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Crop advisor casebook: Mystery of the discoloured canola

A Crop Advisor's Solution from the February 18, 2020 issue of Grainews

Jason Fillion.
photo: Supplied

Back in mid-June, I got a call from Dave, a grain farmer near Elie, Man., who was concerned about the condition of the plants in one of his canola fields.

Dave, who also grows wheat, soybeans and flax on his 3,200-acre farm, said some of the canola plants started exhibiting discoloured leaves shortly after emergence. He asked if I could come out to see if disease was responsible.

When I arrived at Dave’s farm and surveyed the problem field, I could see numerous patches of discoloured canola plants, but there was no discernable pattern. After closely scouting the field, I noted some of the affected plants had entire leaves that had turned white, while for many others, the discolouration was concentrated on the leaf margins in the form of a bleached halo.

After closely scouting the field, I noted some of the affected plants had entire leaves that had turned white.
photo: Supplied

I considered what could be causing the kind of symptoms I was seeing. A bleached halo at the cotyledon stage is often due to rapid uptake and translocation of the seed treatment, however, this crop was already at the five-leaf stage, so this was unlikely to be at the root of the symptoms.

I also considered disease to be the cause of the discolouration of the crop. This field had been planted to the same variety of canola in a three-year rotation; thus, I knew that blackleg was a possibility. Another disease of canola, alternaria, was a second possibility.

I wondered if a nitrogen deficiency could be to blame and asked the grower about his fertility program. Dave said he’d applied 100 pounds of nitrogen per acre in a mid-row band at seeding, and soil tests had indicated there was a reasonable amount of residual nitrogen in the soil due to consecutive years of dry conditions.

In other plants, the discolouration was concentrated on the leaf margins in the form of a bleached halo.
photo: Supplied

When the discussion turned to field history, Dave listed all of the crop protection products that had been sprayed over the last three years, including the in-crop herbicide applied on last year’s wheat crop. It was at this point that I realized what was causing the symptoms in his canola field.

Crop Advisor’s Solution: Herbicide carryover caused discoloured canola

I finally had my answer when I discussed the field history with Dave, as I had been recently informed by some of my colleagues that they were seeing the exact same symptoms in canola fields across the Prairies — all caused by herbicide carryover in the soil.

Due to extremely dry conditions over two consecutive years, this canola crop experienced crop injury due to the application of an herbicide product containing pyrasulfatole in the previous cropping season. Less of the active ingredient had broken down in the soil due to decreased microbial activity caused by the dry conditions. Pyrasulfatole carryover is also influenced by a high soil pH, which in this field was 7.9.

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Pyrasulfatole is xylem-mobile and prevents chlorophyll synthesis within the plant, which is why the carryover symptoms had expressed themselves as bleaching of the leaf margins on the newest leaves. Pyrasulfatole is also very mobile in the soil, so it is easily taken up by canola plants in spring.

The symptoms caused by pyrasulfatole carryover are often transient, meaning the bleached leaves will disappear over time. That’s what happened in Dave’s canola field, and fortunately for the grower, there was no impact on yield.

Pyrasulfatole can normally be safely applied the year prior to canola. However, the adverse conditions caused unforeseeable crop injury symptoms. Dave will continue to be cautious and ensure to evaluate carryover risk, even when following herbicide labels, in extreme conditions such as drought.

Jason Fillion works for Richardson Pioneer Ltd. in Starbuck, Man.

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