Crop advisor casebook: Purple plants perplex canola grower

A Crop Advisor's Solution from the June 1, 2021 issue of Grainews

Some canola plants were turning purple. They were scattered here and there throughout the crop, although there wasn’t a great deal of them.
Amy Prybylski. photo: Supplied

One morning in early July, I got a call from Aaron, a grain farmer in Melville, Sask., who grows canola, wheat and barley. Aaron told me he’d been out scouting canola fields on his 10,000-acre farm when he came across some plants that were turning purple — he asked me to come have a look.

At the farm, I walked out into a canola field with Aaron. Looking around, I could see discoloured plants scattered here and there throughout the crop, although there wasn’t a great deal of them.

When I uprooted a few purple plants and a few healthy ones from the dry, dusty soil to compare the two, I saw all of the plants showed signs of flea beetle feeding.

“Could the flea beetles really have done this much damage?” Aaron asked.

I didn’t think so, since the canola was already past the four-leaf stage, which meant there hadn’t been any flea beetle feeding in a while. Something else must have caused additional damage to the plants more recently.

When I dug up the purple canola plants, I observed their roots didn’t have much, if any, root hairs. The plants also seemed to be pinched right at the soil surface, with stems that looked almost scabbed over from the damage.

Looking down the canola rows, I could also see that some of the purple plants were cut off near the soil surface, and only the stems were sticking out. I wondered if cutworms could be the source of the trouble.

I also pondered whether this had something to do with insufficient nutrients, or if it was a herbicide carryover issue. The previous crop on the field was wheat and I knew carryover from Group 2 herbicides could cause some similar symptoms in canola as I was seeing in the field.

As I talked things over with Aaron, he pointed out it hadn’t been a great year weather-wise, with little in the way of rain and very high winds in recent weeks. However, I didn’t believe this alone could explain what was going on in the canola field.

What was really puzzling to me was why some plants exhibited so many different symptoms, while the rest of the crop appeared unaffected. Adding to the mystery was the problem wasn’t limited to this one field — when I’d scouted a couple of nearby canola fields Aaron had seeded around the same time, I could see they, too, contained purple plants scattered randomly throughout the crop.

Crop Advisor Solution: Purple plants caused by environmental stress, insect feeding and high winds

While I was digging up plants, I noticed there was very little soil moisture. Aaron explained there hadn’t been a good rain on the field all year, and he also mentioned strong winds had buffeted the canola in recent weeks. 

This raised some red flags, and after conferring with some other agronomists about the problem, I thought I had my answer. There wasn’t one cause but a combination of factors that had led to this purple plant dilemma. 

A likely explanation was the environmental stress caused by lack of moisture combined with damage from flea beetle feeding had affected some plants more than others. The high winds that had whipped through the crop must have weakened the stems of affected plants even further causing the pinched stems, which contributed to some plants turning purple and others to break off completely. 

There wasn’t anything Aaron could do to fix this problem; however, because there were only a small number of plants affected, yield damage was minimal. Come harvest time, Aaron’s canola yields were down but that was mostly due to the drought conditions that persisted for much of the growing season. 

There were a number of things Aaron could do to prevent a similar situation in the future. For one, he could consider keeping his stubble a little taller to prevent exposed ground and provide protection for young canola plants. Taller stubble could also trap some snow for springtime and help maintain some subsoil moisture. Aaron can also take steps to minimize flea beetle damage in his canola fields by utilizing seed treatments, monitoring beetle populations following seeding and taking corrective action when needed. 

Amy Prybylski, CCA, AAg, works for Richardson Pioneer Ltd. in Melville, Sask.

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