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Crop Advisor’s Casebook: Deformed wheat spikelets spell trouble

A Crop Advisor's Solution from the February 7, 2017 issue of Grainews

Tess Strand
photo: Supplied

Dan operates a 5,000-acre grain farm south of Canora, Sask. Last summer, he called me mid-July to inquire if any other producers in the area were discovering white heads in their hard red spring wheat crops.

In certain areas of his fields, he said, shortly after heading, some wheat plants had developed a whitish tinge to them. The discolouration appeared in patches and seemed worse along field edges and in low-lying regions, he told me.

That day, I took a trip to Dan’s farm to inspect his wheat fields. When I got there, I found it was mostly the wheat head tips that were affected. The topmost spikelets were shrivelled and bleached-looking, and the awns were also white and bent.

These fields had been planted on barley stubble, Dan told me, whereas the wheat fields seeded after canola were not exhibiting symptoms.

At this point, I suspected the whitened heads were a result of a disease connected with Dan’s rotation. Because cereals had been planted two years in a row, and since the area had received a lot of moisture that growing season, I examined plant crowns and roots for signs of root disease. However, the roots were healthy, and only the head tips were affected.

Next, I examined the wheat heads for signs of fusarium head blight, which is caused by several species of the fungal pathogen Fusarium, as this disease can cause bleached spikelets and reduced seed filling.

Because the plants’ topmost spikelets were consistently affected, and due to the absence of the orange spore clusters typically found near the base of florets of FHB-infected plants, I ruled out FHB as the cause of the whitened heads.

“Could it be wheat stem maggot?” Dan asked. It’s true, these stem-boring insects can cause whitened heads in wheat, but usually the whole head turns white, and not just the head’s top, as we were seeing in Dan’s fields.

However, when checking the plant stems for wheat stem maggot larvae, I unrolled the upper stem and found several small, black specks.

When checking the plant stems for wheat stem maggot larvae, 
I unrolled the upper stem and found several small, black specks.
photo: Supplied

“Here’s your answer,” I said. “They’re rare in wheat, but we’ve been seeing unusually high numbers in barley fields in this area.”

Crop Advisor’s Solution: Monitor your cereal fields for barley thrips

It was while I was examining the plant stems for wheat stem maggot larvae that I found the true cause of the bleached heads. As I unrolled the upper stem, I found several small, black specks — barley thrips!

It’s rare to find these insects in wheat crops, but there had been unusually high thrip numbers in barley fields in this area.

Adult thrips are slender, dark-coloured, and less than two millimetres long, and so appear as brown or black specks to the naked eye. Immature larvae are white, or pale green to yellow in colour. Larvae feed in the upper stem, puncturing cells and sucking out the contents, causing distorted flag leaves and shrivelled kernels at the top and/ or bottom of the wheat head. Thus, thrip feeding by both larvae and adults caused the wheat heads to appear bleached and deformed.

Because adult females overwinter in grass hosts, thrip damage can be worse along field edges, as we were observing in Dan’s fields.

It is important to monitor cereal fields at the flag leaf stage for thrip infestation, especially cereal and barley fields seeded into barley stubble. When the flag leaf is visible, sampling should begin, and continue until heading is complete.

After heading, when symptoms are noticeable, the damage has already been done, and it is too late to warrant control. In Western Canada, it’s not very often these pests reach levels warranting control, however, it is something to watch for in barley after a season with higher numbers. In barley crops, the economic threshold can be calculated per stem as:

(cost of control ÷ expected value per bushel) ÷ 0.4. However, in wheat crops, there is no set economic threshold.

At harvest, Dan’s yield was slightly affected, but because no economic threshold exists for wheat, it is difficult to determine if it would have paid to spray.

Tess Strand is a regional sales agronomist in Canora, Sask.

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