Tar Spot is relatively new in the U.S., but the small, black spots indicative of the disease created big challenges — and significant yield loss — for some farmers last growing season. While it’s too early to know how severe any Tar Spot outbreaks may be during the 2022 growing season, farmers will want to be on the lookout for symptoms so that they can manage accordingly.
Tar Spot can be identified by raised, black, circular fungal structures that resemble specks of tar on the leaf surface. The lesions have a bumpy feel that’s not easily razed off, and they are often surrounded by a small, tan “halo” that is sometimes said to produce a fish-eye appearance.
“The disease begins on the lower corn leaves, then moves to the upper plant and ear husks,” says Todd McRoberts, agronomy manager for NK Seeds. “It’s sometimes confused with common and Southern rust late in the season, but it’s easy to distinguish the two because rust pustules are easy to scrape from the leaf.”
Tar Spot can move limited ranges by wind and plant residue. It overwinters in soil and residue and thrives in cool (60-70 degrees Fahrenheit) and humid conditions with prolonged periods of wet leaves. Infection can occur at any crop stage, although it’s most common throughout the grain fill period.
McRoberts says that farmers should consider several management practices to make their corn crops less susceptible to Tar Spot:
- Select Hybrids Carefully. Hybrids differ in susceptibility to Tar Spot infection. Consult the NK Seed Guide and your local NK agronomist for the most recent hybrid sensitivity ratings when selecting hybrids for fields with a history of Tar Spot.
- Consider Crop Rotation and Tillage. Rotating to crops other than corn and using tillage to bury residue can help to reduce fungus inoculum levels in fields.
- Apply Fungicide When Needed. Early fungicide applications, at or before the first signs of development, have been effective against Tar Spot in trials. (While early fungicide programs applied before the onset of disease may be effective, late-season, curative fungicide applications are not recommended.)
- Consider a Two-Pass Fungicide Program. If conditions are favorable for Tar Spot development early in the season, consider a two-pass fungicide program using Trivapro® or Miravis® Neo fungicide at the V4-V8 corn growth stage and the VT/R1 growth stage. If the risk of Tar Spot is lower, consider an application of Trivapro or Miravis Neo fungicide at the VT/R1 growth stage, which may also combat other yield-reducing corn foliar diseases.
Contact your local retailer or sales representative to learn more about our disease tolerant hybrids.
Look for more seasonal agronomic tips here on The Amplifier throughout the growing season.
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