Management Considerations for Tar Spot in Indiana

In Indiana, tar spot has been an annual concern since 2018 when growers experienced 20-60 bu/A loss. This past season favorable weather conditions led to another severe epidemic where there are reports of fields experiencing 50% reduction in yields across the Midwest. Tar spot has continued to spread and has now been confirmed in 82 of 92 Indiana counties, 14 states, and Ontario Canada. As to say tar spot is a disease has become the number one topic in corn during our winter meetings.

Therefore, I am going to share some points on what we have learned and how to plan for this disease in 2022 and beyond.

My first question to a grower is how severe tar spot was on your farm in 2021? Did you find a few lesions (upper green leaf in Fig 1.) or was it severely blighted and covered with stromata (lower leaf in Fig 1)? In our research trials in central Indiana (West Lafayette), we saw limited tar spot impact. I can find the small black spots (stroma of the fungus), but it has yet to get above 1% severity. Gray leaf spot was our bigger concern. There were extremely dry/drought conditions across central Indiana in 2021 – where lack of water was a bigger concern than disease. If tar spot was not severe on your farm you won’t get a return on investment (ROI) to manage it, but be aware, on the lookout, and prepared to make in-season decisions should the environment become favorable.

 

Figure 1. Severe tar spot causing blighting. Tar spot lesions (stromata) on corn leaves. (Photo Credit: Darcy Telenko)

Figure 1. Severe tar spot causing blighting. Tar spot lesions (stromata) on corn leaves. (Photo Credit: Darcy Telenko)

 

If the farm saw severe tar spot, I suggest a few things for next year.

  1. Watch the tracking map to know when the disease is first active in Indiana. I will worry about the disease staring early if we have a wet June and July like we did in 2021. Otherwise the disease won’t appear to mid- to late- July. (https://corn.ipmpipe.org/tarspot/ or https://extension.purdue.edu/fieldcroppathology/)
  2. Download the Tarspotter app to help with determining if the weather conditions are favorable for tar spot to develop in your fields. (https://ipcm.wisc.edu/apps/tarspotter/)
  3. Scout, scout, and continue to scout your fields.
  4. Make informed fungicide decisions. Only in 2021 did our research trials show a benefit of two application at V10/V14 with a follow up application 3 weeks later. We have seen severe disease every season in Porter County – yield impact will all depend on when the disease starts. In 2019 and 2020, we DID NOT see a benefit of a second fungicide application, that is why it is important to monitor and scout.
  5. As for a fungicide timing window VT-R2 has consistently provided good protection with a single application program.
  6. We need to make an informed decision on our fungicide use not only for ROI, but also for fungicide stewardship to make sure we aren’t increasing risk for fungicide resistance to develop.
  7. No, it will not be cost effective to apply fungicide every year. I suggest being flexible and it is important to understand how severe the disease was on your farm. Moisture plays a significant role in how fast tar spot develops.

A summary of what we have learned thus far.

Tar spot will continue to be an issue in Indiana

  • Severity level will be a function of the hybrid, weather, and when epidemic initiates earlier vs. later in the season.
  • The 2021 epidemic was problematic, because tar spot started in some fields before tasseling.
  • Fungus driven by weather – a wet July in 2021 compared to 2019 and 2020.
  • Varying levels of tar spot occur across region due to weather

The tar spot fungus can overwinter in the upper Midwest

  • We now have high inoculum levels in many locations.
  • Weather is key (water and irrigation management).
  • Rotation may help a bit, not a sole solution.
  • Tillage may help reduce or delay onset of disease (reducing residue).
  • Tar spot inoculum (spores) can travel long distances.

Some hybrids are more resistant than others

  • Strong hybrid resistance can be overcome by a favorable disease environment.

Fungicide application can reduce tar spot severity

  • Product is important, use multiple modes of action (QoI + DMI or QoI + DMI + SDHI) (See resources for details on fungicide efficacy)

Timing very important

  • Application needs to occur close to the onset of the epidemic
  • Number of applications and optimal timing are going to vary by year.
  • Tarspotter isn’t perfect, but a valuable tool to help make the decision, and optimize, fungicide applications
  • If just spraying once and not interested in prediction, VT-R2 has been most consistent timing in Indiana.

Understand your farm – what disease(s) are most of concern in each field.

What you can do?

  1. Assess your risk – is the disease endemic in your area? Did you find it in your fields in 2021? If so, how severe did tar spot get at the end of the season?
  2. Talk to your seed salesmen about hybrid resistance.
  3. Scout and monitor your fields throughout the season.
  4. Use these tools if you have fields at high risk and are going to apply fungicides.
    1. Fungicide efficacy tables and Extension research reports (see links below)
    2. Use the Tarspotter App to monitor for conducive weather conditions
    3. Follow the map to learn when tar spot is active new your county https://corn.ipmpipe.org/tarspot/
  5. Leave check strips if you try a new management strategy.
  6. Don’t forget about the other diseases – new and established (gray leaf spot, southern rust, ear and stalk rots, etc.).

Resources:

Tar spot of corn: Impact and management options. PDF of Dr. Telenko’s 2022 tar spot slide deck from winter meetings.

Purdue Field Crop Pathology Extension Website for in-season updates, the tar spot and southern rust maps and other resources.

Crop Protection Network: tar spot publications and web book.

Applied Research in Field Crop Pathology for Indiana – summaries of research trials in Indiana.  (2021 data will be available Feb 1).

Tarspotter Apps

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