Whoomp! There it is! What to do about Tar Spot of Corn in 2024

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Damon Smith, Extension Field Crops Pathologist, Department of Plant Pathology, University of Wisconsin-Madison

Over the last several weeks we have seen confirmed positives for tar spot in parts of Iowa, Missouri, Kansas, Nebraska, Indiana, Illinois, Michigan, and now Wisconsin (Pepin Co.; Fig. 1). While it has been found in Wisconsin, the severity and incidence are extremely low and does not necessitate spraying fungicide at the moment! So, what should we do now?

My advice is to get prepared and make sure you have the tools in place to deal with this problem. As I said the last few seasons, tar spot is here to stay and we need to simply be prepared and ready to fight the disease. The first line of defense is to know if you have had tar spot before. This will tell you if there is resident inoculum sources present that can initiate epidemics. If you have seen tar spot on your farm before, then assume the pathogen is present and in close proximity to corn (the host). Remember the disease triangle? The last component of the triangle is the weather. If there has been conducive weather then the triangle has been met and risk is high for finding tar spot. So how do you know if the weather is conducive? Well, there is an app for that!

Tarspotter and Field Prophet are both Smartphone applications that can help you determine if the weather has been conducive to put your corn crop at high risk of tar spot development. The app DOES NOT tell you if the pathogen is present. We are working on this part of the triangle to improve our predictions, but you need to determine if the pathogen is present in your field. This tool just tells you if the weather has been conducive.

So what weather is conducive for tar spot development? Yes, precipitation is helpful, but more importantly, we need intermittent wet/dry cycles to give us intermittent leaf wetness. Specifically leaf wetness at night. What gives leaf wetness this time of year other than rain? That would be high dew points and humidity. These variables are included in the models that run in Tarspotter and Field Prophet. We also include temperature which is an influential variable too. These variables are measured over the last 14 days and 30 days and included in each daily run of the tool. We use the GPS on the smartphone to pull down cloud-based weather for a precise location. Thus, these results are site-specific. I also like to the use the Field Prophet version of the models as this version provides a 7-day trend line on how weather has been progressing and also allows for a true 7-day forecast. These additional tools can better help with the decision-making process. If you would like to learn more about the “nuts and bolts” that run behind the smartphone apps, you can find our research publication HERE.

My corn is at V8-V10, should I spray Fungicide?

Figure 2. Tar spot severity diagram indicating various levels of tar spot on corn leaves. Yield loss isn’t typically detectable in the field until severity reaches 10% or more on the ear leaf or leaves above this leaf.

My short answer is no! The disease is just getting started. If you find it in Wisconsin right now, it will be at low severity and is low in the canopy on leaves that are not going to contribute to yield. My advice is to use your prior knowledge of where tar spot occurred and the Tarspotter tool to help guide your scouting efforts. Get out into the fields and know what you are dealing with. Figure 2 shows various severity levels on a corn leaf. We don’t start to see yield loss until we reach about 10% severity on the ear leaves or above. Thus, you have time! Target fields planted to known susceptible hybrids. Get yourself prepared and use those lower leaves to monitor severity and tar spot progress. Be ready to protect (put fungicide on) those leaves that contribute to yield (ear leaf and above), later on especially if the weather becomes increasingly conducive (think wet/dry cycles!) and/or your scouting indicates severity is increasing.

When should I spray fungicide? What should I use?

When making decisions on using a fungicide for tar spot management keep in mind that fungicide active ingredients are important. Products with multiple fungicide classes are preferred (QoI + DMI or QoI + DMI + SDHI). Products with multiple fungicide classes tend to provide better efficacy and delay the development of fungicide resistance. See the CPN Fungicide Efficacy Guide for specific products and their ratings for tar spot and other diseases.

Application timing is very important for tar spot management. It is best to use scouting and/or tar spot risk or profit tools like Tarspotter and Field Prophet to make informed decisions about when to apply fungicides for tar spot management. These apps use weather data to determine if the environmental conditions are favorable for tar spot to develop, and consequently optimize fungicide application timing.

In most years, a fungicide application will not be needed prior to the V10 growth stage. In most years, one well-timed (VT-R3 growth stages) fungicide will be sufficient to manage tar spot. Even in years where two applications appear to improve tar spot control, improved ROI is marginal over a single well-timed application.

The Conclusion

DON’T PANIC! This is just a call to be ready. Download the apps and know what the weather is doing. Use your prior knowledge and scouting in key locations to track tar spot. Get your management plan in place. Have your fungicide of choice available. Communicate with your custom applicator. Be ready to spray between the VT and R3 growth stages if you plan to use just one fungicide application and you are seeing tar spot increase. If you spray between the V8 and VT growth stages, be ready to monitor the smartphone apps and do more scouting as you might have to pull the trigger again later in the season. Get out and SCOUT, SCOUT, SCOUT!

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