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Managing Tar Spot with a Combination of Genetics and Fungicide

Categories: GROWING, CORN
  • Tar spot is a relatively new disease in the U.S., but it has already spread throughout the Corn Belt and become an established pathogen.
  • Hybrid susceptibility or tolerance to tar spot is the key factor in yield loss potential after tar spot infects a cornfield.
  • Dialing in the timing of fungicide application(s) is critical to tar spot management.

Tar spot in corn is a relatively new disease in the United States. It was first reported in northwest Indiana and north-central Illinois in 2015. Prior to 2015, tar spot only occurred in Mexico, Central America and northern parts of South America in cooler, high-elevation environments. In 2018, this disease caused significant yield loss in the U.S. in a multi-state outbreak and recently has spread as far west as Nebraska and as far south as Georgia and Florida (Figure 1).

US map highlighting counties with confirmed tar spot disease in corn.
Figure 1. Counties confirmed with tar spot incidence (Corn ipmPIPE 9/22/22)

While knowledge of how this disease impacts corn production is still limited, the University of Illinois Extension notes that since infections are typically sparse and often occur late in the season, yield loss has been minimal. However, heavy disease pressure during grain fill may compromise grain and stalk quality. We encourage you to know how to identify this late-season disease, since tar spot can overwinter in the soil and residue, potentially becoming a larger problem in coming years.

Fungal pathogen responsible for tar spot

Tar spot observed in the U.S. is caused by a fungus referred to as Phyllachora maydis.1 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 fungi is referred to as tar spot complex and known to cause more severe yield loss when both pathogens are present.2 Monographella maydis and its association with P. maydis has not yet been observed in North America, although significant yield loss is frequently observed.

Tar spot identification

  • Tar spot can be identified by raised black, irregular-shaped fungal structures (Figure 2) called stromata, which appear as specks of tar splattered onto the leaf surface.
  • Lesions have a bumpy feel that is not easily rubbed off.
  • Spots can 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.
  • Tar spot is often confused with:
    1. Common and Southern rust late in the season, as these diseases switch from producing orange-red spores (urediniospores) to black spores (teliospores). However, rust pustules may easily be scraped from the leaf.
    2. Saprophytic organisms that break down dead plant tissue late in season. However, those will not exhibit a bumpy texture.
  • To confirm proper identification, consider submitting samples to a National Plant Diagnostic Network lab.
Tar Spot fungus close up picture on corn stalk.
Figure 2. Phyllachora maydis, the fungus causing tar spot, with and without lesions forming around stromata.

Tar spot development

The repeated occurrence of tar spot in the Midwest indicates it is well adapted to overwintering on soil and residue. Tar spot development and infection is highly dependent on extended periods of cool nighttime temperatures, often observed when average night-day temperatures reduce to 60-70°F. Infection is also highly dependent on having extended periods of leaf wetness resulting from high overnight humidity levels. Infection can occur at any stage of crop development if inoculum is present and favorable environmental conditions exist. After infection it can take 14-21 days before tar spot lesions begin to appear. Symptoms generally start on lower leaves and rapidly move up the plant if favorable environmental conditions persist. Typical hot and dry July weather patterns delay tar spot infections until later in the growing season. However, tar spot can develop earlier in the season, resulting in severe loss of leaf area, reducing yield potential and standability. Late season disease development, occurring just prior to or at crop maturity, is more frequently observed and depends on the infection timing, so it may have minimal impact on yield.

Part of what makes tar spot a challenging disease is that it is polycyclic, meaning that within a 21-day period it can complete a growth cycle and form new spores that can spread and cause secondary infections. Spores move by wind and plant residue that can be carried by field equipment to other fields.

Five key factors drive yield loss potential

  1. Hybrid tolerance to tar spot
  2. Presence and quantity of inoculum in a field
  3. Environmental conditions for infection and spread
  4. Growth stage of corn when it is infected and when lesions form
  5. Effectiveness of management practices

The timing of tar spot infection is a driver in potential yield loss from this disease. Temperature and moisture influence when the infection from present inoculum begins. The later in the season that infection occurs, the lesser the potential is for yield loss. Since tar spot is polycyclic and can produce new spores in overlapping cycles, the disease pressure can grow and spread rapidly.

Tar Spot infection and symptom influence on corn yield loss timeline
Figure 3. Tar spot infection/symptom timeline

Tar spot management practices

Due to the newness of this disease in the U.S., best management practices are still being developed. Employing multiple management practices is critical for dealing with tar spot.

  1. Hybrid selection:

    Hybrids differ in susceptibility to tar spot infection, making hybrid selection one of the first tools for managing this disease. Hybrid differences observed in Table 1 can be used in hybrid placement decisions for fields with known histories of tar spot.
  2. Crop rotation and tillage:

    Burying residue with tillage and rotating crops to avoid exposure of overwintering pathogens are a common cultural practice used to manage many diseases. Recent research on tar spot has shown small reductions in disease severity from using these cultural practices. The ability of tar spot spores to spread within the growing season is likely minimizing most benefits of using rotation or tillage. Rotation and tillage may help in some situations but are likely not effective enough to use as a standalone management practice.
  3. Fungicide application:

    Early fungicide applications at or before first signs of development have been effective at reducing tar spot in previous trials.3 However, infection often occurs weeks before symptoms, making timing of preventative fungicide applications challenging. Late-season curative applications of fungicides are typically not effective. Scouting, along with using the Tarspotter app to understand when conditions are most conducive for disease development, can help better time spray applications.
    • If conditions are favorable for tar spot development early in the season, an application at V4-V8 corn growth stage and/or the V10-VT growth stage with a registered product could reduce infection within fields confirmed with tar spot in prior years.
    • A second application may be warranted if conditions persist for infection around the VT-R3 growth stage.
    • If a susceptible hybrid was planted, a conducive environment for infection continues, or the field has a history of significant tar spot, a second application could be needed.
    • Fully registered Syngenta fungicide options to manage tar spot include Trivapro® fungicide and Miravis® Neo fungicide.
    • It is important to also consider other disease risks. Potential for other diseases like gray leaf spot, Northern corn leaf blight, Northern corn leaf spot and rust can help simplify decisions to treat fields with a broad-spectrum fungicide that can manage multiple diseases simultaneously.
  4. Other considerations:

    Observations of reduced tar spot severity in dryland corners of irrigated fields have highlighted the potential influence of irrigation on tar spot development. Current research is further investigating limiting irrigation to daytime hours to determine if that can help minimize disease development.
Table showing corn hybrids rated 1-9 on dealing with Tar Spot disease.
Table 1. Golden Harvest Hybrid Tar Spot 1-9 Rating. 1= best 9 = worst
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