Crop advisor casebook: Leaf yellowing and uneven stand in wheat

A Crop Advisor's Solution from the March 27, 2018 issue of Grainews

Shawna Lajoie.
photo: Supplied

When I pulled up to Don’s wheat field last year on July 6, the first thing I noticed was how yellow the crop was. The crop’s development was also very uneven. For example, some plants were still at the five-leaf stage, while others had the flag leaf out with the head in the boot. There was no noticeable pattern to this uneven crop development.

Don farms around 1,500 acres of wheat, canola, barley and oats near Bjorkdale, Sask. He asked me to visit his farm because he thought his crop was looking unhealthy.

Throughout the field, the plants’ newest leaves were yellowing, and in some instances, were already turning brown. When I gently tugged on the newest leaves they came away easily from the plants due to pinching at the base of the leaves. I dug up some severely injured plants and I found a lot of browning at the plants’ bases. In fact, some leaves in this area were turning to brown mush.

“Could this damage be the result of a nutrient deficiency?” asked Don. “Or, perhaps, a disease has set in?”

Throughout the field, the plants’ newest leaves were yellowing, and in some instances, were already turning brown. The newest leaves came away easily from the plants due to pinching at the base of the leaves.
photo: Supplied

Initially, I thought the leaf yellowing could be due to nitrogen deficiency. However, after examining several plants I found the yellowing only occurred on the newest leaves. Nitrogen is a mobile nutrient within plants, thus yellowing will always occur on the oldest leaves first when there’s a deficiency.

Additionally, after inquiring about Don’s fertility program, I thought his fertilizer blend was more than adequate to produce a healthy wheat crop.

A week prior to my visit, it had rained for a period of three days. Leaf diseases brought on by moisture usually have some sort of lesions present on the plants’ leaves. Typically, these lesions aren’t limited to the newest leaves. I also examined the root system for root rot, however, the roots appeared to be healthy. Thus, I didn’t think the plants’ symptoms were caused by disease.

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That season, Don applied a tank mix of glyphosate and florasulam (Group 2) for the pre-seed burndown. A tank mix of tralkoxydim (Group 1), clopyralid (Group 4) and fluroxypyr (Group 4) was applied in-crop a few weeks prior to my visit.

Had something gone wrong with Don’s herbicide applications? If so, what?

Crop Advisor’s Solution: Watch out for low temperatures when spraying herbicides

It was when I checked the weather reports for the time frame surrounding Don’s in-crop herbicide application that I realized how this wheat crop had been damaged.

Normally, wheat plants metabolize the herbicide tralkoxydim rapidly, preventing plant injury. Although the daytime temperatures, at around 20 C, were good for spraying the in-crop herbicides, the temperatures were low, around 3 C, at night.

Cold temperatures, even if only at night, can limit the rate at which wheat metabolizes tralkoxydim, decreasing the plant’s tolerance to the herbicide. This intolerance produces the symptoms typical of Group 1 herbicide injury, such as yellowing of the newest leaf and the pinching and browning of the leaf base.

After explaining my theory to Don, he told me he used the surfactant rate recommended for 10 gallons of water, but he’d only used eight gallons. The additional surfactant in the tank mix exacerbated the wheat crop’s herbicide injury symptoms.

When applying herbicides, it’s important to examine the weather forecast for the daytime and night-time temperatures before spraying because those conditions can affect a herbicide’s performance.

Some herbicide modes of action are more sensitive to adverse conditions, such as low temperatures, than others. When in doubt, it’s a good idea to review environmental conditions with a crop advisor before spraying to ensure optimal herbicide performance.

In addition, use correct surfactant rates when applying herbicides — concentrations higher than recommended label rates could put more stress on the crop and exacerbate herbicide injury symptoms.

Although Don’s wheat crop looked very uneven and was difficult to manage for the duration of the growing season, due to fungicide and herbicide staging, at harvest the yield loss was negligible.

Shawna Lajoie works for Richardson Pioneer Ltd., in Crooked River, Sask.

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