Crop advisor casebook: Patchy growth in canola presents a puzzle

A Crop Advisor's Solution from the September 24, 2019 issue of Grainews

Janell Rempel. photo: Supplied

The canola in Rob’s field had just emerged when he contacted me this past spring about concerns of insect pressure and a patchy plant stand. In addition to canola, Rob grows wheat, barley and peas on 2,500 acres near Wakaw, Sask.

At the time of his request for me to visit his farm, the region was experiencing dry environmental conditions and heavy flea beetle pressure. Rob had found flea beetle damage in his canola while scouting the previous evening. He wanted me to help him determine if flea beetle feeding had reached the action threshold in his field.

Rob had also noticed that there was some spotty emergence with areas of poor stand establishment, especially in the low spots. He described the low-lying areas of the field as blacker soils with good surface moisture. These areas had been worked prior to seeding.

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Rob had expected to see the highest germination rates and the best stand establishment in those low-lying areas because the soils there held more moisture than in the other regions of the field. The crop on the rest of the field was struggling due to the dry conditions and the higher levels of crop residue from the previous year.

When I arrived at the field and began my evaluation of the crop, the canola varied in stage from just starting to emerge from the soil to late cotyledon stage. Some areas were covered with heavy crop residue, which slowed crop emergence. There were also areas where extremely low soil moisture inhibited some germination.

In addition to challenging environmental conditions, there was heavy flea beetle pressure and feeding damage, particularly on the headlands that bordered fields seeded to canola the previous year. While flea beetle pressure was definitely at threshold levels on the headlands, that was not the case in the low-lying areas of the field. We also didn’t find any plant remains in these areas, which is typically an indication of mortality due to flea beetle feeding and damage.

It was possible, Rob said, that he’d made an error with seeding depth. “I had made some adjustments to my seeding equipment on this particular field,” he said. To address Rob’s concern about a possible seeding error, we examined the seed rows in the low-lying areas to determine if he’d seeded too shallow or deep, which could have affected germination.

While digging around we found the odd seed in the seed row but there were not enough to suggest seeding depth issues. These seeds had also been planted around the targeted one-inch depth, thus ruling out seeding error.

The stand establishment in the low-lying regions was also not caused by the dry conditions as soil moisture in these areas was higher than the field’s average.

We moved to the edge of the region with the thin stand, hoping to determine a difference between the area of good and poor plant establishment. The first thing I noticed was a few plants were lying on the soil’s surface as if they had been knocked over. However, the real giveaway to solving this mystery lay just below the soil’s surface.

Crop Advisor’s Solution: Scout early and watch your fields for the classic signs of cutworm damage

While digging in the soil near a canola plant that looked like it had fallen over, we discovered a plant severed just below the soil surface. This is typical of cutworm damage. As we continued to examine the area at the edge of the poor plant stand, we found more stems severed below the soil surface and small plants clipped off as they emerged. We dug a little deeper and found cutworms.

While digging in the soil near a canola plant that looked like it had fallen over, we discovered a plant severed just below the soil surface. We dug a little deeper and found cutworms. photo: Supplied

We had already determined that the field needed an application of an insecticide to control the flea beetles after reaching the action threshold. Rob decided to spray the area where we found the cutworms as well because they were still actively feeding and the affected area was growing quickly. Luckily, there were products available that would control both pests in a single application.

After the insecticide’s labelled re-entry period, Rob reseeded the cutworm-affected areas. The reseeded plants were able to catch up with the rest of the crop because of the higher moisture levels in the low-lying soils.

At the time of writing, Rob’s crop had not been harvested, however, I believe he found significant benefit from an early diagnosis and the application of an insecticide. By diagnosing the issue early, Rob was able to reseed the cutworm-affected areas and the second-pass canola seed was able to catch up to the earlier seeded crop. The application of the insecticide was applied early and controlled both pests in the field, which can contribute to significant yield damage if not controlled.

In the future, Rob will likely use an insecticidal seed treatment to help control cutworms early in the season and limit the damage they can do. He will also continue to scout his fields early to ensure timely discovery of cutworms and flea beetles, allowing the time to take action if required.

Janell Rempel, MES, CCA, AAg, works for Richardson Pioneer Ltd. in Wakaw, Sask.

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