Tar spot is a relatively new disease to the Corn Belt. It was first found in the United States in 2015 and was then considered to be only a minor, cosmetic infection. Farmers realized tar spot was much more serious after a severe outbreak in 2018.
Because it’s not as well-known as other diseases, tar spot has taken some farmers by surprise, leading to significant corn yield loss. This happened in several Midwest states in 2021, with farmers reporting yields losses of up to 50 bushels per acre in some cases.
Experts say tar spot is here to stay and will likely start spreading to new areas. So, here’s what you need to know to protect your corn yield potential in 2021.
An Introduction to Tar Spot
Tar spot is not an entirely new disease, having been first identified in Mexico in 1904. For a long time, the disease was limited to high elevations in cool, humid areas of Latin America until spreading to the southern United States and, eventually, farther north.
As previously mentioned, 2015 was the year tar spot was identified in the United States. An outbreak in 2018 caused widespread corn yield losses in the Midwest and, last year, the disease hit hard once again in several states, including Illinois, Indiana, Iowa, Michigan and Wisconsin. At this point, it’s difficult to say just how severe a tar spot outbreak will be, but high yield losses are possible, particularly in vulnerable corn varieties.
Tar spot is caused by the fungus Phyllachora maydis, which experts say can reproduce and spread very quickly. So, that’s why it’s so important for farmers across the Corn Belt to be aware of the disease signs, symptoms and favorable conditions for development.
Watching for Tar Spot
Early tar spot manifests as small, raised stromata on plant leaves. The stromata look like little black spots – or flecks of tar – that turn into black ovular or circular lesions. These symptoms will start on the lowest leaves, spreading upward to upper leaves, leaf sheathes and the husks of developing ears.
Under favorable conditions, tar spot can spread rapidly through the corn canopy, causing premature senescence of corn leaves. This is because the lesions the disease creates reduce the photosynthetic capacity of the leaves. Eventually, the disease can cause stalk weakening, rotting, lodging and even premature plant death.
When it comes to conditions for development, tar spot favors the following:
- Cool temperatures, about 60 to 70 F
- High relative humidity (>75%)
- Frequent cloudy days
- Seven-plus hours of dew at night
Controlling Tar Spot
Start scouting cornfields for tar spot early in the season and do so on a regular basis. When scouting, it’s best to look for symptoms on lower leaves first, because the disease manifests at the bottom of corn plants and spreads upward from there.
Other precautions you can take against tar spot include:
- Selecting hybrids with good genetic tolerances against the disease.
- Establishing a healthy stand at planting.
- Managing crop residue at harvest, as the disease can overwinter in residue.
Finally, a timely application of a strong fungicide can help prevent tar spot. Aproach® Prima fungicide offers two powerful modes of action against the disease. Work with your local ag retailer or Corteva Agsriscience territory manager to determine proper timing of Aproach Prima for maximum protection against tar spot.
Even if tar spot hasn’t hit your state or area yet, it’s a good idea to take precautions against it sooner rather than later. Talk with your trusted advisors to see how best you can protect your cornfields.
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