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Crop advisor casebook: What’s to blame for this yellowing durum crop?

A Crop Advisor's Solution from the June 4, 2019 issue of Grainews

Shiela Miller.
photo: Supplied

Ed, who grows more than 6,000 acres of durum, canola and lentils on his farm near Rouleau, Sask., called me in July to tell me about a problem in one of his durum fields.

Ed had recently driven by the field and noticed the crop didn’t appear all that healthy — he could see there were yellowing leaves on quite a few durum plants, and a closer look revealed it was the oldest plant leaves that were exhibiting chlorosis and, in some instances, had died off.

“It must be some sort of disease setting in, or maybe I didn’t spray my fungicide at the right time,” Ed told me over the phone.

I went to Ed’s farm to have a look, and when I surveyed the affected crop it was evident that the yellowing and senescence of older plant leaves was occurring all across the field. Overall, the crop didn’t look as vigorous as it should for that time of year, although plants in the lower areas of the field appeared to be somewhat healthier.

Even though it had been relatively dry in the area for the past two years, I wondered if disease was at work here. I checked some affected plants and found no dark brown lesions or chlorotic halos around lesions, which can indicate the presence of a fungal pathogen such as tan spot. I asked Ed about his disease control efforts and he said he’d applied a fungicide at 75 per cent head emergence under ideal conditions and he had used an adequate water volume.

I also considered whether a nutrient deficiency could be to blame. Since nitrogen and potassium are both mobile within plants, a deficiency of either nutrient can cause the older leaves of plants to show signs of chlorosis. When I inquired about his fertility program, Ed informed me that he’d applied 385 kilograms (175 pounds) of 46-0-0 (urea) fertilizer as well as a 7-36-18 blend of phosphate and potash at seeding.

At this point, I thought I had everything I needed to determine what was going on in Ed’s durum field.

Crop Advisor’s Solution: Moisture deficiency in durum

I had just scouted the same field the week prior and found no signs of disease development. It had been very hot and dry over the weekend and I knew there hadn’t been any significant rainfall in the area since June, which helped me clue in to the reason why the durum was healthier in the lower parts of the field — there was more available soil moisture for plants in those areas.

The agricultural soils in the Rouleau area are generally high in clay content and have a high capacity to hold moisture. In the previous year, there had been little rain and the crop would have depleted the stored soil moisture.

Most nutrient uptake by plants occurs through the translocation of water from the soil into the plants through the root system. As a result of continued dry soil conditions this year, the durum plants would not have been able to take in enough of the required nutrients for growth, so they translocated the nutrients from their lower leaves to the heads to produce seed. My diagnosis was confirmed when the same plant symptoms started showing up in other fields on Ed’s farm and in surrounding farmers’ fields within the next few days.

Unfortunately, this was an environmental issue and there wasn’t much that could be done to help Ed’s crop. Although drought symptoms can only be improved by timely rains or irrigation, a few agronomic practices that may help your crop in acute drought conditions are balanced fertility to promote early season root growth and early weed control.

Shiela Miller works for Richardson Pioneer Ltd. in Corinne, Sask.

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