Identification and Symptoms of Tar Spot
- Tar spot is the physical manifestation of circular-sharped, tar colored fungal fruiting bodies, called ascomata, developing on corn leaves.
- Initial symptoms are small brown lesions that darken with age.
- The texture of the leaf becomes bumpy and uneven when the fruiting bodies are present.
- Tar spot lesions cannot be rubbed away completely or dissolved in water.
Figure 1: Corn leaves infected with tar spot in a field in Illinois in 2018
- Under favorable conditions, tar spot spreads from the lowest leaves to the upper leaves, leaf sheathes, and eventually the husks of the developing ears.
- Severe infection can cause leaf necrosis.
- Affected ears can have reduced weight and loose kernels, and kernels at the ear tip may germinate prematurely.
Figure 2: Corn leaf showing tar spot lesions
Figure 3. Counties with confirmed incidence of tar spot, 2015-2020 (as of 10-12-20). Source: Corn ipmPIPE, 2020.
Tar Spot Occurrence in the U.S and Canada
- 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, Missouri, Minnesota, Pennsylvania, and southern Ontario (Figure 1).
- If you suspect Tar Spot in your corn crop, please contact your Pioneer Area Agronomist or Sales Representative
Tar Spot Epidemiology
- 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 more likely to develop during 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 environ-
mental 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.
Figure 4: Microscopic view of fungal spores of P. maydis.
Yield Impact of Tar Spot
- 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).
- Severe tar spot infestations have been associated with reduced stalk quality. If foliar symptoms are present, monitor stalk quality carefully to determine harvest timing.
- There is no evidence that tar spot causes ear rot or produces harmful mycotoxins (Kleczewski, 2018).
Differences in Hybrid Response
- Observations in hybrid trials indicate that hybrids differ in susceptibility to tar spot (Kleczewski and Smith, 2018).
- 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).
- 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.
- Specific management recommendations for fungicides in the Canada are still being developed.
- Research suggests that tar spot may be challenging to control with a single fungicide application due to its rapid reinfection cycle, particularly in irrigated corn.
Agronomic Practices to Manage Tar Spot
- The pathogen that causes tar spot overwinters in corn residue. How the amount of residue on a field’s soil surface affects disease severity the following year is unknown.
- 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.
The foregoing is provided for informational use only. Please contact your Pioneer sales professional for information and suggestions specific to your operation. Product performance is variable and depends on many factors such as moisture and heat stress, soil type, management practices and environmental stress as well as 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.
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. http://bulletin.ipm.illinois.edu/?p=3423
Kleczewski, N. 2018. Tar Spot on Corn: Setting the Record Straight. Illinois Field Crop Disease Hub. University of Illinois Extension. http://cropdisease.cropsciences.illinois.edu/?p=822
Kleczewski, N. and D. Smith. 2018. Corn Hybrid Response to Tar Spot. The Bulletin. University of Illinois Extension. http://bulletin.ipm.illinois.edu/?p=4341
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.
Telenko, D., M.I. Chilvers, N. Kleczewski, D.L. Smith, A.M. Byrne, P. Devillez, T. Diallo, R. Higgins, D. Joos, K. Kohn, J. Lauer, B. Mueller, M.P. Singh, W.D. Widdicombe, and L.A. Williams. 2019. How tar spot of corn impacted hybrid yields during the 2018 Midwest epidemic. Crop Protection Network.
Wise, K. 2020. Fungicide Efficacy for Control of Corn Diseases. Crop Protection Network. CPN-2011-W.