Crop advisor casebook: Why are rows of these lentils yellowing and stunted?

A Crop Advisor's Solution from the April 23, 2019 issue of Grainews

Chad Scott.
photo: Supplied

Cory owns a 4,000-acre grain farm near Cupar, Sask. His crop rotation consists of lentils, wheat, durum and canola. It was around the middle of June when I received a call from Cory, who was concerned about his lentil crop after he noticed that in certain rows the plants were stunted and the leaves of those plants were turning yellow and shriveling. Some plants in the affected rows were already dead.

“The plants seem like they have stopped growing,” he said. “The leaves are yellowing and the roots have a brownish-caramel colour to them instead of white.”

When I arrived at the field, I immediately checked the roots to determine if they were nodulating. The nodules were just starting to turn from pink to green, indicating little nitrogen fixation was happening in them. The plants with poor nodulation occurred mainly in low-lying areas of the field, but it looked like the damage was creeping uphill.

“I’ve been checking this field regularly and these symptoms began after my herbicide application,” said Cory.

This information was interesting to me because pulse crops can be sensitive to herbicide application and I was curious if there could be a risk of herbicide injury. I examined the affected areas to determine if any plants were showing symptoms of chemical injury.

I didn’t find any physical symptoms of herbicide injury. In addition, Cory had used adequate water volumes and a stress-mitigating product that is marketed to decrease the risk of herbicide injury. I concluded herbicide injury wasn’t the issue.

The injured and dead plants were located in low-lying areas; they may have been damaged as a result of water stress. But that wouldn’t explain why only certain rows had damaged plants and why the symptoms were spreading uphill.
photo: Supplied

Considering the poor nodulation and that there were not any clear patterns in the field, I reasoned the problem was not caused by residue contamination in the sprayer tank due to improper tank cleanout — the damage was not consistent with that type of chemical injury.

We also examined seeding depth, which we found was normal and consistent across the field. Since the injured and dead plants were located in low-lying areas, I wondered if they had been damaged as a result of water stress. However, that wouldn’t explain why only certain rows had damaged plants and why the symptoms were spreading uphill.

Cory has been implementing a good fertility plan and he had used a seed treatment on the lentils at planting. It had been a dry year for the second year in a row. From the start of May, Cory’s field had received 1.5 inches of rain on this particular field, where the soil is high in clay content.

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All of this data was leading us closer to the reason for the damaged plants in Cory’s lentil crop.

Crop Advisor’s Solution: Fields infected with aphanomyces may need a 6-year break

Aphanomyces typically shows up in clay soils first because of the higher water-holding capacity of the fine-textured soil. Aphanomyces is known as a water mould and is transported by water movement in the soil. Unfortunately, the timing of the rainfall event was conducive to the spread of aphanomyces infection in Cory’s field.

Unfortunately, there is a lack of control options for aphanomyces. Cory really wanted to try something to minimize the level of stress that the plants were experiencing in this field. While there is no treatment or control measures that can effectively manage aphanomyces, we decided to try a biostimulant to see if it would help the plants manage the stress of the compromised root systems.

The biostimulant seemed to positively affect the areas of the field in question. Cory was happy with the outcome and relieved that we caught the problem when the affected plants were limited to the low-lying areas of the field and he wasn’t dealing with widespread infection. Because the area of infection was small with respect to the size of the field, the yield wasn’t noticeably affected.

Since there are no chemical control measures for aphanomyces, and because the spores survive for long periods of time in the soil, Cory will need to take at least a six-year break from planting a lentil or pea crop to let them die out. If he continues to grow lentils or peas on that land without a six- to eight-year break, he will increase inoculum levels in the soil and will continue to struggle with root rot infection.

Aphanomyces is a serious disease that shouldn’t be taken lightly, especially on clay soils which are able to hold water for longer periods of time than coarser-textured soils.

Chad Scott works for Richardson Pioneer Ltd. in Southey, Sask.

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