An Overview of Tar Spot

Symptoms and Signs

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. These structures form on both the upper and lower surfaces of corn leaves (Figure 2). In severe cases, stromata may also be observed 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 the disease development. Leaves of infected plants prematurely die when severity is approximately 30 percent or more.

Occasionally, tan to brown lesions with dark borders can develop around the stromata (Figure 3). The lesions are referred to as fisheye lesions because of their appearance. 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. 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. In any case, the cause of fisheye lesions observed in United States tar spot outbreaks is currently unknown.

Disease Cycle

The tar spot fungus (P. maydis) is an obligate pathogen, which means that it requires a living host to grow and reproduce. Although there are many species of Phyllachora that infect various grass species, P. maydis is only known to infect corn. Phyllachora maydis can overwinter in Midwestern states where the disease has been confirmed. Spring ascospore viability can range from as low as 2.5 percent to as high as 25 percent on corn leaves that overwinter in Midwest U.S. fields. Rain and high humidity cause the stromata to release spores (ascospores and conidia; Figure 4) 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.

Conditions that Favor Disease

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, disease increases when there is at least seven 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 and Impact

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. 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.

Diagnosis

You can diagnose corn tar spot in the field by examining corn leaves for the presence of circular to diamond-shaped, black, tar-like spots, which may 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 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 and senesced tissues. Occasionally, necrotic brown tissue may surround the black stromata, 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.

Diseases with Similar Symptoms

Management

Our understanding of this disease in the United Statesis 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.

1. Avoid highly susceptible hybrids. Speak to your seed dealer or crop adviser and check universitycorn 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.

2. 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 tarspot, timing of fungicide applications is importantin successfully managing this disease. We have little consistent data regarding the optimal time to apply fungicides for tar spot management. Determiningthe 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. For more informationon fungicides available for tar spot management consult CPN-2011 Fungicide Efficacy for Control of Corn Diseases (doi.org/10.31274/cpn-20190620- 002).

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. 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 toreduce inoculum.

5. 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 tar spot inoculum in a field, but will not reduce the risk of infection from locally dispersed inoculum.

6. Scout for tar spot 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 //corn.ipmpipe.org/tarspot.

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Acknowledgements

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; Albert Tenuta, Ontario Ministry of Agriculture, Food and Rural Affairs; and Kiersten Wise, University of Kentucky.

Reviewers

Tom Allen, Mississippi State University; Gary Bergstrom, Cornell University; Kaitlyn Bissonnette, University of Missouri; Carl Bradley, University of Kentucky; Emmanuel Byamukama, South Dakota State University; Alyssa Collins, Pennsylvania State University; Nicholas Dufault, University of Florida; Paul Esker, Pennsylvania State University; Travis Faske, University of Arkansas; Andrew Friskop, North Dakota State University; Tamra Jackson-Ziems, University of Nebraska; Doug Jardine, Kansas State University; Alyssa Koehler, University of Delaware; Dean Malvick, University of Minnesota; Hillary Mehl, Virginia Tech University; Pierce Anderson Paul, The Ohio State University; Trey Price, LSU AgCenterRichard Raid, University of Florida; Ed Sikora, Auburn University; Adam Sisson, Iowa State University; Lindsey Thiessen, North Carolina State University; and Heather Young-Kelly, University of Tennessee.

Photo Credits

All photos were provided by and are property of the authors and contributors, except Figure 2, by Edward Zaworski, Iowa State University.