Last year (2018) saw a widespread epidemic of corn tar spot across the western side of Michigan, with losses of 50 bushels per acre in particular fields. The fungus (Phyllachora maydis) responsible for this disease can overwinter on corn residue and release spores to initiate new infections. To date in this 2019 season, tar spot has been confirmed in Kent and Gratiot counties at low levels.
Similar reports are coming out of other states surrounding Lake Michigan. It appears as though the disease is off to a slower start in 2019 compared to last year, however it will be important to continue monitoring for this disease.
Initial tar spot lesions are easy to confuse with insect frass, however frass will dissolve and wipe off the leaf with some water, as seen in the short video below. Tar spot lesions will be embedded in the corn leaf tissue and have a slight raised feel to them. If you look closely, there will also be a small area or dead (necrotic) or yellow (chlorotic) leaf tissue close to the lesion.
n addition to walking fields as we head into mid- to late August, drones can become helpful for spotting areas of heavy infection in a field. Areas of heavy infection may very well lodge, and in the case of silage may dry early. If you suspect tar spot, submit a sample or at least a high quality image to either Michigan State University Plant & Pest Diagnostic Services or to email@example.com.
Visit the Corn ipmPIPE website for real-time tracking of tar spot occurrence.
Fungicides, if used, are most likely to see a return on investment when applied at the VT/R1 growth stages. Early vegetative applications are less likely to see a return. When considering a fungicide application for any disease, the current disease levels at late vegetative stages are a good indication of what is likely to develop as ears develop and fill.
Fungicide timing, fungicide efficacy and epidemiology trials are being conducted to fine-tune tar spot management recommendations. Fungicides labelled for tar spot include Trivapro, and those with FIFRA 2(ee) recommendations allow Aproach Prima, Delaro, Headline AMP, Lucento, Miravis Neo, Quilt Xcel and Topguard EQ.
Visit the Crop Protection Network website for more information on how tar spot impacted the 2018 crop and considering a fungicide for late-planted crops.