Tar spot finally made it to the Corn Commentary field in south-central Indiana. However, the disease arrived so late that it will not be a factor in corn yields this year. When spotted, corn was already well into R5, dent stage, heading toward black layer and maturity.

“If you have corn at dent stage and you find tar spot, you don’t need to worry about spraying,” says Dan Quinn, Purdue Extension corn specialist. “You are way past the point of expecting an economic return on your investment for spraying a fungicide.”

And if you’ve already sprayed fungicide once, as was the case in the Corn Commentary field, you could find it tougher to earn an ROI on a second application anyway. A typical fungicide application in the Midwest — whether by drone, airplane or ground rig­ — can run up to $14 per acre plus the cost of fungicide, which typically starts around $18 per acre for generics and increases from there.

Recap of 2024 season

Here is a brief recap on this season’s Corn Commentary field. The goal is to cover what happens in this field all year, with the hopes that others can gain insights they can apply to their fields, no matter where they farm across the Midwest.

Planting. The field was planted during the first week of May into good conditions. Temperatures were on the warm side. By flagging plants as they emerged, it was determined that corn emerged eight to nine days after planting, and 95% of all seedlings emerged with 24 to 36 hours of each other.

Related:Will 2024 start new era in corn yield trends?

Warm early. June turned out to be a warm, dry month. But by the time weather forecasters were beginning to talk about a flash drought, rains returned in many areas, Quinn recalls. Once it rained in very late June in this field, moisture was adequate during the rest of the season, to date.

Cooler July. Illinois state climatologist Trent Ford at the University of Illinois says July averaged about 1 degree F below normal in Illinois, and that trend extended into central Indiana. That provided good pollination weather.

Fungicide. Gray leaf spot was not noticed until pollination and silking were wrapping up. Within a couple of weeks, a low to moderate number of lesions were noticeable up to, but not including, the ear leaf. The farmer chose to spray fungicide, hiring application done by drone. Tar spot was not visible at that time.

Diseases. Finally, tar spot symptoms appeared in mid- to late August, but only on scattered leaves and mostly in small amounts. After the fungicide application, gray leaf spot did not advance higher on stalks. By Aug. 22, there were a few tar spot specks on scattered upper leaves, but no gray leaf spot lesions. An occasional northern corn leaf blight lesion was spotted as well.

Related:Yield estimates hinge on kernel size fudge factor

“Tar spot just arrived too late to be a factor in many fields,” Quinn concludes. “It is definitely worth recording if you find it so you can plan for the future. But at this point, it is past doing anything more for this year in terms of spraying.”

Yield estimates holding strong

What about yield estimates for the Corn Commentary field? The first yield estimate was issued during the first week of August, at more than 250 bushels per acre.

At that time, it was less clear which kernels might fill and which might abort, or how deep kernels might become. Three weeks later, the picture is clearer. And it’s a positive for strong yields. A minimum number of ears were checked, but each one was filled to the tip. Kernels that were still suspect as far as filling or aborting three weeks ago are now in the “fill and count” column.

Remember that the yield formula Quinn uses boils down to this: Ears per 1/1,000 acre times (number of kernel rows times number of kernels per row) divided by a fudge factor for number of kernels per a bushel volume.

For many years agronomists figured 90,000 kernels per bushel. Today, with modern hybrids, most use 80,000 as the standard. One seed company, Wyffels Hybrids, rates its hybrids for this factor, with one popular hybrid averaging right at 60,000 kernels per bushel.

Here is the updated estimate for this field as of Aug. 22: 32 ears per acre × (17 rows on average × 42 kernels per row) = 22,848 ÷ 80 = 285.6 bushels per acre

What if the estimate is off by 5% either way? Then the range would be 271.3 to 299.9 bushels per acre.

What if it is off 10% either way? The range becomes 257 to 315.2 bushels per acre.

Too high? Time will soon tell — stay tuned!