Crop advisor casebook: What’s causing extreme stem discolouration in this wheat crop?

A Crop Advisor's Solution from the March 13, 2018 issue of Grainews

Alaina Stoesz. photo: Supplied

While checking some crops, Ron, a Saskatchewan producer, was alarmed to find his wheat crop’s plant stems had turned a vivid reddish-purple colour within a very short period of time.

It was August 9 when Ron asked me to investigate the cause of the stems’ colour change. Ron farms 3,000 acres of wheat, canola, barley and peas, and runs a small cow-calf operation, west of Hague, Sask. I had just visited this field a couple of days ago. The plant stems, at that time, were a normal straw colour.

From the road, the crop had a purplish tinge to it. A closer examination revealed the plant stems were extremely reddish-purple in colour. However, there were some stems that were half straw-coloured and half reddish-purple.

Although they appeared consistently throughout the field, the symptoms were more pronounced in drier areas, such as hilltops.

Only the stems appeared to be affected; the stem joints remained a green colour, and the heads and leaves also looked normal.

“Do you think it could be a phosphorus or potassium deficiency?” asked Ron.

A nutrient deficiency or imbalance generally shows up earlier in a wheat crop, at the two- to four-leaf stages, rather than at the beginning of maturity. Also, the reddish-purple colour would be evenly distributed on the plant stem, and not on half of it.

Only the stems appeared to be affected; the stem joints remained a green colour, and the heads and leaves also looked normal. photo: Supplied

In addition, the leaves and/or leaf tips would exhibit symptoms if the cause was a nutrient deficiency or imbalance. Soil test results also indicated phosphorus and potassium levels were adequate for normal growth and development.

“Could I have made a mistake with the herbicide application timing?” Ron asked.

Herbicide injury due to incorrect timing was improbable because the wheat heads would be kinked by the specific herbicide group that was sprayed. In addition, I recommended the herbicide application at the five-leaf stage, which was well within the timeframe for the crop.

Ron asked me if the symptoms could’ve been caused by a root disease. After digging up some plants we determined the roots were normal and healthy, and there were no lesions or discolouration at the root crowns.

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If not a nutrient deficiency, herbicide injury or root disease, what was causing the reddish-purple stem colouring?

Crop Advisor’s Solution: Drought stress conditions cause extreme anthocyanin expression in wheat

The fact that some stems were half-affected and half-normal, strongly suggested the condition was caused by environmental stress on the plants.

And, I’d seen this phenomenon before. While scouting fields in southern Manitoba years ago, I came across a variety named Kane, a hard red spring wheat, which would sometimes exhibit purple stems. This purple stem colouration was more pronounced during hot, dry conditions at maturity.

Kane is a half-sibling of Cardale, the wheat variety Ron planted in this field. Cardale is derived from a cross between the varieties McKenzie and Alsen.

Both McKenzie and Kane are known to exhibit strong anthocyanin (reddish-purple plant pigments) colouration under certain conditions, such as drought-stressed environments. The expression of anthocyanin is a physiological response to drought stress, and, in this case, is an inherited trait.

Under the drought-stressed environmental conditions in Ron’s field, sugars accumulated in the plants. Within these sugars are purple anthocyanin pigments, which produced the extreme colour change. The hot, dry conditions at plant maturity made the symptoms much more pronounced.

Happily, studies performed in Manitoba and the United States indicate the stem discolouration doesn’t affect yield. Also, the symptoms occurred late in the season, after yield had already been determined.

Alaina Stoesz works for Richardson Pioneer Ltd. at Carlton Crossing, Saskatoon, Sask.

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