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Crop advisor casebook: What’s causing leaf chlorosis and yellow spots in this wheat?

A Crop Advisor's Solution from the April 10, 2018 issue of Grainews

Wyatt Kieper.
photo: Supplied

Neil owns a 3,500-acre grain farm north of Rossburn, Man. At the end of May, while checking one of his wheat fields for crop staging after seeding, Neil noticed some spots and abnormal yellowing on the plants’ leaves. Since Neil wasn’t sure what had caused the damage, he asked me to examine the crop and get some answers for him.

The yellow spots and chlorosis (yellowing leaf tissue) developed shortly after emergence during the plants’ vegetative phase. Affected plants were scattered throughout the field, and in no particular pattern.

After visually inspecting the crop, I wanted to explore three possible injury sources: environmental stress, equipment malfunction and nutrient deficiency.

From a distance, it seemed the plants in the affected field may have experienced early-season stress from an air seeder malfunction. We toured fields seeded before and after the one in question. Because the plants in the other fields were developing normally, an air seeder malfunction was not the cause of the plant injury.

After ruling out equipment failure, Neil and I discussed the nutrient plan and application practices for the affected field, as well as the environmental conditions after seeding the crop. These details provided the information I needed to solve the case.

For example, prior to seeding, Neil broadcast 40 pounds of potash on this field. Also, the region received limited precipitation that spring. In addition, Neil thought he remembered some cool temperatures after seeding.

I also collected some plant samples for tissue testing to determine the plants’ nutrient levels in this field.

I knew what was happening in Neil’s wheat field, and the tissue tests would confirm my diagnosis.

Crop Advisor’s Solution: Incorporate nutrients with limited mobility into the soil

 

Two factors were causing the symptoms in Neil’s crop: a nutrient deficiency and environmental stress.

From Neil’s choice of fertilizer application method and the plants’ symptoms, I suspected the plants were suffering from a chloride deficiency. Tissue test results confirmed this diagnosis.

Potash is made up of both potassium and chloride. Potash doesn’t move freely in the soil, as it’s usually adsorbed onto soil surfaces. Typically, potash is delivered to the plant via mass flow; however, because of the limited precipitation that spring, movement of free potassium particles to the root zone was restricted.

Plant-available potassium would also be limited because most of the soil-applied potassium particles are bound to the soil or in the organic form.

This restricted movement of potassium to the plants’ roots caused the chloride deficiency, resulting in the development of yellow spots on the leaves. Thus, it’s important to incorporate nutrients with limited soil mobility, such as potash, into the soil.

It was environmental stress that caused the leaf chlorosis in this crop. Weather records indicated temperatures dipped below freezing shortly after emergence, causing frost damage to the plants. With time, the plants would outgrow this injury.

Because the region experienced record yields that season, and the damaged plants were scattered throughout the field, yield loss was negligible at harvest.

Wyatt Kieper works for Richardson Pioneer Ltd. in Shoal Lake, Man.

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