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Crop advisor casebook: Soybean disease or chemical injury?

A Crop Advisor's Solution from the February 20, 2018 issue of Grainews

Shantelle May.
photo: Supplied

Ryan owns a mixed dairy and grain farm north of Saskatoon, Sask. At the beginning of June, we were scouting Ryan’s soybean field when we discovered brown mottling on the plants’ leaves.

These dark brown spots were present largely on the unifoliate leaves as well as on some of the cotyledons. The soybean plants’ trifoliate leaves were just unfolding. The speckling effect didn’t look random as the spots occurred uniformly on the leaves’ margins. Most plants in the field had some speckling on their leaves, however, some plants had more spots than others.

“Could this be disease in our soybeans?” Ryan asked. “Something like bacterial blight or septoria brown spot?”

Although the brown spots looked similar to those found on bacterial blight- or septoria brown spot-infected leaves, there were several key differences, ruling these diseases out.

For example, bacterial blight infection produces small, water-soaked spots on leaves, which turn from yellow to brown in colour. These dark spots are surrounded by a yellowish-green halo. When this tissue dies, the leaves take on a ragged appearance. The leaf spots on these plants didn’t exhibit any of these characteristics.

Leaves infected with septoria brown spot also develop small, brown spots. However, these spots often grow together to form irregular brown lesions with tiny, black, spore-producing pycnidia in them. Septoria brown spot was not injuring these plants, either.

When I examined the other parts of the affected plants, they were free of necrotic tissue and their growing points were healthy. The plants’ stems were also healthy, with no tissue damage or brown discolouration. In addition, the plants’ roots looked normal.

Brown spots on leaves can also be caused by surfactant burn following glyphosate application. However, Ryan hadn’t applied the in-crop herbicide to this field yet. When plants experience surfactant burn, the spots appear in a droplet pattern where the herbicide has landed on the leaves. Furthermore, the leaves’ centres would also contain spots when burned by surfactant.

That season, only a soil-applied Group 14 residual herbicide had been sprayed with the glyphosate burndown.

So how did Ryan’s soybean plants end up with what looked like a chemical injury?


Crop Advisor’s Solution: The problem looked like disease. It was herbicide injury

In mid-May Ryan sprayed a soil-applied Group 14 residual herbicide with the glyphosate burndown. The residual activity for this Group 14 herbicide is four to six weeks. Sometimes, water from the treated soil can splash up onto the emerging crop, causing temporary injury.

The injury’s shape and placement on the soybean leaves was exactly what I would expect from water droplets splashing up from the soil onto the leaves’ edges. In addition, the most recently emerged leaves were uninjured, as the event that generated the brown spots occurred before the newly emerged leaves started to unfold.

Eventually, the crop outgrew the injury and yield wasn’t affected. Because it increases the risk of crop injury, I recommended that Ryan not use the soil-applied Group 14 residual herbicide in his burndown under the following circumstances:

  • on soils with organic matter greater than five per cent, or
  • on fields that are poorly drained, or
  • under cool, wet conditions.

It is also important to follow cropping deadlines before and after spraying this herbicide type.

Continual crop scouting is essential. Early detection and correct identification of crop issues reduce panic and mitigate yield loss. Furthermore, knowing the symptoms of crop injury for the herbicide in use before the chemical is applied, helps in the recognition and identification of herbicide injury early on.

Shantelle May works for Richardson Pioneer Ltd. at Carlton Crossing, Saskatoon, Sask.

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