Managing tar spot in corn
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EmailThe start of the 2024 season has seen weather favorable for tar spot development, and reports of disease are starting to come in. Read on to understand the risk and what you can do to combat this disease.
Take-aways
- Understand your corn hybrid weaknesses, be it tar spot, northern leaf blight or gray leaf spot.
- Scout fields regularly, especially as we come into reproductive stages.
- Most years a single fungicide application from tassel (VT/R1) to milk (R3) provides adequate suppression of tar spot.
- Leave a check strip if you are making a fungicide application so you can understand your return on investment.
- Use an app such as Tarspotter to assess risk conditions and aid in fungicide application timing.
Start of the 2024 season
Initial conditions for the 2024 season have seen frequent moisture events across the state and region. This coupled with increasing awareness has led to early detections of tar spot across the region including Michigan. If these conditions continue as they did in 2021, we may be in for a severe tar spot year. Weather conditions throughout the remainder of the season will be the primary factor for disease development. Farmers and crop consultants should make efforts to understand their corn hybrid susceptibility and be scouting for disease.
What causes tar spot?
Tar spot is caused by the fungal pathogen Phyllachora maydis. P. maydis can only infect green corn tissue and to date has not been found on any other crops of cover crops. The fungus survives on infected corn residue through the winter and releases spores in the spring that can infect new plants. It is not seed transmitted, though can be moved on infected leaf tissue.
Should I use tillage and crop rotation?
In short, no. Although tar spot does survive on residue, the spores can travel many miles on the wind. In our severe tar spot epidemic years of 2018 and 2021, there were numerous fields that hadn’t been in corn for years that were “smoked” with tar spot. Any value in tilling fields will be short lived as inoculum blows in from outside fields. Crop rotation is an important tenant for disease and insect management, however variety susceptibility and weather conditions play a much larger role in disease development.
What about other agronomic factors?
To date, research has demonstrated that nitrogen rates did not influence tar spot development. Tar spot is caused by a fungal pathogen, not by a lack of nutrients. Tar spot can develop just as happily on healthy plants as it can on those that are mismanaged. Although we have seen an inverse relationship between plant population and tar spot development, i.e., less disease in higher plant populations. Increasing plant density to lower disease did not improve the economics of production. However, in the studies hybrid susceptibility had a strong effect on tar spot severity and yield and should be prioritized for disease management.
Scout, scout, scout!
You can’t manage what you don’t know. It’s critical to be aware of disease development, especially as those final leaves have emerged and we move into reproductive stages.
How do I identify tar spot?
Tar spot lesions are relatively distinct, consisting of a matte black raised spot (1/16 – 3/4-inch diameter) that is embedded within the leaf and will not rub off the leaf surface. Tar spot lesions form on the top side of the leaf but will protrude through the bottom side of the leaf. Lesions are typically drawn to some extent with veins in the leaf, and often have a slight brown margin around them. Be sure of your diagnosis; fields have been sprayed when no tar spot was present. If in doubt, send a sample into the Michigan State University Plant and Pest Diagnostics, or send good quality pictures via email to chilvers@msu.edu or via X (Twitter) to @MartinChilvers1.
Insect frass (bug poop) is often confused with tar spot, however these are easy to distinguish as frass will dissolve and wipe off the leaf with some water while tar spot will not. Initial gray leaf spot lesions may look similar to tar spot, but typically they will have a dark margin but light-colored center of dead leaf tissue. Physoderma brown spot can also look similar but lesions are not raised. Late season rust pustules can also take on a dark color and look similar to tar spot, but upon close inspection rust pustules erupt through the leaf leaving a torn margin. Rust spores will wipe off the leaf onto your finger.
How should I manage tar spot?
Use of hybrids with tolerance/resistance to disease is the cornerstone of disease management. A hybrid with partial resistance to tar spot will greatly assist in slowing disease development and has the potential to negate the need for a fungicide. Unfortunately, no hybrid is immune to tar spot, but there is significant variation in hybrid susceptibility. Talk to your seed dealer about your hybrid’s risk potential. We continue to work with colleagues and industry to screen for resistance to tar spot.
If I apply a fungicide, what should I use?
In general, Michigan State University Extension recommends using a fungicide product with mixed modes of action, as this typically provides better disease suppression than a single mode of action product. We are also concerned with the development of fungicide resistance in the tar spot fungus (Phyllachora maydis). Using a mixed mode of action fungicide should slow the development of fungicide resistance. Each year, we conduct fungicide efficacy trials to assess current and new products and combinations. Those trials are summarized in fungicide efficacy charts that are updated annually and can be found at: Fungicide Efficacy for Control of Corn Foliar Diseases.
When should I apply a fungicide, and should I make two applications?
Depending on the timing of disease onset and weather, a fungicide application somewhere from VT/R1 to R3 will likely be the best timing. Early vegetative applications tend to have little effect on suppressing tar spot epidemics. Although we have seen greater disease suppression with two fungicide applications, this is not always profitable, especially with today’s lower corn prices.
Is there an app to help with fungicide timing?
There certainly is! We continue efforts to develop data sets to train and test the tar spot disease forecasting app Tarspotter developed with our colleagues at the University of Wisconsin-Madison. The Tarspotter app is free to download and can be found on smartphone app stores (a version is also available via Field Prophet). Tarspotter can be used to assess tar spot disease risk based on weather variables for multiple pinned locations within the app. When using the app, keep in mind it is still important to scout fields to see if disease is present and be mindful of crop growth stage. We typically see the best fungicide timing for tar spot suppression from silking (VT/R1) to milk (R3) or dough stage (R4).
Leave a check strip!
If you do decide to make a fungicide application, be sure to leave check strips and be sure to pencil out the costs and benefits, otherwise you will never know what your return on investment is. It’s always important to pay attention to what is working for you and what is not.
If you have questions about corn disease management, don’t hesitate to contact me at chilvers@msu.edu.
This work was supported in part by the Corn Marketing Program of Michigan, National Predictive Modeling Tool Initiative, Project GREEEN and MSU AgBioResearch.