. 

Corn is susceptible to infection at all growth stages; however, disease onset prior to reproductive stages poses the greatest risk for yield losses. Early infections may be observed at V5-V6, but may not cause significant losses if environmental conditions are not favorable for repeated spore dispersal and disease progress.

Conditions that Favor Disease

Cool temperatures, approximately 60-74°F (16-23°C), and moisture events are key drivers of tar spot development. Research has demonstrated a complex relationship between moisture and disease development, with prolonged periods of high humidity or excessive rainfall negatively associated with disease development or spore release. Periods of intermittent wet and dry appear to influence disease development. For more details on the relationship between temperature and moisture, see Tar Spot Prediction in Corn: The Weather Matters. Corn production under irrigation can be at a much greater risk for yield losses compared to non-irrigated corn. Overhead irrigation can increase leaf wetness duration, thereby making conditions more conducive for disease development and spread when otherwise environmental conditions may not be favorable for the disease.

Yield Losses and Impact

Yield losses from tar spot are highly variable and influenced by disease onset timing, environmental conditions, and hybrid susceptibility. Research across the U.S. Corn Belt and Canada indicates the greatest losses occur when disease develops before or during early reproductive stages (VT-R3).

• When disease develops late, especially after R4-R5, yield impacts are often minimal

• Moderate epidemics typically result in approximately 10-30% yield loss

• Severe epidemics can result in 50% yield losses

Yield losses result from reduced phytosynthetic leaf area due to rapid disease progression, poor kernel fill, reduced ear weight and premature plant senescence. In some cases, severe infections have been associated with vivipary (kernel germination while still on the cob). High disease severity may also increase the risk of stalk rot and lodging, particularly when plants senesce prematurely.

Severe tar spot also reduces silage corn feed quality by rapidly reducing moisture, decreasing digestible components, and reducing available energy. To date, no mycotoxins have been associated with tar spot directly. High levels of tar spot can reduce whole plant moisture, making silo bunker management difficult and increasing the length of time to complete fermentation. During prolonged periods of poor fermentation, additional fungi can continue to grow, including those that produce mycotoxins. Care should be taken to harvest silage at optimum moisture, prioritizing fields with high levels of tar spot first.

Diagnosis

Tar spot can be diagnosed in the field by examining corn leaves for circular to diamond-shaped, black, tar-like spots called stromata. These structures are slightly raised, embedded in the leaf tissue, and often feel bumpy to the touch. A key diagnostic feature is that stromata cannot be rubbed or wiped off the leaf surface. In many cases, stromata develop through the leaf, emerging on both the adaxial (upper) and abaxial (lower) leaf surface. 

In the U.S., tar spot is most commonly observed from silking through to late grain fill (growth stages R1-R6), although infections can occur earlier under favorable conditions. Initial stromata may develop on either lower or upper canopy leaves, depending on when infection is initiated, and can be found on both green and senesced plant tissues. Occasionally, brown, necrotic tissue may surround the stromata, producing a fisheye appearance. While this symptom can aid identification, it is not always present and should not be required for diagnosis. 

Because tar spot can be confused with other conditions, confirmation may be necessary, especially during early disease development or in areas where tar spot has not been previously reported. If the diagnosis is uncertain, submit samples to a state diagnostic lab or contact an extension specialist for verification.

Diseases with Similar Symptoms

Common and Southern Rust (Puccinia spp.)

Usually, pustules from rust diseases are initially orange or red, but as they mature, the rust fungi shift into a different phase of their lifecycle, and pustules become dark brown or black fungal structures called telia, which can be mistaken for tar spot (Figures 5 and 6). Telia will also be raised and feel bumpy to the touch, similar to the stromata of tar spot.

How to distinguish corn rusts from tar spot:

It is difficult to distinguish the telia produced by rust fungi from the stromata of tar spot without laboratory confirmation. Trained diagnosticians can examine the spores produced by the lesions and determine the causal fungus.

Physoderma Brown Spot (Physoderma maydis)

Physoderma brown spot forms purple-to-brown circular spots along the leaf midrib and leaf sheath, and small, light-brown to orange circular spots that often appear in bands within the leaf tissue (Figure 7).

