Corn leaf samples collected in Harrison, Carroll, and Marion counties and submitted to the MU Plant Diagnostic Clinic were confirmed for tar spot this week.

Figure 1 Tar spot on green leaf tissue. Sometimes the black tar lesions can have brown necrotic 'fish eyes' surrounding them (white arrows).

Figure 2 Tar spot on senesced corn leaves. Saprophytic fungi might be confused with tar spot lesions on senesced tissue.

Tar spot (Figures 1 and 2) was first confirmed in Missouri in Marion County in 2019. The pathogen that causes tar spot, Phyllachora maydis, can survive over the winter in corn residue, so disease presences in subsequent years is not surprising. Confirmations in Carroll and Harrison County suggest that the pathogen is likely present in much of northern Missouri (Figure 3).

Figure 3 Red stars indicate counties where tar spot of corn has been confirmed one or more times between 2019 and 2022.

Scouting at this point in the season can help with management decisions for 2023. Management practices can be found in our previous newsletter on tar spot.

On corn leaves that have already senesced, the 'black dust' of saprophytic fungi might be mistaken for tar spot lesions. Usually, those fungi can be wiped off the leaf unlike tar spot lesions. If you believe you have tar spot, please submit samples to the MU Plant Diagnostic Clinic for confirmation.