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Crop Advisor’s Casebook: Why are these yellow peas in a twist?

A Crop Advisor's Solution from the July 18, 2017 issue of Grainews

Jodi Christopher.
photo: Supplied

On June 6 of last year, I visited Mike’s 3,000-acre farm, near Morse, Sask., where he grows green lentils, yellow peas, mustard and durum. Mike was concerned about some yellowing, unhealthy-looking plants in an area of his pea crop.

Two days before he noticed the damaged plants, the crop had been sprayed by a custom applicator with an in-crop herbicide containing Group 1 (cyclohexanedione) and Group 2 (imidazolinone) modes of action, to control broadleaf and grassy weeds.

The plant symptoms included stunted development, twisted and bent stems and chlorosis at the growing point. In addition, I could see a distinct line in the field indicating a difference in the plants’ health and development on the headlands, when compared with the rest of the field.

“Could the damage be a result of an evening application of herbicide?” Mike asked.

It’s true, application of a Group 2 imidazolinone herbicide under certain conditions, such as cool temperatures, plants already under stress, or a crop past recommended staging, can cause yellow flash in plants. However, because of the plants’ abnormal growth (i.e., twisted stems and stunted development), I thought there was more going on in the affected area than yellow flash caused by the Group 2 herbicide application.

I ruled out the obvious first. If the cause of the damage had been a nutrient imbalance or fertilizer mishap, I would expect to see symptoms expressed more consistently across the entire field, or at least a section of it.

In addition, during fertilizer application, Mike hadn’t noticed any blockage of machinery, and the distinct lines in the field didn’t match up with the width of the drill. Furthermore, twisting and bending of stems is not a common symptom of nutrient imbalance or an error in fertilizer application.

The distinct geometric pattern created by the affected plants allowed me to eliminate herbicide drift from a neighbouring field as the cause of the damage. Also, these straight lines did not support pest pressure as the causal factor. Symmetrical areas, or straight lines like this, don’t normally occur in nature and are not usually created by natural factors, such as pest damage.

Mike wasn’t dealing with an inoculant error, either, as the formation, number and colour (pink to red) of nodules, in both affected and unaffected plants, were normal. Thus, the pea plants had healthy, nitrogen-fixing nodules.

However, I had a good idea of what went wrong in Mike’s pea crop. The stunted growth and twisted stems of the affected plants were significant. Also, I compared the weeds present in both affected and unaffected areas. Lastly, to confirm my suspicion, I called the custom applicator who sprayed Mike’s field two days prior to my visit.

I had enough evidence to provide Mike with a clear-cut diagnosis of the problem in his pea field.

The damage was in a distinct geometric pattern, eliminating herbicide drift or pest pressure as causal factors.

Crop Advisor’s Solution: Avoid Group 4 injuries with tank cleanout

 

One phone call to the custom applicator settled the matter. The operator had sprayed a tank mix that included a Group 4 herbicide on a cereal crop before applying the in-crop herbicide on the pea field. The symptoms were caused by Group 4 residue in the sprayer tank.

In fact, we could see exactly where the first tank was sprayed, and where the booms were shut off. The first sprayer tank pass left twisted and stunted pea plants in its wake, whereas the second and remaining sprayer tank passes did not injure the plants.

Pea plants and weeds exposed to Group 4 herbicides will have twisted and bent stems, chlorosis of the growing points, stunted development, and wilting, as observed in this field. I notified the custom applicator about the damage to the pea field due to improper tank cleanout procedures.

Performing tank cleanout correctly will prevent the unnecessary headaches and yield and financial losses caused by herbicide residue left in sprayer tanks, booms and filters. For example, spray out any solution remaining in the tank before switching to a different chemistry. Thoroughly rinse the tank, booms and filters several times with a strong detergent and adequate amounts of ammonia.

Furthermore, although it was not responsible for the damage in this field, when it comes to Group 2 (imidazolinone) herbicide application, avoid spraying in cool temperatures, if the plants are under stress, or if the crop is past staging, to reduce yellow flash.

Those peas that did survive the Group 4 herbicide injury were severely stunted, and maturity was delayed, resulting in significant yield loss when compared with the rest of the field. In addition, with less crop competition, a second flush of weeds created further headache for Mike at harvest, and a larger weed seedbank for the following season.

Fortunately, though, only the first tank of herbicide was harmful to the crop — the rest of the field performed well. However, Mike continued to monitor the field, and the damaged sprayer pass remained conspicuous for the duration of the season.

— Jodi Christopher, AAg, works for Richardson Pioneer Ltd. at Reed Lake (Herbert), Sask.

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