Tar Spot in Corn in the Midwestern U.S.
Crop Insights written by Mark Jeschke, Ph.D., Agronomy Manager
Crop Insights written by Mark Jeschke, Ph.D., Agronomy Manager
Tar spot is a foliar disease of corn that has recently emerged as an economic concern for corn production in the Midwestern U.S. It is not a new disease, having been first identified in 1904 in high valleys in Mexico. Historically, tar spot’s range was limited to high elevations in cool, humid areas in Latin America, but it has now spread to South American tropics and parts of North America. It first appeared in the U.S. in 2015. During the first few years of its presence in the U.S., tar spot appeared to be a minor cosmetic disease that was not likely to affect corn yield. However, widespread outbreaks of severe tar spot in multiple states in 2018 proved that it has the potential to cause a significant economic impact.
With its very limited history in the U.S., much remains to be learned about the long-term economic importance of this disease and best management practices. This Crop Insights discusses tar spot’s appearance and spread in the U.S., the epidemiology of the tar spot pathogen, identification and symptoms of the disease, and management considerations.
Corn leaves infected with tar spot in a field in Illinois in 2018.
Tar spot in corn is caused by the fungus Phyllachora maydis, which was first observed in high valleys in Mexico. Historically, P. maydis was not typically associated with yield loss unless a second pathogen, Monographella maydis, was also present, the combination of which is referred to as tar spot complex. In Mexico, the complex of P. maydis and M. maydis has been associated with yield losses of up to 30% (Hock et al., 1995). In some cases, a third pathogen, Coniothyrium phyllachorae, has been associated with the complex. Only P. maydis is known to be present in the United States but it has proven capable of causing significant yield losses, even without the presence of the second pathogen.
The first confirmations of tar spot in the U.S. were in Illinois and Indiana in 2015 (Bissonnette, 2015; Ruhl et al., 2016). It has subsequently spread to Michigan, Wisconsin, Iowa, Ohio, and Minnesota. Its presence was also confirmed in Florida in 2016 (Miller, 2016). Tar spot reappeared in 2016 and 2017 but remained a relatively minor cosmetic disease of little concern.
2018 Outbreak
In 2018, tar spot established itself as an economic concern for corn production in the Midwest, with severe outbreaks reported in Illinois, Indiana, Wisconsin, Iowa, Ohio, and Michigan. Significant corn yield losses associated with tar spot were reported in some areas. University corn hybrid trials conducted in 2018 suggested potential yield losses of up to 39 bu/acre under the most severe infestations (Telenko et al., 2019). Growers in areas severely impacted by tar spot anecdotally reported yield reductions of 30-50% compared to 2016 and 2017 yield levels. Yield losses specifically attributable to tar spot were often difficult to determine, however; because of the presence of other corn diseases due to conditions generally favorable for disease development. Instances of greatest tar spot severity in 2018 were largely concentrated in northern Illinois and southern Wisconsin, where other foliar diseases and stalk rots were also prevalent.
2019 Observations
In 2019, tar spot severity was generally lower across much of the Corn Belt and appeared later and more slowly compared to 2018, although severe infestations were still observed at some locations. There is no clear explanation for why tar spot severity was lower in 2019 in areas where it was severe 2018. Less favorable conditions for disease development during the latter part of the growing season in 2019 may have played a role. Reduced winter survival may have been a factor as well. Winter temperatures in some tar spot-affected areas oscillated between warm periods and extreme cold, which may have affected fungal dormancy and survival (Kleczewski, 2019)
Figure 1. Counties with confirmed incidence of tar spot prior to 2019 and new confirmations during 2019 (as of 10-21-19) showing the expanding range of disease incidence (Corn ipmPIPE, 2019; Pioneer agronomist observations).
Despite the generally lower disease severity, tar spot continued to expand its geographic range in 2019. In Iowa, tar spot presence was limited to around a dozen eastern counties in 2018, but expanded to cover most of the state in 2019 (Figure 1). Tar spot was confirmed in Minnesota for the first time in September of 2019 (Malvick, 2019). Tar spot spread to the south and east as well, with new confirmations in parts of Indiana, Ohio, and Michigan. Tar spot has likely spread into northeast Missouri as well.
Tar spot is the physical manifestation of fungal fruiting bodies, the ascomata, developing on the leaf. The ascomata look like spots of tar, developing black oval or circular lesions on the corn leaf (Figure 2). The texture of the leaf becomes bumpy and uneven when the fruiting bodies are present. These black structures can densely cover the leaf and may resemble the pustules of rust fungi (Figure 2 and Figure 3). Tar spot spreads from the lowest leaves to the upper leaves, leaf sheathes, and eventually the husks of the developing ears (Bajet et al., 1994).
Figure 2. A corn leaf with tar spot symptoms.
Figure 3. Corn leaf under magnification showing dense coverage with tar spot ascomata.
Figure 4. Microscopic view of fungal spores of P. maydis.
Under a microscope, P. maydis spores can be distinguished by the presence of eight ascospores inside an elongated ascus, resembling a pod containing eight seeds (Figure 4).
