Tar Spot in Corn in the Midwestern U.S.
Causal Pathogen and Occurrence in the U.S.
- Tar spot is a relatively new disease of corn in the Midwestern U.S., first appearing in Illinois and Indiana in 2015 (Bissonnette, 2015; Ruhl et al., 2016) and subsequently spreading to Michigan, Wisconsin, and Iowa (Figure 1). Its presence was also confirmed in Florida in 2016 (Miller, 2016).
- Tar spot in corn is caused by the fungus Phyllachora maydis, which was first observed in high valleys in Mexico.
- P. maydis has not typically been associated with yield loss by itself; however, it can form a complex with another pathogen, Monographella maydis, the combination of which is referred to as tar spot complex. In Mexico, the tar spot 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.
- Tar spot reappeared in 2016 and 2017 but remained a relatively minor cosmetic disease of little economic concern.
- In 2018, however, it became much more severe with significant outbreaks reported in Illinois, Indiana, Wisconsin, Iowa, and Michigan.
Figure 1. Counties with confirmed incidence of tar spot as of October 2018 (University of Illinois, Purdue University, University of Wisconsin, Iowa State University, and Michigan State University Extension).
Identification and Symptoms
- 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. 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).
- 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).
- P. maydis alone produces small, round, dark lesions; M. maydis causes a brown necrotic ring around the P. maydis ascomata. Together, they produce the characteristic “fish-eye” symptom of tar spot complex (Figure 3).
- 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).
Figure 2. A corn leaf with symptoms of P. maydis (tar spot).
Figure 3. A corn leaf demonstrating “fish-eye” symptoms of tar spot complex (P maydis + M. maydis).
Figure 4. Microscopic view of fungal spores of P maydis.
Tar Spot Arrival in the U.S.
- 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 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 associated with tar spot complex, M. maydis, into the U.S. via people and/or materials is believed to be high (Mottaleb et al., 2018).
Figure 5. Corn leaf under magnification showing dense coverage with tar spot ascomata.
Tar Spot Epidemiology
- Much remains unknown 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).
- Tar spot has been reported every year since its initial confirmation, which suggests that P. maydis is overwintering in the Midwestern U.S.
- P. maydis 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.
- It appears to have windborne spores and tends to release them in 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 some Midwestern fields in 2018 (Smith, 2018; personal observation).
- M. maydis was not detected in association with fish-eye symptoms in these cases. The cause of the fish-eye symptoms and why they showed up in some fields but not others remains undetermined.
Figure 6. Corn leaves infected with tar spot in a field in Stephenson Co., IL; September 1, 2018. Tar spot was prevalent in this field, but symptoms appeared late in the season when senescence was already beginning. Stalk lodging was minimal in this field, and yield data suggested that tar spot likely had little to no impact.
- The potential yield impact of tar spot in corn in the Midwestern U.S. is undetermined at this point.
- Anecdotal reports associated tar spot infection with yield loss and increased rates of stalk lodging; however, weather conditions were highly conducive for foliar diseases, reduced stalk quality, and stalk rots in many areas in 2018, so it is not clear how much tar spot may have caused or exacerbated these issues relative to other factors.
Differences in Hybrid Response
- Observations in hybrid trials in 2018 showed that hybrids differed in severity of tar spot symptoms (Kleczewski and Smith, 2018).
- The extent to which differences in leaf symptoms may correspond to differences in yield is unknown at this time.
- Pioneer agronomists and sales professionals collected data on disease symptoms and hybrid performance in locations where tar spot was present in 2018 and will use those findings to assist growers with hybrid management in 2019.
- Research in Mexico has shown that fungicide treatments can be effective against tar spot (Bajet et al., 1994), although no fungicides are currently labeled for tar spot control in the U.S.
- Efficacy of fungicides in managing tar spot in the Midwestern U.S. is still undetermined. University trials were conducted in 2018 and will continue in 2019 to help determine if and how fungicides may be used to help manage tar spot.
Tillage and Rotation
- The pathogen that causes tar spot appears to be overwintering 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.
- If spores are able to disperse over large distances within a region (also unknown), local effects of crop residue in continuous corn and/or reduced tillage systems may be relatively inconsequential.
- There is no evidence at this point that tar spot causes ear rot or produces harmful mycotoxins (Kleczewski, 2018).
Will Tar Spot Continue to Spread in the U.S.?
- 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 indicate the areas beyond the current range of infestation at highest risk for spread of tar spot are central Iowa and northwest Ohio.
- Results indicate 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.
Figure 7. Corn husk and leaf with tar spot symptoms (Stephenson Co., IL; September 1, 2018).
- Bajet, N.B., B.L. Renfro, J.M. Valdez Carrasco. 1994. Control of tar spot of maize and its effect on yield. International Journal of Pest Management, 40:121-125.
- Bissonnette, S. 2015. CORN DISEASE ALERT: New Fungal Leaf disease “Tar spot” Phyllachora maydis identified in 3 northern Illinois counties. The Bulletin. University of Illinois Extension.
- Chalkley, D. 2010. Systematic Mycology and Microbiology Laboratory, ARS, USDA. Invasive Fungi. Tar spot of corn - Phyllachora maydis.
- Hock J., J. Kranz, B.L. Renfro. 1995. Studies on the epidemiology of the tar spot disease complex of maize in Mexico. Plant Pathology, 44:490-502.
- Kleczewski, N. 2018. Setting the Record Straight on Tar Spot in Corn. Successful Farming. Sept. 28, 2018.
- Kleczewski, N. and D. Smith. 2018. Corn Hybrid Response to Tar Spot. The Bulletin. University of Illinois Extension.
- Miller, C. 2016. Tar spot of corn detected for the first time in Florida. University of Florida Extension.
- Mottaleb, K.A., A. Loladze, K. Sonder, G. Kruseman, and F. San Vicente. 2018. Threats of tar spot complex disease of maize in the United States of America and its global consequences. Mitigation and Adaptation Strategies for Global Change. Online May 3, 2018:
- Robertson, A., E. Zaworski, and E. Stoetzer. 2018. Tracking Tar Spot in Corn 2018. Iowa State Univ. Extension. Integrated Crop Management News.
- Ruhl G., M.K. Romberg, S. Bissonnette, D. Plewa, T. Creswell, and K.A. Wise 2016. First report of tar spot on corn caused by Phyllachora maydis in the United States. Plant Dis 100(7):1496.
- Smith, D. 2018. Holy Tar Spot Batman! Badger Crop Doc Blog. University of Wisconsin.
Author: Mark Jeschke