Tar Spot of Corn Web Book
CPN 2015. Published March 16, 2021. DOI: doi.org/10.31274/cpn-20201214-2
Darcy Telenko, Purdue University; Martin Chilvers, Michigan State University; Nathan Kleczewski, University of Illinois; Daren Mueller, Iowa State University; Diane Plewa, University of Illinois; Alison Robertson, Iowa State University; Damon Smith, University of Wisconsin; Adam Sisson, Iowa State University; Albert Tenuta, Ontario Ministry of Agriculture, Food and Rural Affairs; and Kiersten Wise, University of Kentucky.
Characteristic tar spot symptoms on corn foliar tissue.
Live Tar Spot Distribution Map from the Corn ipmPIPE
For current tar spot updates:
#tarspot on Twitter
Pest and Crop Newsletter from Purdue University
Michigan State University Extension Corn Newsletter
Download theTarspotter app, a tar spot forecasting managment tool for iOS and Android.
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Use the Table of Contents on this page to navigate between chapters and resources available as part of this text.
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In the United States, tar spot of corn is caused by the fungus Phyllachora maydis. The fungus produces small (1/16-3/4 inch), round to irregular diamond-shaped, raised black structures called stromata (Figure 1). These structures form on both the upper and lower surfaces of corn leaves. Stromata may also develop on leaf sheaths, husks, and tassels.
Tar spot severity on ear leaves at growth stage R5 (dent stage) can exceed 50 percent in susceptible hybrids when conditions are favorable for disease development. Leaves of infected plants prematurely die when severity is approximately 30 percent or more.
Figure 1. Tar spot stromata on corn foliar tissue.
Ed Zaworski
Occasionally, tan to brown lesions with dark borders can develop around the stromata. The lesions are referred to as fisheye lesions because of their appearance (Figure 2). Fisheye lesions are frequently observed in areas of Mexico and Central America. When fisheye lesions occur in these areas, the disease is called tar spot complex, because a second fungus (Monographella maydis) is thought to be associated with these lesions.
Although fisheye lesions have been observed in the United States, M. maydis has not been detected. The cause of fisheye lesions observed in United States tar spot outbreaks is currently unknown. Fisheye lesions could potentially be related to hybrid genetics, the genetics of the tar spot fungus, the environment, a different microbe forming a complex with P. maydis, or some unknown factor.
Figure 2. Tar spot fisheye lesions on corn foliar tissue.
Martin Chilvers
Tar spot on corn leaf. Image: Martin Chilvers
Tar spot on corn ear. Image: Martin Chilvers
Heavy tar spot infestation. Image: Martin Chilvers
Aerial view of field with tar spot infestation. Image: Martin Chilvers
Tar spot on corn leaf. Image: Kiersten Wise
Early tar spot development. Image: Ed Zaworski
Tar spot symptoms and signs.
Tar Spot Identification
Tar spot can also be confused with several other corn foliar diseases. This includes southern rust, common rust, and Physoderma brown spot. Insect frass and soil can also appear similar to tar spot on leaves. See Tar Spot of Corn (CPN 2012) for additional details on commonly confused diseases.
The tar spot fungus (P. maydis) is an obligate pathogen, which means that it requires a living host to grow and reproduce. Phyllachora maydis overwinters in Midwestern states where the disease has been confirmed. Viability of ascospores in stroma in the spring can range from 2.5 to 25 percent on corn leaves that overwinter in Midwest U.S. fields. Rain and high humidity cause the stromata to release the spores that are dispersed by rain splash or wind. Spores can be dispersed in-field and locally.
According to data from Central America, ascospores are released as single spores or in bunches. After infection, new stromata form within infected tissue in 12-15 days. The stromata can produce spores soon thereafter. When conditions are favorable, multiple spore release events and infection cycles can occur during the growing season. Corn is susceptible to infection at any developmental stage.
Tar spot disease cycle
Iowa State University Integrated Pest Management
In Central America, cool temperatures 60-70°F (16- 21°C) and high relative humidity (greater than 75 percent) favor tar spot development. In addition, the disease increases when there is at least 7 hours of free moisture on the leaves due to rain, fog, or high relative humidity. Corn production under irrigation is at a much greater risk to yield losses compared to non-irrigated corn. Overhead irrigation can increase leaf wetness duration, thereby making conditions more conducive for disease development and spread.
Yield losses due to tar spot can be variable, depending on the time of disease onset, weather conditions, and hybrid susceptibility. Losses can be minimal to none, and in severe cases, losses of 50 bushels per acre or more have been observed on susceptible hybrids. Yield losses are a function of reduced ear weight, poor kernel fill, and vivipary (a condition in which the seed germinates while still on the cob). Stalk rot and lodging may increase when tar spot severity is high. Severe tar spot also reduces silage corn feed quality by reducing moisture, decreasing digestible components and reducing energy. No associated mycotoxins have been reported for this disease.
Corn tar spot can be diagnosed in the field by examining corn leaves for the presence of the circular to diamond-shaped, black, tar-like spots, which usually have a slightly raised appearance and feel bumpy to the touch. Tar spot stromata cannot be wiped off the leaf. Tar spot has been observed most often in the United States during or after corn silking through to late grain fill (growth stages R1-R6), but may appear earlier. Initial stromata can form on lower or upper leaves depending on the onset of disease development, and have been observed on green (Figures 1 and 2) and senesced tissues. Occasionally, necrotic brown tissue may surround the black stomata, which produces a fisheye appearance. If you suspect tar spot, send a sample to your state diagnostic lab or contact your extension state specialist to confirm the diagnosis.