Tar Spot Look-Alikes
Common rust (Puccinia sorghi) and southern rust (Puccinia polysora) can both be mistaken for tar spot, particularly late in the growing season when pustules on the leaves produce black teliospores (Figure 5a). Rust pustules can be distinguished from tar spot ascomata by their jagged edges caused by the spores breaking through the epidermis of the leaf (Figure 5b). Rust spores can be scraped off the leaf surface with a fingernail, while tar spot cannot. Saprophytic fungi growing on senesced leaf tissue can also be mistaken for tar spot.
Figure 5a. Southern rust in the teliospore stage late in the season, which can resemble tar spot (left). Figure 5b. Corn leaf with common rust spores showing jagged edges around the pustules (right).
Figure 6. Corn leaf with tar spot symptoms.
Numerous reports have speculated that P. maydis spores may have been carried to the U.S. via air currents associated with a hurricane in 2015, the same mechanism believed to have brought Asian soybean rust (Phakopsora pachyrhizi) to the U.S. several years earlier. However, Mottaleb et al. (2018) believe that this scenario is unlikely and that it is more plausible that spores were brought into the U.S. by movement of people and/or plant material. Ascospores of P. maydis are not especially aerodynamic and are not evolved to facilitate spread over extremely long distances by air.
Tar spot was observed in corn in Mexico for over a century prior to its arrival in the U.S., during which time numerous hurricanes occurred that could have carried spores into the U.S. Chalkley (2010) notes that P. Maydis occurs in cooler areas at higher elevations in Mexico, which coupled with its lack of alternate hosts would limit its ability to spread across climatic zones dissimilar to its native range. Chalkley also notes the possibility of transporting spores via fresh or dry plant material and that the disease is not known to be seedborne. The risk of importation of the second pathogen of tar spot complex, M. maydis, into the U.S. via people and/or materials is believed to be high (Mottaleb et al., 2018).
Much is still being learned about the epidemiology of tar spot, even in its native regions, and especially in the U.S. P. maydis is part of a large genus of fungal species that cause disease in numerous other species; however, P. maydis is the only Phyllachora species known to infect corn, and it appears to only infect corn (Chalkley, 2010).
P. maydis is an obligate pathogen, which means it needs a living host to grow and reproduce. It is capable of overwintering in the Midwestern U.S. in infected crop residue on the soil surface. Tar spot is favored by cool temperatures (60-70 ºF, 16-20 ºC), high relative humidity (>75%), frequent cloudy days, and 7+ hours of dew at night. Tar spot is polycyclic and can continue to produce spores and spread to new plants as long as environmental conditions are favorable. P. maydis produces windborne spores that have been shown to disperse up to 800 ft. Spores are released during periods of high humidity.
So far, M. maydis has not been detected in the U.S. “Fish-eye” lesions, consistent in appearance with those caused by tar spot complex in Mexico, were observed in many Midwestern fields in 2018 and 2019 (Figure 7) (Smith, 2018; personal observation). M. maydis was not detected in association with fish-eye symptoms in any of these cases. The cause of the fish-eye symptoms and why they showed up in some fields but not others remains undetermined. Currently, fisheye symptoms in U.S. corn are believed to be a result of interactions among the host, pathogen, and environment.
Figure 7. Tar spot lesions with necrotic halos resembling the “fish-eye” lesions characteristic of the two-pathogen tar spot complex in Mexico. Fisheye lesions have been observed in many Midwestern fields even though the second pathogen of the complex has not been detected. Photo: Karen Zuver, Pioneer Field Agronomist.
Yield Impact
2018 was the first time that corn yield reductions associated with tar spot were documented in the U.S. University corn hybrid trials conducted in 2018 suggested potential yield losses of up to 39 bu/acre under heavy infestations (Telenko et al., 2019). Pioneer on-farm research trials, along with grower reports, showed yield losses of up to 50% under the most extreme infestations during the 2018 growing season.
Differences in Hybrid Response
Observations in hybrid trials in 2018 showed that hybrids differ in susceptibility to tar spot (Kleczewski and Smith, 2018). Tar spot affects yield by reducing the photosynthetic capacity of leaves and causing rapid premature leaf senescence. Longer maturity hybrids for a given location have been shown to have a greater risk of yield loss from tar spot than shorter maturity hybrids (Telenko et al., 2019).
Pioneer agronomists and sales professionals continue to collect data on disease symptoms and hybrid performance in locations where tar spot is present to assist growers with hybrid management. Pioneer hybrid trials have shown differences in canopy staygreen among Pioneer® brand corn products* and competitor products under tar spot disease pressure (Figure 8). Genetic resistance to tar spot should be the number one consideration when seeking to manage this disease, as it appears to have a greater impact on symptoms and yield loss than either cultural or chemical management practices.
Severe tar spot infestations have been associated with reduced stalk quality (Figure 9). If foliar symptoms are present, stalk quality should be monitored carefully to determine harvest timing.
Figure 8. Pioneer on-farm trial in Ottawa County, Michigan with high tar spot pressure showing differences in canopy staygreen among hybrids (September 27, 2019).
