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Crop Advisor’s Casebook: Puzzling patterns in a pea field

A Crop Advisor's Solution from the October 21, 2014 issue of Grainews

Back in late spring, Allan was alarmed to see some strange patterns showing up in his crop of field peas at his 2,000-acre mixed grain farm near Aylsham, Sask. Irregular sections of pale green peas were appearing in the field, in contrast to the lush green growth of the rest of the crop.

Mike Wassill is an area marketing representative with Richardson Pioneer Ltd. at Crooked River, Sask.

Mike Wassill is an area marketing representative with Richardson Pioneer Ltd. at Crooked River, Sask.
photo: Supplied

The pea field had been seeded the second week of May. Allan called me in mid-June, just before staging of early bloom. The peas in the lush green sections were all starting to show early blooms, while those in the pale green areas were not.

Allan told me he initially thought the problem could be a result of higher-than-usual spring rainfall. “It seems like the field is going backwards. Could it be caused by moisture?” he asked. “If so, why not the whole field instead of only certain areas? It’s starting to show up on some of the higher spots in the field as well, where the moisture isn’t as significant.”

When I drove out to Allan’s farm and had my first look at the pea crop, the first thing I noticed was that the field had a great stand and there were lots of plants. However, the difference in crop colour and development was stark. Pea plants in the pale green areas were small and struggling compared to the larger, healthier-looking plants in the lush green areas.

Upon closer inspection, there didn’t appear to be any aphid damage, a problem that can sometimes affect peas at an early bloom stage. When I asked about the fertility package for the field, Allan informed me he typically uses little fertilizer on his peas and that only 20 pounds of 11-52 product per acre had been placed with seed row.

The only chemical applied to the field up to this point was a common herbicide package called Odyssey DLX, which had been utilized before the sixth node stage. Allan told me the field’s prior crop had been Roundup Ready canola, and that the only chemical sprayed in the previous year was glyphosate.

As I was leaving the field, I made sure to collect some plant samples from both the pale green and lush green areas so that I could compare the roots and plant structure of the peas. A close look at these samples provided some telling clues as to the source of the problem.

Crop Advisor’s Solution: Root rot

Back in late spring, I got a call from Allan concerning a crop of field peas at his 2,000-acre mixed grain farm near Aylsham, Sask. Some troubling patterns were showing up in the crop, with pale green areas containing small, struggling plants appearing in stark contrast to larger, healthier-looking peas in lush green parts of the field.

Allan called me in mid-June, just before staging of early bloom. He said the peas in the lush green areas were all starting to show early blooms, while those in the pale green areas were not.

When I arrived at the farm, Allan told me he initially thought the reason plants in affected areas were not progressing was because of higher-than-usual spring rainfall. However, that did not explain why the problem appeared in only certain areas of the field, or why it was also showing up in higher areas of the field as well.

The answers to my questions about Allan’s fertility package and herbicide treatment for the crop didn’t sound any alarm bells, and the plants (pale or not) didn’t seem to be displaying any chemical damage either. I began to suspect that the answer lay with something in the soil or plant roots that was affecting nutrient uptake — which was confirmed upon closer inspection of plant samples from both the pale green and lush green areas of the field.

When I washed the soil off the roots of the plant samples in a testing pail, it was clear by the state of the healthy nodules and white roots of the unaffected plants that they were getting adequate nutrients. In contrast, the roots of affected plants were brown and shriveled and contained very few nodules.

The roots were obviously being affected by some sort of disease, either in the stem or soil-borne. Upon review and consultation with Richardson Pioneer agronomist, Dean McDermott, it was determined that root rot caused by a fungal infection was to blame.

This disease group included the pythium and fusarium species as well as rhizoctonia. The clear line between infected and healthy plants in irregular patches in the field indicated that soil-borne fungi was at work, which was confirmed when the plants were rinsed off with water.

I informed Allan that warm, moist weather conditions in the spring had allowed the disease to become active and infect the pea roots. Unfortunately, at this stage of crop growth, there was no effective treatment for limiting the spread of the disease and its damage. The only advice I could give Allan was to rotate away from peas for at least four years, and hope for a drier cycle of environmental conditions.

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