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Pay attention to late-planted corn to protect yield

Tom J. Bechman gray leaf spot on corn plants in the field
SCOUT AND INVEST: With high commodity prices, this is not the year to let insects or diseases such as gray leaf spot (pictured) steal valuable bushels.
Corn Illustrated: Insects and diseases could rob a crop of valuable bushels.

With high commodity prices, this is not the year to take the season off from scouting. Even if your corn was planted late, agronomists say you should continue scouting throughout the entire season.

“There may still be some things you can do this season to protect yield potential,” says Dave Nanda, director of genetics for Seed Genetics Direct. “If insects come in during silking in big numbers, or if diseases come in late, you must be ready.”

Remember these scouting objectives:

Silk clipping. Weather conditions may determine whether Japanese beetles or corn rootworm beetles become an issue clipping silks during pollination, Nanda says. While you don’t want to spray if it’s not necessary, be alert. Be aware of what’s happening inside each field as silking unfolds.

According to the Purdue University Corn & Soybean Field Guide, treatment may be necessary if silks are clipped to one-half inch or less before 50% of the plants are pollinated with beetles present. Pull back husks and do the shake test to determine degree of pollination. Silks that already have pollinated ovules will fall away. Silks that remain are still attached to ovules that haven’t been pollinated.

Foliar diseases. Gray leaf spot is still a major disease threat throughout the Corn Belt, especially if it’s warm and wet with sufficient moisture to favor disease. Northern corn leaf blight tends to be more of a problem when summers are on the cool side.

Southern rust must blow in each year from Southern states, so it’s highly dependent on weather conditions. However, it caused some yield loss in 2020, and can even come in late — as late as very early September — and still cause significant yield loss if corn is still in the grain fill state.

Tar spot can also come in late and multiply rather quickly if conditions are right. This relatively new disease tends to be more of a problem in northern counties, but it’s not limited to northern areas.

Beck’s did a study in 2020 aimed at determining if it still pays to invest in fungicide for a later-planted crop, which might have lower yield potential. They applied either no fungicide or 13.7 ounces of Trivapro per acre on corn planted in three windows: April 16-30, May 1-15 and May 16-30. The study was repeated in Kentucky, southern Illinois and Wisconsin.  

Beck’s agronomists note that in two of the three locations, Southern rust came in late. Applying a fungicide and still investing in late-planted corn paid off. In fact, fungicide paid off for all three planting dates in 2020, using a corn price of $3.72 per bushel. The return on investment for the three planting windows was, respectively: $38.11, $26.58 and $63.78. If you use the same yield increases at $5-per-bushel corn, ROIs would be: $57.95, $43.04 and $92.45, respectively, assuming all costs were the same as in 2020.

Nutrition concerns. One of the Beck’s Practical Farm Research studies in 2020 turned up a surprising result, which could apply this year in any areas that are dry around the VT stage when fungicide applications are made. Agronomists note there is a sharp increase in boron uptake at flowering. Boron uptake can be limited in dry soils. Two of three locations where a boron additive was included with the fungicide were dry, and showed a payoff for the additive in 2020.

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