Crop advisor casebook: Why the discoloured canola plants in this Sask. field?

A Crop Advisor's Solution from the October 12, 2021 issue of Grainews

This photo shows the affected area of discoloured plants fading into an unaffected area with healthy plants.
Kailyn Krchov. photo: Supplied

In June 2020, I was doing a routine herbicide efficacy assessment for Rick, who farms 13,000 acres of durum, spring wheat, canola, barley, lentils, peas and flax near Kyle, Sask.

While walking a 320-acre canola field, I noticed some severe discolouration of the canola plants. The affected plants were pale green to yellow in colour in a strip in the field. Although the discolouration was present throughout the strip, the yellow colour seemed to be concentrated on the older leaves and even more so on the leaf margins. The affected plants were also suffering from reduced biomass and some slight cupping.

“I noticed a little discolouration but things got busy and I didn’t think it was worth mentioning,” Rick told me.

My first thought was it could be a fertilizer issue, but the fertilizer program we had planned was adequate to sustain the crop throughout the growing season.

When looking at NDVI maps, we could see the symptoms appeared gradually and were becoming more pronounced now the canola was in the rosette to budding stage.

The surrounding areas of the field looked normal, with canola plants at the budding stage. Soil conditions and moisture were adequate. When approaching the discoloured area, you could see the margins of the affected area were very straight on the sides but were abnormally shaped on the front and back edges.

There were several possible causes, however, the straight lines pointed to something mechanical. Spray damage could be the issue with either herbicide carryover from previous years or a previously sprayed chemical pulled into the solution from the sprayer tank, which could explain the wavy margins on the end of the strip. Variance in seeding depth in this section could potentially explain why the margins were straight — the canola seed could have been placed too deep reducing the plant vigour in the pass. Field variation and environmental stress were also possibilities.

After some more investigation, we ruled out all the suspected possibilities. Chemicals in the tank prior to spraying would not have caused the damage present in the field, and the chemical rotation in place ruled out any carryover concerns.

After digging up some plants, I verified the depth of this pass was consistent with the rest of the field. Since the affected strip spanned the length of the field, this ruled out elevation differences. Also, there had been no abnormal weather patterns in the past few weeks, which ruled out environmental stress.

Some further investigation was in order.

Crop advisor solution: Fertilizer miss causes discoloured canola

When walking the affected area again, I noticed it did align with the width of the drill. The symptoms indicated deficiencies of both nitrogen and sulphur with the cupping of newer leaves, the pale green colour of the plants, yellowing of the lower leaves, stunted maturity and reduced biomass.

Going back over seeding data and application maps, we discovered Rick had run out of the side-banded nitrogen and sulphur fertilizer blend when he made his pass through this part of the field. This explained the straight-lined margins on the sides and the wavy pattern on the front and back due to the fertilizer runs running out of fertilizer at different times.

Nitrogen and sulphur deficiencies are often distinguished by the location of chlorosis on the plant but with a deficiency of both nutrients, we had to rely on additional symptoms and the application maps.

We discussed a top-dress application of nitrogen and sulphur to make up for the deficiencies. However, given the size of the affected acres, it was decided it would work as a good trial for fertilizer efficiency and importance of application.

This part of the field did suffer from a decrease in yield, and because the plants were behind in staging, the affected plants were hit harder by the heat wave that arrived some days later.

Even in drought conditions, there can be a significant response to fertilizer, or lack of fertilizer, applications. I emphasized that communicating with an agrologist on any concerns or mishaps at the time of application is crucial so they can be addressed and monitored before it’s too late to remediate the issue. We concluded that reviewing information in a timely matter is essential and using all the digital tools available to us can be a huge asset in a fast-moving agriculture environment.

Kailyn Krchov, AAg, CCA works for Richardson Pioneer Ltd. in Elrose, Sask.

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