Crop advisor casebook: What caused this stunted and purpling canola?

A Crop Advisor's Solution from the March 5, 2019 issue of Grainews

The canola field was exhibiting uneven germination and the plants that had germinated were stunted and purpling. Eric thought these symptoms could 
be attributed to the frost that occurred a few days prior to my visit.
Trista Jonescu. photo: Supplied

Eric, who farms 10,000 acres north of Assiniboia, Sask., asked me to visit his farm around mid-May, which was a couple of weeks after he seeded his canola crop. He wanted me to assess if it was necessary for him to reseed the field.

The field was exhibiting uneven germination and the plants that had germinated were stunted and purpling. Eric thought these symptoms could be attributed to the frost that occurred a few days prior to my visit.

“The canola plants are purpling and stunted from the frost,” he said. I wasn’t so sure that was the case.

First, I dug in the seed rows to determine if the uneven germination was a result of seeding issues, such as uneven seed depth or equipment malfunction. I found that the seeds were there at the correct depth, but they just hadn’t germinated.

The area had received very little precipitation that spring and the region was experiencing drought conditions. The dry conditions could be responsible for the uneven germination in the canola field.

I thought I would observe the field for a few days to look for any changes in plant symptoms. After two days, many of the plants’ leaves were turning yellow or even becoming bleached. This kind of leaf colour change can occur after frost, especially under drought conditions.

However, damage from frost typically doesn’t cause plant stunting and purpling. At this time, the canola plants had reached the cotyledon stage, and the area was experiencing a large flea beetle outbreak.

We kept scouting Eric’s canola field to monitor flea beetle populations. Once flea beetle levels reached the economic threshold, around May 26, Eric sprayed an insecticide for pest control. Although flea beetle damage can cause crop thinning, delayed crop development, and seedling death, it usually doesn’t cause the purpling or stunting we observed in Eric’s crop.

After applying insecticide, the crop was no longer being damaged by flea beetles, yet the symptoms remained, including poor germination.

At this point, the field received a couple tenths of an inch of rain. This precipitation helped germinate the remaining canola seeds that were dormant in the soil. The uneven germination had been caused by the dry conditions.

Eric and I sat down and examined the field’s history. A herbicide-tolerant lentil variety had been planted the previous year, which was sprayed in-crop with a Group 2 herbicide (imazamox)*.

As per its registration, this product was safe for use in the field a season prior to planting canola. So, what was causing the plants to be stunted and purpling?

Crop Advisor’s Solution: Drought conditions can increase risk of herbicide carryover damage in canola

In many cases, purpling and stunted growth is a symptom of Group 2 herbicide injury, however, a Group 2 herbicide had not been applied to the field. When we checked the field’s history, a Group 2 herbicide (imazamox)* had been applied in-crop to the herbicide-tolerant lentils the year before; however, the product was deemed safe to use one year** prior to planting canola according to its registration details.

Under certain conditions, such as drought, the breakdown of herbicides is inhibited — as it was in this case. Although the Group 2 herbicide should have broken down by the time Eric planted his canola crop, it had not. Thus, Group 2 herbicide carry-over was responsible for injuring the canola plants one year following application.

To be certain, the soil was tested for the residual herbicide. The test results confirmed the injury to the canola plants was caused by herbicide carry-over damage. There was nothing Eric could do but wait for the crop to grow out of the injury.

Having enough rainfall can prevent carry-over issues, but when it is limited, or drought conditions exist, residual herbicide injury can occur.

Note: The original version of this post reported that both imazamox + imazethapyr had been applied. This was an error. When both imazamox (Solo, Viper) + imazethapyr (Generics and Odyssey, which contain both imazamox and imazethapyr) are applied, growers must wait until the second growing season after the season of application to plant these crops. Thanks to Clark Brenzil, weed specialist for Saskatchewan Agriculture, for getting in touch to point this out.

Note: The original version reported that the product was deemed safe to use prior to planting canola. Dan Packer, BASF pulse crop manager, points out that it cannot be used pre-seed, but must have been used the year before.


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