Jerry Clark
Crops & Soils Educator for Chippewa, Dunn, & Eau Claire Counties
UW-Madison Extension
In the past few years, Tarspot in corn has risen to the top of the list for disease management in corn. Chippewa, Dunn, and Eau Claire counties have all recently experienced the disease at some level. Now is the time to get prepared and make sure you have the tools in place to deal with this problem, when or if it arises. Tar spot is here to stay, and we need to 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 near corn. We have been dry over the past month so how do we know if the weather is conducive for tar spot? 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. Scouting corn fields is the only way to know if the pathogen is present. It does overwinter in Wisconsin corn residue so, if your fields had it before, the risk of seeing it again is higher.
The temperature tar spot thrives in is in the high 60s to low 70s-degree Fahrenheit. Specifically leaf wetness at night is the best condition which is driven by high dew points and humidity. Tar spot is just getting started lower in the canopy. Scout fields and target fields planted to known susceptible hybrids. Be prepared and use those lower leaves to monitor severity and tar spot progress. Be ready to protect those leaves that contribute to yield which are the ear leaf and above.
If you select the correct fungicide, one application can generally control tar spot to the point to preserve your yield potential. Research has shown the best timing to be between the VT (tasseling) and R3 (milk) growth stages. Most of the protection of yield from tar spot will occur before the R3 stage. So, if planning one application, between these two stages is best. If applied too early, a second application later in the season may be needed.
Regarding fungicide selection, there are many options. Check out the report “Fungicide Efficacy for Control of Corn Diseases” on the Crop Protection Network website for information about fungicide selection.
Get out and scout for Tar Spot and be ready. If possible, download the apps and know what the weather is doing. Prepare a management plan and check on fungicide availability if applicable. Be ready to manage between the VT and R3 growth stages if you plan to use just one fungicide application and Tar Spot is increasing. If fungicides are applied between V8 and VT growth stages, be ready to monitor the smartphone apps and do more scouting as another application may be needed later in the season.