How to distinguish Physoderma brown spot from tar spot:

Tar spots are often slightly raised or feel bumpy when touched, while Physoderma spots are embedded in the plant tissue. Physoderma brown spot symptoms are often observed near the base of the leaf, while tar spot stromata can be observed along the entire leaf with higher density from the middle toward the tip of the leaf blade and are not observed on the midrib.

Insect Frass and Soil

How to distinguish insect frass and soil from tar spot:

Insect frass and soil will rub off the corn leaf and usually occur only on one side of the leaf. Tar spot stromata are embedded in both sides of the leaf tissue and will not rub off the leaf surface.

Management

1. Avoid highly susceptible hybrids. Consult with your seed dealer or crop adviser concerning tar spot hybrid tolerance and check university corn performance trial data. To date, all hybrids have some level of susceptibility to tar spot, although some are less susceptible than others.

2. Consider fungicides. Several fungicides are available for tar spot management, although their efficacy may vary. While fungicides effectively manage tar spot, timing of fungicide applications is important to successfully manage this disease. A single fungicide application at the tassel to milk (VT-R3) growth stage has generally provided the most consistent protection of the canopy from tar spot and the greatest potential for positive return on investment (ROI). A fungicide application at dough (R4) may also protect yield and provide positive ROI under high tar spot severity (≥5%). See the spray benefit table for tar spot. A 2-pass fungicide program may not always result in a positive ROI for tar spot, but may be necessary if the first application was made prior to tassel before tar spot was active in the canopy. A second application might be necessary under these circumstances, especially if the disease is active during the R3 to early R4 growth stages. Remember that a fungicide is only active for 2-5 weeks after application, depending on active ingredients applied. Therefore, early applications may miss the optimal timing of protection that typically occurs during the mid-reproductive growth stages.

For more information on fungicides available for tar spot management consult CPN-2011 Fungicide Efficacy for Control of Corn Diseases.

A Tar Spot Crop Risk Tool is available to help determine if environmental conditions are favorable for tar spot to help decide if a fungicide application is warranted.

A Fungicide ROI Calculator is also available.

3. Manage irrigation. Reducing the frequency and duration of leaf wetness may reduce disease. Anecdotal evidence indicates that excessive irrigation or frequent, light irrigation events may increase disease. However, there is limited research on the impact of irrigation on tar spot, and farmers who rely on irrigation should consult a local Extension specialist to determine how irrigation may influence disease development.

4. Nitrogen and planting density. Replicated research trials in Michigan have demonstrated that nitrogen applications, or lack thereof, do not influence tar spot severity. However, tar spot severity was significantly influenced by hybrid susceptibility and planting density. Interestingly, lower seeding densities resulted in significantly greater tar spot levels, especially on susceptible hybrids. With that said, farmers are encouraged to use plant populations that result in the economic optimal yield on their farm.  

5. Rotate to other crops. Crop rotation seems to only play a minor role in reducing risk of tar spot. However, this practice will allow residue to decompose and reduce the primary inoculum. At present, it is not yet known how many years of rotation away from corn are needed to reduce inoculum.

6. Manage residue. Tillage appears to only play a minor role in reducing risk of tar spot. Tilling fields buries infected residue and increases the rate of decomposition, which may help reduce the amount of overwintering inoculum of the tar spot fungus in a field, but will not reduce the risk of infection from locally dispersed inoculum.

7. Scout for stalk rot?  and be prepared to harvest heavily diseased fields early if push tests indicate that stalk integrity is impacted to avoid lodging. In-season confirmations of tar spot can be monitored at the Corn IPM Pipe.

Earn Certified Crop Adviser CEUs

Successfully complete a quiz for this publication to earn 0.5 CCA CEUs.

Find Out More

Other publications in the Corn Disease Management series are available on the Crop Protection Network website.

Check, J. C., Byrne, A. M., Singh, M. P., Steinke, K., Widdicombe, W. D. and Chilvers, M. I. 2023. Effects of nitrogen application rate and plant density on severity of tar spot of corn. 

