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Crop Advisor’s Casebook: Mystery of the ailing soybeans

A Crop Advisor's Solution from the March 17, 2015 issue of Grainews

The lowest leaves on the soybeans plants were sloughing off.
Russell Thompson is a sales agronomist with Richardson Pioneer Ltd. in Dauphin, Man. photo: Supplied

It was the first week in July when I got a call from Aaron, who grows wheat, canola, barley and soybeans at his farm northeast of Dauphin, Man. Soybeans account for 1,000 acres of Aaron’s 3,300-acre operation, but plants in two of his soybean fields were really struggling.

“The crop isn’t doing so well in those fields, compared to the rest,” he said. “The plants are stunted, and their leaves are yellowing and dropping off.”

I paid Aaron a visit to have a look at his soybean crop. I could clearly see that the affected fields were not as green and lush as the nearby soybean fields. I also observed that while plant populations were appropriate in the affected fields, individual plants in them were smaller and weren’t as bushy as those in the healthy fields.

Plant populations were fine, but individual plants were smaller and not as bushy as in healthy fields. photo: Supplied

A closer look revealed the affected plants had unifoliate and first trifoliate leaves that were not only yellow and but also covered with small brown spots. Soybeans’ cotyledon leave emerge out of the ground first. Then the unifoliate leaf, followed by the first trifoliate leaf.

The lowest leaves were sloughing off, and most of the cotyledons had already dropped off too, much earlier than they usually do. We dug up plants to check the condition of the roots, but they were well nodulated and there was no evidence of root disease.

The soybean plants were stunted. Their leaves were yellowing and dropping off. photo: Supplied

It had been a late spring with cold, wet weather, but the affected fields, which were on light soils, were quite dry. I asked Aaron about the history of those fields and was told this was the second consecutive year he’d planted soybeans on them and that no fertilizer had been applied.

Aaron thought some kind of leaf disease might be to blame for the crop’s poor condition, but I wasn’t so sure. I arranged to have plant tissue samples sent to Manitoba Agriculture, Food and Rural Development’s Crop Diagnostic Lab as well as another agriculture testing service to help shed some light on the issue.

Crop Advisor’s Solution: Nutrient deficiencies behind ailing soybeans


As Aaron suspected, there was leaf disease, as the Crop Diagnostic Lab confirmed the presence of brown spot. However, this disease wasn’t unusual for soybeans in this area and typically wasn’t a huge concern.

The real problem lay with nutrient depleted soil. The plant tissue analysis by the second lab confirmed phosphorus and potassium deficiencies. The nutrient imbalance was the result of growing soybeans for two years straight with no fertilizer added. Because the soybeans were under stress, they were more susceptible to a leaf disease like brown spot.

As a rescue treatment for Aaron’s soybean crop, I recommended that he apply a foliar fungicide as well as a nutritional product with the next glyphosate spray. I hoped this would help the plants fight disease as well as establish more robust root systems which enable them to access more water and nutrients.

Fortunately for Aaron, the weather got better and plant growth improved in the affected fields. The fungicide and nutritional products definitely helped, and generated enough extra yield to pay for the treatments.

Ultimately, though, the long-term fix for Aaron’s soybean fields is a longer crop rotation and adequate fertilization, particularly since soybeans remove so much phosphorus and potassium in the seed. Better scouting for diseases like brown spot is also something to consider, particularly if rotational options are limited.

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