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Crop Scene Investigation - 37: What caused the yellow spots in that winter wheat?

Monday, February 20, 2012

by BERNARD TOBIN

Every spring, Peter Johnson sees yellow spots. It's just one of those things that happens in winter wheat, says the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) cereals specialist.

The reason for those lighter-coloured areas in wheat fields is almost always manganese deficiency, particularly on sandy soils, muck soils and high organic matter soils. But, in 2011, many of the spots left Johnson scratching his head, searching for an explanation.

"We saw the same yellow spots develop again this summer in fields, especially down in the London-Mitchell-Chatham area," recalls Johnson. "But there was no way it should have been manganese-related, because manganese becomes more available under anaerobic or saturated soil conditions.

"With all the rain in that area this spring (more than twice the normal rainfall) there should have been ample manganese available in those fields," he says, explaining that under wet conditions manganese reduces and oxidizes, so it's much more available.

Johnson also notes that the plants did not exhibit interveinal chlorosis – yellowing of tissue between the veins – which is often associated with manganese problems. "These plants were really just yellow," he recalls.

He thought the problem may have been linked to water-logging – wheat does not like wet feet – but the positioning of the spots in the field ruled that out. "Sometimes we were seeing the spots over the tile drain or over the slope or face of a hill. It just didn't make sense."

The cereal specialist also wondered if field shape may have played a role. He was often seeing yellow spots in hollows where organic matter was probably lower due to some form of tillage erosion. "There was one field where the face of every slope was extremely yellow. It would just go across the top of the hill until it broke at the crest."

Most of the spots in question showed up around May 1, and there was quite a range. "In some fields, they were the size of your kitchen table and they were not really yellow, just a lighter green. In others, you could get spots that were an acre in size."

Johnson thought of other possible explanations. "You might have said it was nitrogen deficiency, but two weeks earlier, in some of those fields, we had just gone in and put on 150 pounds of nitrogen, so we ruled that out."

Johnson surmised that the plants had to be suffering from a micronutrient deficiency. One way to find out would be to conduct flag strip testing in the spots. So he gathered together a collection of spray bottles full of different micronutrient treatments, sprayed each of them on a metre of the crop and noted the location of each test by writing the name of the micronutrient applied on marker flags.

He also took tissue samples from healthy green-coloured plants in the field and had them compared to the yellow plants he found in the spots. When the test results arrived 48 hours later, they revealed that the yellow plants scored well on their manganese levels, but they were perilously low on a different micronutrient.

With results in hand, Johnson then went back to his test strips and, sure enough, the plot sprayed with the deficient micronutrient identified in the tissue tests had already greened up. "That's when it all came together," says Johnson.

Do you know the missing micronutrient? Send your solution to Better Farming at: rirwin@betterfarming.com or by fax to: 613-678-5993. Be sure to include your address and phone number.

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

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