Figure 1. A single tar spot lesion on corn foliar tissue.
Darcy Telenko
Figure 2. Early tar spot lesions developing on corn foliar tissue.
Darcy Telenko
Example of fisheye lesion severity training
Tar spot stroma severity rating charts, ranging from 1 to 25 percent leaf coverage.
Iowa State University Integrated Pest Management
Tar spot fisheye severity rating charts, ranging from 1 to 25 percent leaf coverage.
Iowa State University Integrated Pest Management
Our understanding of tar spot in the United States is limited because of its recent establishment. Most of what we know about tar spot has originated from Mexico and Central America; however, differences in regional environments, fungal populations, hybrid genetics, and cropping systems may influence disease development and management practices.
Several management practices may help reduce tar spot development and severity.
Avoid highly susceptible hybrids. Speak to your seed dealer or crop adviser and check university corn performance trial data. Due to the recent establishment of tar spot, there have been limited opportunities to screen hybrids and breeding material for resistance to tar spot. To date, all hybrids have some level of susceptibility to tar spot, though some are less susceptible than others.
Consider fungicides. Some fungicides may reduce tar spot, and there are several fungicides with 2ee labels that can be used to manage tar spot. While fungicides have shown efficacy in managing tar spot, proper timing of fungicide applications is important to manage this disease succesfully. We have little consistent data regarding the optimal time to apply fungicides for tar spot management. Determining the optimum timing has been difficult due to year-to-year variability in disease onset and severity observed thus far. Efforts are underway to understand the biology and epidemiology of this disease, which may help to develop better fungicide application timing recommendations. The Tarspotter app is a tar spot disease forecasting management tool developed for iOS and Android designed to help with fungicide application decisions. For more information on fungicides available for tar spot management consult CPN-2011 Fungicide Efficacy for Control of Corn Diseases.
Manage irrigation. Reducing the frequency and duration of leaf wetness may reduce tar spot. 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.
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 tar spot inoculum.
Manage residue. Tillage appears to 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 tar spot inoculum in a field, but will not reduce the risk of infection from locally dispersed inoculum.
Scout for tar spot and be prepared to apply fungicides or harvest heavily diseased fields early if push tests indicate that stalk integrity is impacted to avoid lodging. In-season confirmations of tar spot across the U.S. can be monitored at the Corn ipmPIPE.
Earn 0.5 Certified Crop Advisor CEUs after reading this web book. Click here for the quiz.
Tar spot susceptibility varies by corn hybrid, as seen in this aerial image where the green parts of the field were planted to a less susceptible hybrid.
Martin Chilvers
Disease forecasting
Download the Tarspotter app, a tar spot forecasting management tool for iOS and Android.
Information and updates
#tarspot on Twitter
Pest and Crop Newsletter from Purdue University
Michigan State University Extension Corn Newsletter
University of Wisconsin Tar Spot Archive
Tar Spot (CPN-2012) by CPN
Peer-reviewed manuscripts
Tar Spot: An Understudied Disease Threatening Corn Production in the Americas by Valle-Torres et al. in Plant Disease. 2020.
Documenting the Establishment, Spread, and Severity of Phyllachora maydis on Corn, in the United States by Kleczewski et al. in Journal of Integrated Pest Management. 2020.
Corn Yield Loss Estimates Due to Diseases in the United States and Ontario, Canada, from 2016 to 2019 by Mueller et al. in Plant Health Progress. 2020.
Phyllachora maydis Ascospore Release and Germination from Overwintered Corn Residue by Groves et al. in Plant Health Progress. 2020.
First Report of Tar Spot on Corn Caused by Phyllachora maydis in the United States by Ruhl et al. in Plant Disease. 2016.
Tools
Tarspotter 3.0. Coming Soon.
Earn 0.5 Certified Crop Advisor CEUs after reading this web book. Click here for the quiz.
Authors
Darcy Telenko, Purdue University; Martin Chilvers, Michigan State University; Nathan Kleczewski, University of Illinois; Daren Mueller, Iowa State University; Diane Plewa, University of Illinois; Alison Robertson, Iowa State University; Damon Smith, University of Wisconsin; Adam Sisson, Iowa State University; Albert Tenuta, Ontario Ministry of Agriculture, Food and Rural Affairs; and Kiersten Wise, University of Kentucky.
Citation
Telenko, D., Chilvers, M., Kleczewski, N., Mueller, D., Plewa, D., Robertson, A., Smith, D., Sisson, A., Tenuta, A., and Wise, K., 2021. Tar Spot of Corn. Crop Protection Network. CPN 2015. doi.org/10.31274/cpn-20201214-2.
Reviewers
Andrew Friskop, North Dakota State University and Dean Malvick, University of Minnesota.
Images
Darcy Telenko, Purdue University; Martin Chilvers, Michigan State University; Kiersten Wise, University of Kentucky; and Ed Zaworkski, Iowa State University. Illustrations by Emily Poss, Iowa State University Integrated Pest Management.
Sponsors
Support for the creation of this web book was provided by the Foundation for Food and Agricultural Research (FFAR).
This information in this publication is only a guide, and the authors assume no liability for practices implemented based on this information. Reference to products in this publication is not intended to be an endorsement to the exclusion of others that may be similar. Individuals using such products assume responsibility for their use in accordance with current directions of the manufacturer.
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