Crop advisor casebook: Poor pea stands puzzle Saskatchewan producer

A Crop Advisor's Solution from the July 14, 2020 issue of Grainews

Kent Gress.
photo: Supplied

It was a fine June morning when Conner set out to see how his pea crop was doing on his farm near Canora, Sask. However, his day turned sour when he realized the peas in several of his fields were in trouble.

Conner reached out to me shortly after that. “The pea stand is poor in some of my fields,” he said. “I’m wondering if it has something to do with the quality of the seed or, maybe, herbicide drift.”

I assured Conner I’d come out to see what I could do. When I arrived at Conner’s farm and walked into one of the affected fields, I could see there were large areas where the pea plants had very poor top growth. I also observed within the field there were places with yellowing peas, but they didn’t appear in distinct patches.

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When I dug up a few of the yellowing plants, I saw the roots were honey-brown in colour and it was evident they hadn’t developed properly — there weren’t very many lateral roots and, generally, there was much less root growth than you would expect.

It was the same story in two other pea fields Conner showed me, which was a stark contrast to his remaining unaffected pea fields, where the crop was healthy, lush and green. I learned from Conner that the same seed and inoculant had been used across all of the fields.

All of Conner’s pea fields appeared to be relatively clean, with little weed pressure. None of the sickly-looking plants in the affected fields exhibited any signs of herbicide injury that I could see, and previous applications would not have been expected to cause injury through residual activity.

The symptoms didn’t look like a nutrient issue, and there wasn’t anything to suggest this when I reviewed Conner’s fertility program for his pea crops.

At this point, though, I thought I had a good idea what was behind the problem peas.

Crop Advisor’s Solution: Aphanomyces at root of pea problem

At this point, all signs pointed to disease as the culprit.

Based on the symptoms, the ailing pea plants appeared to be suffering from some kind of root rot, and although the weather had turned dry over the past few days, conditions had been wet for a number of weeks and were, therefore, conducive to fungal development in the soil.

I also learned from Conner that some of his fields were being seeded to peas with a crop every two or three years. This short rotation made it more likely the soil would contain higher populations of soil-borne pathogens like Aphanomyces, which can seriously damage the roots of pea plants, causing them to wilt and die prematurely.

In addition, Conner informed me the fields where the pea plants appeared to be unaffected had not been sown to peas for many years, making them less likely to be affected by Aphanomyces.

We sent some samples of the diseased roots in for DNA testing and the lab results confirmed the presence of Aphanomyces.

At this point, there wasn’t anything that Conner could do to stop the root rot infection, which continued to contribute to a poor plant stand and lodging and led to a reduced yield at harvest time.

Conner admitted he was disappointed that we couldn’t fix what went wrong with his pea crop, but he was thankful for a diagnosis so he could take steps to avoid a similar problem in the future.

I told him the first step was to avoid the temptation to shorten up his crop rotations. Longer rotations reduce the chances of diseases like Aphanomyces from occurring, and Conner assured me he would increase the length of his rotation.

I also recommended that he not grow peas again on the affected land for at least six years, which would help break the pest cycle of Aphanomyces.

Kent Gress, PAg, CCA, works for Richardson Pioneer Ltd. in Canora, Sask.

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