Crop advisor casebook: Chlorotic spring wheat plants perplex farmer

A Crop Advisor's Solution from the March 2, 2021 issue of Grainews

Wyatt Kieper. photo: Supplied

During the first week of July last year, I received a call from Leonard, who grows 7,000 acres of spring wheat, winter wheat, canola, flax and peas near Shoal Lake, Man. He’d been out doing his evening crop check the day before when he spotted something wrong with his spring wheat crop. Rather than a sea of green, the field was awash with yellowing, chlorotic plants.

Leonard was worried he’d be facing a major yield loss and asked that I come out to have a look. Once I arrived and surveyed the field from the road, I observed the chlorosis wasn’t consistent throughout the wheat crop but followed a pattern that was aligned with the path taken by the combine in previous harvests.

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Outside of these chlorotic areas, the plants looked to be symptom-free and the crop seemed to be doing very well. I could also see lots of winter wheat volunteers, which were growing only in those parts of the field that were yellowing.

The symptomatic spring wheat plants were stunted and had yellow streaks along their flag leaves, which were also quite brittle. Leonard assumed that some kind of nutrient deficiency was to blame, and based on the symptomology, I thought it could be zinc that the plants were lacking.

I knew more proof was required to support this diagnosis, so Leonard and I collected some tissue samples from chlorotic plants and sent them off for a tissue nutrient analysis and plant pathology tests.

A chlorotic flag leaf from a spring wheat plant. photo: Supplied

At this point, I had another idea it might be something else causing the chlorosis symptoms in Leonard’s spring wheat crop, but it wasn’t until the laboratory results came back that we had our answer.

Crop advisor’s solution: Wheat streak mosaic disease and not a nutrient deficiency caused chlorosis in spring wheat

There was one important clue that led me to believe that disease could be the culprit — the large population of winter wheat volunteers growing in those areas of the field with chlorotic wheat. Winter wheat volunteers are known host plants for wheat curl mites, which can transmit wheat streak mosaic disease and can cause the kind of chlorosis we were seeing in Leonard’s spring wheat field.

The spring wheat was grown on canola stubble, but I learned from Leonard there’d been winter wheat grown there two years previously. This would account for the winter wheat volunteers, which were growing from the remnants of the chaff blown out of the back of Leonard’s combine when he had harvested his winter wheat crop.

In order to get a definitive diagnosis, we collected tissue samples and sent them off for plant pathology and nutrient analysis testing. The results confirmed that it was wheat streak mosaic, and not a nutrient deficiency, that was causing the chlorosis symptoms in the spring wheat.

While waiting for the lab results, we decided the best course of action was to spray the field with a tank mix containing a flag leaf fungicide along with a foliar zinc fertilizer product on the off chance the plants were deficient in zinc.

The fungicide helped reduce any further infection from other pathogens on the plants damaged by the wheat streak mosaic disease, and the application appeared to help keep the crop green.

In the end, the yield was only slightly affected — a much better result than we imagined when evaluating the field back in July. As it turned out, the wheat streak mosaic infection occurred later in the season and the spring wheat plants were advanced enough to have minimal yield reduction.

To avoid this problem in the future, I recommended that Leonard adjust his rotation to try to maximize the time between his winter wheat and spring wheat crops, since this would reduce the chances of one of them acting as a green bridge for any pathogens that might affect cereal crops.

We also discussed how a pre-seed burndown with glyphosate could have prevented the winter wheat volunteers from turning up in the field. Leonard told me he hadn’t thought it necessary to do a burnoff due to the very dry conditions and an absence of weeds in the field at planting time.

Leonard could have also seeded a little bit later in the season to ensure the volunteers had fully emerged prior to planting so they could be controlled with a pre-seed burndown as there are no in-crop options.

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