Alyssa K. Betts, Extension Field Crops Pathologist; akoehler@udel.edu.
We knew it was coming, but we have officially confirmed Tar Spot in all three counties this week (Figure 1). Most of these fields have been near MD state lines. Last week I shared some information on Tar Spot. This year first sightings are earlier than the past two years (October dates), which has been observed in other states in the years after first discovery. Although we are seeing a bit of Tar Spot out there, grey leaf spot is still the main disease I am seeing along with some cases of northern corn leaf blight. Since tar spot is around you will likely want to be more active in scouting late season corn. Most fields are at or past the R3 stage and getting close to black layer, so notable yield loss due to tar spot should stay limited. If you already put out a VT-R2 fungicide, that should provide protection through grain fill. In fields where no fungicide was applied, continue to monitor and fungicide decisions could be made through R3 (milk stage ~18-22 days after silking). Late plated corn often faces increased disease pressure from grey leaf spot and northern corn leaf blight and should be scouted. If you are seeing these foliar diseases in late plated corn a fungicide application may be warranted, particularly if tar spot is also in the field.
If you suspect tar spot in your field and need help with identification or other questions you can contact me with questions, akoehler@udel.edu.
Figure 1. Distribution of Tar Spot by county as of August 8, 2025 https://cropprotectionnetwork.org/maps/tar-spot-of-corn