Tar spot of corn, a disease caused by the fungus Phyllachora maydis, has now been confirmed in Doniphan (6/26), Atchison (6/30), Jefferson (6/30), Brown (7/05), and Nemaha (7/28) counties, Kansas (Figure 1).
Figure 1. Tar Spot of Corn (Phyllachora maydis) in Kansas and surrounding states in 2023. Source: https://corn.ipmpipe.org/tarspot/
What am I scouting for?
Tar spot develops as small, black, raised spots (circular or oval) that develop on infected plants, and may appear on one or both sides of the leaves, leaf sheaths, and husks. Spots may be found on both healthy (green) and dying (brown) tissue. Tar spot can be easily confused with insect poop, which can appear as black spots on the surface of the leaf (Figure 2). For assistance in confirming tar spot, please contact your local county extension office or the K-State plant diagnostic clinic at https://www.plantpath.k-state.edu/extension/plant-disease-diagnostic-lab/.
Figure 2. Tar Spot of Corn. Purple arrows are indicating a few of the tar spot lesions. Photos courtesy of Rodrigo Onofre, Department of Plant Pathology, K-State Research and Extension.
Is there a history of disease in this field or neighboring fields?
Tar spot overwinters on infested corn residue on the soil surface, which serves as a source of inoculum for the subsequent growing season. Spores can be dispersed by wind and rain splash and can move to nearby fields if conditions are favorable.
What growth stage is the field?
Research has shown that making an application just after first detection and at or after VT is effective if lesions are detected early. If you wait until there is significant disease in the upper canopy, then a fungicide application may be too late. Here you can find a guide for growth stages in corn: https://bookstore.ksre.ksu.edu/pubs/MF3305.pdf.
How does moisture influence disease development?
The recent rains likely helped to promote tar spot development. Additionally, irrigated corn may be at particularly high risk for yield or silage loss. Forecasted rainfall and high humidity will favor tar spot development and spread.
Should I apply a fungicide?
Fungicides are an effective tool for controlling tar spot if they are timed well. Research has shown the best return on investment from a fungicide application on corn occurs when fungal diseases are active in the corn canopy. A well-timed, informed fungicide application will be important to reduce disease severity when it is needed, and we recommend holding off until the disease is active in your field and corn is at least nearing VT/R1 (tassel/silk) or even R2 (blister). Scouting will be especially important if wet weather continues. There are several fungicides that are highly effective at controlling tar spot when applied from tassel (VT) to R2 (blister). I would recommend picking a product with multiple modes of action. The National Corn Disease Working Group has put together efficacy ratings for fungicides labeled for the control of tar spot can be found at the Crop Protection Network website, link: https://cropprotectionnetwork.org/publications/fungicide-efficacy-for-control-of-corn-diseases.
If there is high disease pressure early in the season, a second application may be warranted. Fields should be scouted 14-21 days after the first application to see if tar spot has become active again. Fungicides will not provide benefits after R5. Always consult fungicide labels for any use restrictions prior to application.
Please help us track tar spot, you can contact me (785-477-0171) directly if you suspect a field has tar spot and/or submit a sample to the K-State Plant Disease Diagnostic Lab at https://www.plantpath.k-state.edu/extension/diagnostic-lab/documents/2021_PP_DiseaseLabChecksheet.pdf.pdf. This will help us monitor the situation in the state.
Rodrigo Onofre, Row Crop Plant Pathologist