Figure 9. Field with severe tar spot infection and extensive stalk lodging in Wisconsin in 2018. Photo: Scott Rowntree, Pioneer Field Agronomist.
Fungicide Treatments
Research in Mexico on tar spot complex has shown that fungicide treatments can be effective against tar spot (Bajet et al., 1994). Specific management recommendations for the use of fungicides in managing tar spot in the Midwestern U.S. are still in development as more research is done. A limited number of university trials conducted in 2018 in locations where tar spot was present provided evidence that fungicides can reduce tar spot symptoms and potentially help protect yield. However, initial work also suggests that tar spot may be challenging to control with a single fungicide application due to its rapid reinfection cycle, particularly in irrigated corn. DuPont™ Aproach® and Aproach® Prima fungicides have both received FIFRA 2(ee) recommendations for control/ suppression of tar spot of corn.
Agronomic Practices
The pathogen that causes tar spot overwinters in corn residue but to what extent the amount of residue on the soil surface in a field affects disease severity the following year is unknown. Spores are known to disperse up to 800 ft, so rotation or tillage practices that reduce corn residue in a field may be negated by spores moving in from neighboring fields. Observations so far suggest that rotation and tillage probably have little effect on tar spot severity.
Duration of leaf surface wetness appears to be a key factor in the development and spread of tar spot. Farmers with irrigated corn in areas affected by tar spot have experimented with irrigating at night to reduce the duration of leaf wetness, although the potential effectiveness of this practice to reduce tar spot has not yet been determined.
Yield potential of a field appears to be positively correlated with tar spot risk, with high productivity, high nitrogen fertility fields seeming to experience the greatest disease severity in affected areas. Research on P. maydis in Latin America has also suggested a correlation between high nitrogen application rates and tar spot severity (Kleczewski et al., 2019).
Mycotoxins
There is no evidence at this point that tar spot causes ear rot or produces harmful mycotoxins (Kleczewski, 2018).
Mottaleb et al. (2018) used climate modeling based on long-term temperature and rainfall data to predict areas at risk of tar spot infection based on the similarity of climate to the current area of infestation. Model results indicated the areas beyond the then current range of infestation at highest risk for spread of tar spot were central Iowa and northwest Ohio. Observations in 2019 were consistent with model predictions, with further spread of tar spot in counties in northwestern Ohio and a dramatic expansion of tar spot across Iowa. Results indicated the potential for further expansion to the north and south but primarily to the east and west, including New York, Pennsylvania, Ohio, Missouri, Nebraska, South Dakota, eastern Kansas, and southern Minnesota.
Tar spot has not been detected in Canada yet but, given the spread of tar spot into eastern Michigan in 2019, pathologists expect it will show up in southern Ontario in the near future.
AM - Optimum® AcreMax® Insect Protection system with YGCB, HX1, LL, RR2. Contains a single-bag integrated refuge solution for above-ground insects. In EPA-designated cotton growing counties, a 20% separate corn borer refuge must be planted with Optimum AcreMax products. Q (Qrome®) - Contains a single-bag integrated refuge solution for above- and below-ground insects. The major component contains the Agrisure® RW trait, the YieldGard® Corn Borer gene, and the Herculex® XTRA genes. In EPA-designated cotton growing counties, a 20% separate corn borer refuge must be planted with Qrome products. Qrome® products are approved for cultivation in the U.S. and Canada. For additional information about the status of regulatory authorizations, visit http://www.biotradestatus.com/. HX1 - Contains the Herculex® I Insect Protection gene which provides protection against European corn borer, southwestern corn borer, black cutworm, fall armyworm, lesser corn stalk borer, southern corn stalk borer, and sugarcane borer; and suppresses corn earworm. YGCB - The YieldGard® Corn Borer gene offers a high level of resistance to European corn borer, southwestern corn borer and southern cornstalk borer; moderate resistance to corn earworm and common stalk borer; and above average resistance to fall armyworm. LL - Contains the LibertyLink® gene for resistance to Liberty® herbicide. RR2 - Contains the Roundup Ready® Corn 2 trait that provides crop safety for over-the-top applications of labeled glyphosate herbicides when applied according to label directions.
Herculex® Insect Protection technology by Dow AgroSciences and Pioneer Hi-Bred. Herculex® and the HX logo are registered trademarks of Dow AgroSciences LLC. YieldGard®, the YieldGard Corn Borer Design and Roundup Ready® are registered trademarks used under license from Monsanto Company. Liberty®, LibertyLink® and the Water Droplet Design are trademarks of BASF. Agrisure® is a registered trademark of, and used under license from, a Syngenta Group Company. Agrisure® technology incorporated into these seeds is commercialized under a license from Syngenta Crop Protection AG.
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The foregoing is provided for informational use only. Contact your Pioneer sales professional for information and suggestions specific to your operation. Product performance is variable and subject to any number of environmental, disease, and pest pressures. Individual results may vary. Pioneer® brand products are provided subject to the terms and conditions of purchase which are part of the labeling and purchase documents.
November 2019