Crop Scene Investigation - 47: What's stalking Stan's corn?

Sunday, November 3, 2013

by BERNARD TOBIN

When DeKalb field agronomist Bob Thirlwall arrived at Stan's cornfield just outside Dresden, he had a mental picture of what the field looked like, but he had to see it for himself.

Stan had called in late May to ask Thirlwall to help him get to the bottom of what was happening to his corn. Both DeKalb hybrids he had planted in the field started well with strong emergence and vigorous growth. But at the five- and six-leaf stage, the story was much different. Now the corn on the north side of the field was looking thin and many of the corn plants were cut off at the base of the plant near the soil level. As well, many of the injured plants had been completely eaten down to the midribs.

Thirlwall immediately suspected black cutworm, which typically cuts plants in this manner. However, he could find no evidence of the pest near the freshly cut plants.

Scanning the field, he was puzzled by the pattern of the damage. Stan said he had planted two different corn hybrids. It was obvious that the hybrid on the north side of the field was under attack, but the hybrid on the south side was standing well with very few cut-off plants. "Plant counts confirmed what I was seeing," says Thirlwall. "The healthy hybrid checked in at 28,000 to 30,000 plants per acre, while the other hybrid scored 10,000 to 20,000 plants in most of the counts."

As Thirlwall walked the perimeter of the corn, he noticed that an adjoining field farther south, next to the healthy corn, had been recently worked. When he asked Stan about it, the farmer said it was a field of rye he had disked down about 10 days earlier, intending to plant corn.

"When these 'green bridges' are disked or plowed in the spring, insects will migrate to other fields once the green vegetation starts to die," explains Thirlwall. But if that were the case in this situation, why would the corn immediately next to the bridge escape damage? Why would the hybrid planted farther north be under attack?

Thirlwall had an idea, but he had to do some research and scouting to see if the puzzle fit together. The undamaged corn was DKC50-45, a Genuity SmartStax hybrid. The injured corn was DKC52-59, a hybrid with YieldGard VT3 protection. The situation was now beginning to make sense. Thirlwall was pretty sure that a closer look at the plants in the poor-performing hybrid would give him the proof he was seeking.

Sure enough, he found what he was looking for in the whorl of several plants, amongst the crop debris on the soil surface and under clods of soil.

Do you know what was stalking Stan's corn? Send your solution to Better Farming at: rirwin@betterfarming.com or by fax to: 613-678-5993.    

Correct answers will be pooled and one winner will be drawn for a chance to win a Wireless Weather Station. The correct answer, along with the reasoning followed to reach it, will appear in the next issue of Better Farming. BF