Tar Spot in Midwestern Corn

Categories: GROWING, CORN

Figure 1. Counties confirmed with Tar Spot incidence in 2019 (Corn ipmPIPE 10/21/19)

Tar spot of corn is a relatively new disease to the U.S. It was first reported in northwest IN and north-central IL in 2015 by Kiersten Wise, Gail Ruhl and Tom Creswell from Purdue University. Since 2015, this disease has spread and can now be found in several states (Figure 1). Prior to 2015, tar spot only occurred in Mexico, Central America and northern parts of South America in cooler, high elevation environments.

Fungal Pathogen Responsible
Tar spot observed in the U.S. is caused by a fungus referred to as Phyllachora maydis. In Latin America, where P. maydis was first observed, it has been known to form a complex with a secondary fungal pathogen, Monographella maydis. The combination of the 2 fungus are referred to as tar spot complex and known to cause more severe yield loss when both pathogens are present. In Latin America where M. maydis has been observed, it appears as a secondary “fisheye” shaped lesion surrounding the original P. Maydis tar spot. Although similar lesions surrounding the tar spot lesion have been observed in the U.S., to date M. Maydis has not been formally identified.

Figure 2.  Phyllachora maydis, the fungus causing tar spot, with and without lesions forming around stromata.​​​​​​​

  • Tar spot can be identified by raised black, circular fugal structures (Figure 2), which appear as specks of tar splattered onto the leaf surface. 
  • Lesions have a bumpy feel that is not easily rubbed off.
  • Spots can also be surrounded by a small, tan halo, giving a “fish-eye” appearance.
  • The disease begins on the lower corn leaves and moves to the upper plant and ear husks.
  • Tar spot is found on both healthy and dead plant tissue on upper and lower surfaces of leaves.
  • It can be confused with common and southern rust later in the season, which switch from producing orange-red spores (urediniospores) to black spores (teliospores). Rust pustules differ in that they may easily be scraped from the leaf.
  • Tar spot may also be confused with saprophytic organisms that break down dead plant tissue late in-season. However, these organisms will not exhibit a bumpy texture.
  • Laboratory diagnosis may be required to correctly diagnose the disease.

Much about how this fungal pathogen behaves in the U.S. is still unknown, although it has been well studied in Latin America. Tar spot can move limited ranges by wind and plant residue. It appears to be capable of overwintering in soil and residue, due to its reappearance in years following its first introduction. This disease thrives in cool (60-70 F) and humid conditions with prolonged periods of wet leaves. Visual symptoms of tar spot generally start on the lower leaves and rapidly move up the plant.  Infection can occur at any crop stage, although it is most commonly observed throughout the grain fill period.

Management Practices 
  • Hybrid Selection: Hybrids differ in susceptibility to tar spot infection. Opportunistic field evaluations were collected at multiple fields in 2018 and 2019. Hybrid differences observed in Table 1 can be used in hybrid placement decisions for fields with known history of tar spot. 
  • Crop Rotation and Tillage: Rotating to crops other than corn and utilizing tillage to bury residue could help reduce fungus inoculum levels in fields. Due to the newness of tar spot, much is still unknown of the magnitude of the reduction that comes from increased residue management practices. 
  • Fungicide Application:  Early fungicide applications at or before first signs of development have been effective against tar spot in previous trials. Early fungicide programs applied prior to the onset of disease can be effective, however late season curative applications of fungicides are not recommended.
  • If conditions are favorable for tar spot development early in the season, an application at V4-V8 corn growth stage and/or the VT/R1 growth stage with a registered product could reduce infection within fields previously confirmed with tar spot in prior years. 
  • Fully registered Syngenta fungicide options include Trivapro® along with EPA Section 2 (ee) special labels for use of Miravis® Neo and Quilt Xcel® to manage tar spot.    
  • It is important to consider all potential disease issues you could be facing when making fungicide decisions for your corn crop. Other diseases like gray leaf spot, Northern corn leaf blight, Northern corn leaf spot and rust may also be present, further improving chances of economic response.

For more information on tar spot or for help with scouting and decision making, contact your local Golden Harvest Seed Advisor or agronomist. 

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©2020 Syngenta. The trademarks or service marks displayed or otherwise used herein are the property of a Syngenta Group Company. All other trademarks are the property of their respective owners.


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