Crop advisor casebook: Purple corn leaves pose a puzzling problem

A Crop Advisor's Solution from the March 12, 2019 issue of Grainews

Mike Gemmill.
photo: Supplied

In late May of last year, I received a call from Joe, who owns a 1,500-acre mixed grain and dairy farm near Stonewall, Man. He said he’d been walking his early-planted corn fields and noticed purpling of the leaves at the three-leaf stage across the entire field.

I drove out to Joe’s farm and immediately saw what he was taking about. The problem was widespread throughout the field. Other than the purpling leaves there was nothing unusual about the field, except for one unaffected strip which barely overlapped another piece of land Joe told me had been planted to wheat the previous year. The rest of the corn field had been seeded to canola in 2017.

“Could this be weather related, or possibly herbicide damage?” Joe asked.

When corn plants emerge, it’s not uncommon for seedlings to have a purplish tint or some uneven early growth. These observations often worry some corn growers. However, when the purple colour disappears — usually after the six-leaf stage — so do growers’ concerns.

We looked at records to determine weather conditions from planting through emergence, and if any extreme temperature fluctuations had occurred. The answer was no. Weather conditions for that period were normal and no cold temperatures were observed that could have caused environmental stress. Neighbouring corn fields had no profound injuries and appeared healthy.

Other than the purpling leaves there was nothing unusual about the field, except for one unaffected strip which barely overlapped another piece of land that had been planted to wheat last year.
photo: Supplied

We examined soil test results and the field’s fertility program to see if any nutrient deficiencies could explain the symptoms. We didn’t find anything suspicious — the fertility program was sufficient and no specific area was worse than another.

We looked into herbicide damage from residual carryover due to the dry conditions the region had experienced. We went back into the field to see if any areas were worse than others, such as areas of overlap, but there was no evidence that suggested a carryover issue. Joe also confirmed he never used any residual chemicals the previous year.

We were baffled. What could be causing these purple leaves in a field that seemed otherwise normal and healthy?

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Crop Advisor’s Solution: Watch for phosphorus deficiency when planting corn after canola

I thought back to what Joe told me about the field’s history. The only uninjured area of the corn field was a single strip that overlapped onto soil planted to wheat the previous year.

After further inquiry, Joe reminded me he had planted his corn in soil seeded to canola the previous year. I felt a soil test was warranted, even though we had ruled out lack of nutrients as the problem.

Test results revealed we were dealing with a lack of phosphorus in Joe’s corn field.

Why was this the case? Since Joe grew canola on the field the previous year, there was no mycorrhizae colonization left in the soil, limiting phosphorus uptake in the corn. These bacteria can efficiently take up phosphorus from the soil solution and transport it to the plants. Unfortunately, canola is not a suitable host for these bacteria, meaning fields planted with canola can suffer from low levels of mycorrhizae.

That’s why the only unaffected strip was the one planted on wheat stubble. Since wheat plants are ideal hosts for mycorrhizae, there was plenty of the bacteria in that single strip.

In addition, tillage can also decrease mycorrhizae colonization, thus multiple practices could have contributed to the severity of this issue.

I advised Joe to try to avoid planting corn on canola stubble in future. If that wasn’t possible, he must make sure that applied phosphorus was readily accessible in the seed trench to limit the symptoms. In the meantime, his corn would grow out of the purpling stage as the mycorrhizae colonized. Additional phosphorus wouldn’t be beneficial at this point, I told him.

Fortunately, there was no yield drag on his corn crop, just a delay in maturity. I advised Joe to be sure to watch his cropping rotations closely and address any potential concerns prior to planting.

Mike Gemmill works for Richardson Pioneer Ltd. in Stonewall, Man.

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