By Alison Robertson
It’s hard to believe it has been 10 years since tar spot was first observed on corn in the U.S. in Illinois and Indiana. Over the past decade, we have learned much about this economically important corn disease. Below are 10 facts about tar spot. Visit the Crop Protection Network (CPN) for more information.
- Tar spot is here to stay. The fungus that causes tar spot, Phyllachora maydis, can survive Midwest winters in infested corn residue and cause disease the following growing season.
- Tar spot continues to spread north, south, east and west. Every year, corn pathologists across the U.S. upload observations of tar spot reported to them by local agronomists to the tar spot map.
- The irregular black spots characteristic of tar spot are fruiting structures of the fungus. They are called stromata and are embedded in the leaves and, occasionally, husks.
- More than 30 days of moderate (64 to 73 degrees F) daily average temperatures and two to three weeks of relative humidity (more than 90%) are required for tar spot development. Interestingly, wet-dry cycles are more favorable for the disease than extended wet periods. You can read how important weather is for tar spot development.
- Tar spot can cause substantial yield loss, particularly on susceptible hybrids and/or when the disease is severe. Yield losses were documented for the first time in the U.S. in 2018, when a severe epidemic of tar spot occurred in Illinois, Indiana, Michigan and Wisconsin. In experimental field plots, when 50% of the ear leaf was covered in tar spot stromata at dent (R5), yield losses of approximately 20 to 40 bushels per acre were measured.
- Corn hybrids vary in their susceptibility to tar spot. Many seed companies publish tar spot ratings in their seed catalogs. If your farm has a history of tar spot or you are concerned about tar spot, plant a hybrid that has some resistance to tar spot.
- Fungicides may be used to manage tar spot and protect yield. Products vary in their efficacy against tar spot. Every year, corn pathologists from across the U.S. rank fungicides for their efficacy against tar spot and other diseases. This information can be found at cropprotectionnetwork.org/publications/fungicide-efficacy-for-control-of-corn-diseases.
- In most years, one application of a fungicide at tasseling (VT) to milk (R3) is sufficient to manage tar spot. An application of fungicide before V12 is not recommended for effective tar spot management.
- When tar spot severity is high (more than 5% on the ear leaf at R5), a positive partial net return is likely. When tar spot severity is low (more than 5% on the ear leaf), a negative partial net return is probable. CPN has a Corn Fungicide ROI Calculator that can be used to calculate the potential return on investment for a fungicide application.
- A tar spot prediction tool has been developed through the National Predictive Modeling Tool Initiative, the National Corn Growers Association and the Wisconsin corn checkoff. The tool will be available on the CPN website this summer.
Robertson is an Iowa State University Extension field crops pathologist.