Plant Health Progress 24: 416-423 doi.org/10.1094/PHP-12-22-0125-RS 

Groves, C.L., Kleczewski, N.M., Telenko, D.E.P., Chilvers, M.I., and Smith, D.L., 2020. Phyllachora maydis ascospore release and germination from overwintered corn residue. Plant Health Progress 21:26-30. https://apsjournals.apsnet.org/doi/pdf/10.1094/PHP-10-19-0077-RS 

Ross, T. J., Allen, T. W., Shim, S., Thompson, N. M., and Telenko, D. E. P. 2023. Investigations into economic returns resulting from foliar fungicides and application timing on management of tar spot in Indiana hybrid corn. Plant Disease.. https://doi.org/10.1094/PDIS-05-23-0932-RE 

Ross, T. J., Chilvers, M. I., Byrne, A. M., Smith, D. L., Mueller, B., Shim, S., and Telenko, D. E. P. 2023. Integration of disease tolerance and fungicide application for management of tar spot on hybrid corn in North Central United States. Plant Health Progress. 24:439-444. https://doi.org/10.1094/PHP-10-22-0103-RS 

Telenko, D. E. P., Chilvers, M. I., Ames, K., Byrne, A. M., Check, J. C., Da Silva, Jay, W. S., C. R., Ross, T. J., Smith, D. L., and Tenuta, A. 2022. Fungicide efficacy during a severe epidemic of tar spot on corn in the United States and Canada. Plant Health Progress. 23:342-344. doi.org/10.1094/PHP-02-22-0012-BR  

Telenko, D. E. P., Chilvers, M. I., Byrne, A. M., Check, J. C., Da Silva, C. R., Kleczewski, N. M., Roggenkamp, E., Ross, T. J., and Smith, D. M. 2022. Fungicide efficacy on tar spot and yield of corn in the Midwest. Plant Health Progress. 23:281-287. doi.org/10.1094/PHP-10-21-0125-RS 

Waibel, K., Goodnight, K. M., Rocco da Silva, C., Bonkowski, J., Creswell, T., Poudel, P., Quinn, D. J., Ruhl, G., Shim, S., Weaver, J. C., Wise, K. A., and Telenko, D. E. P. 2025. Tracking the distribution and risk of tar spot of corn in Indiana from 2015 to 2022. Plant Health Progress.. https://doi.org/10.1094/PHP-08-24-0082-S 

Webster, R. W., Nicolli, C., Allen, T. W., Bish, M. D., Bissonnette, K., Check, J. C., Chilvers, M. I., Duffeck, M. R., Kleczewski, N., Luis, J. M., Mueller, B. D., Paul, P. A., Price, P. P., Robertson, A. E., Ross, T. J., Schmidt, C., Schmidt, R., Schmidt, T., Shim, S., Telenko, D. E. P., Wise, K., and Smith, D. L. 2023. Uncovering the Environmental Conditions Required for Phyllachora maydis Infection and Tar Spot Development on Corn in the United States for Use as Predictive Models for Future Epidemics. Scientific Reports. 13: 17064. https://doi.org/10.1038/s41598-023-44338-6

Acknowledgements

Authors

Darcy Telenko, Purdue University; Mandy Bish, University of Missouri; Boris Camiletti, University of Illinois Urbana-Champaign, Martin Chilvers, Michigan State University; Tamra Jackson-Ziems, University of Nebraska-Lincoln; Diane Plewa, University of Illinois; Alison Robertson, Iowa State University; Madalyn Shires, South Dakota State University; Damon Smith, University of Wisconsin-Madison; Albert Tenuta, Ontario Ministry of Agriculture, Food and Agribusiness; Kiersten Wise, University of Kentucky, and Yuan Zeng, Virginia Tech

Reviewers

Tom Allen, Mississippi State University; Travis Faske, University of Arkansas; Dean Malvick, University of Minnesota; Daren Mueller, Iowa State University; Trey Price, LSU AgCenter; Jean Williams-Woodward, University of Wyoming

How to cite: Telenko, D., Bish, M., Camiletti, B., Chilvers, M., Jackson-Ziems, T., Plewa, D., Robertson, A., Shires, M., Smith, D., Tenuta, A., Wise, K., Zeng, Y. 2026. An Overview of Tar Spot. Crop Protection Network. CPN-2012 (Revision). doi.org/10.31274/cpn-20190620-008.