Tar spot confirmed in Monroe County: Should you be concerned?

Given the recent tar spot epidemic of 2021, tar spot has been on everyone’s mind for the past few seasons. Tar spot lesions are hard, black, raised spots that are typically 1/16 to 3/4 inches in length which form on leaves. Lesions form on the upper side of the leaf and can protrude through the bottom.

Tar spot is distinct from other diseases but can be confused with early gray leaf spot, common rust or southern rust and insect frass (bug poop). Insect frass will wipe off with a little water, while tar spot lesions are embedded in the leaf and will not rub off.

If in doubt, symptomatic leaf samples can be sent to the MSU Plant and Pest Diagnostic Services.

Pictures of suspected tar spots can also be sent via email to chilvers@msu.edu or via twitter to @MartinChilvers1. Samples and pictures of suspected tar spot help us track the development of the disease which is documented at ipmPIPE.

Since Michigan detected tar spot in 2016, the greatest disease severity and losses have been in 2018 and 2021. These years were markedly wetter on average than other years which favors tar spot infection.

Morning dews, lingering fogs and rainy days provide extended leaf wetness durations that drive disease severity. The prolonged dryness throughout May and June appears to have slowed tar spot development thus far in 2023.

To date, tar spot has only been confirmed in Monroe County. A MSU Extension Virtual Breakfast discussion on tar spot management in 2023 can be found by clicking here.

Figure 1: Tar spot confirmations in yellow as of August 1, 2023. Gray color denotes counties in which tar spot has been found in previous years. A live map can be viewed at https://corn.ipmpipe.org/tarspot/

How should I manage tar spot?

Fungicide application

Fungicide applications can help slow disease development. Multiple modes of actions have been found to be effective against tar spot. Products with two or three modes of action have also demonstrated better performance than single mode of action products.

In addition to providing better control than a single mode of action a product with multiple modes of action will also help slow the development of fungicide resistance. The figures below are from trials replicated across Michigan, Illinois, Indiana, Wisconsin and Ontario.

We have seen that all products are able to suppress disease and protect yield, with some products having a slight edge over others. A handy fungicide efficacy chart is available at the Crop Protection Network.

Figure 2: Disease severity following different fungicide product applications from a 2021 multistate uniform fungicide trial. Fungicide applications made at R1 growth stage.

Figure 3: Yield following different fungicide product applications from a 2021 multistate uniform fungicide trial. Source: Telenko, D.E.P., Chilvers, M.I., Ames, K., Byrne, A.M., Check, J.C., Da Silva, C.R., Jay, W.S., Mueller, B., Ross, T.J., Smith, D.L., Tenuta, A.U. 2022. Fungicide efficacy during a severe epidemic of tar spot on corn in the United States and Canada. Plant Health Progress. https://doi.org/10.1094/PHP-02-22-0012-BR

Fungicide timing

Fungicide timing is critical in tar spot management. A single well-timed application can offer protection similar to two applications. In most years the best timings are likely between VT/R1 and R3. Early vegetative applications are likely too early to protect for the duration of the season.

Particularly in a year like this year, where disease is late to develop, fungicide applications beyond R3 might be too late; however, even at the beginning of dent (R5) there is still considerable dry matter accumulation in kernels, so keeping plants healthy up to physiological maturity is important.

To aid in determining if weather conditions are suitable for disease development the smart phone app ‘Tarspotter’ aims to assist in fungicide timing for tar spot by predicting periods of increased risk of disease. Be sure to scout your fields to know if tar spot is present.

Fungicide timing trial results from Michigan 2021: Yield and relative area under the disease progress curve (a measure of disease severity throughout the growing season) at different application timings

What are the deciding factors for a fungicide application?

  • Scout, scout, scout! We consistently see a stronger return on investment of foliar fungicides in years of high disease pressure. Get out and scout! To date 2023 has seen low levels of tar spot or other diseases.
  • Cup growth stage: In most years when disease is present a fungicide applied between VT/R1 to R3 sprays tend to suppress disease and protect yields best
  • Hybrid susceptibility: Response to fungicide is dictated by hybrid susceptibility. A highly tar spot resistant hybrid will often respond to a lesser extent than a susceptible hybrid.
  • Cost of application and logistics: Logistics and cost of application should be factored into the decision. Do you own application equipment, or do you need to schedule an applicator? If necessary, fungicide products can be kept until next year if stored correctly.
  • Check strips: Be sure to leave check strips. You’ll never know how things would have worked out if you don’t have check strips.

Hybrid resistance

Utilizing hybrids tolerant to tar spot will continue to be an important factor in management. Unfortunately, no hybrids are completely immune to tar spot, but there are differences in hybrid susceptibility.

Together with industry colleagues, we are working to screen hybrids to assess tar spot tolerance and provide growers resources for hybrid selection. We are also working with other researchers such as MSU’s Addie Thompson to identify genetic sources of resistance to tar spot to guide breeding efforts. Be sure to discuss tar spot tolerance with your seed salesperson.

Thank you to commodity groups that make our research possible including the Corn Marketing Program of Michigan.

Marty Chilvers, MSU; Farm News Media