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Curly Wheat: An Exercise in Troubleshooting

May 10, 2002
Some very odd leaf symptoms have been showing up in winter wheat fields in south-central Illinois. Gregg Barnard, of Barnards Soil Service in Wayne City, and Dennis Epplin, Extension crop systems educator of Mt. Vernon, alerted me to the situation. Upper leaves on affected plants are curly.

Leaf curl symptom. (Photo courtesy of G. Barnard.)

The curl is reminiscent of curling ribbon:

Leaf curl with white striping. (Photo courtesy of D. Epplin.)

Or, for those of you who don't spend a lot of time decorating packages, the curl looks like an unfurling wood shaving. So naturally this unusual symptom begs the question, "What is it?"

Good question! And I admit it has strained my troubleshooting skills, in particular. Thanks to R. Kiesling and H. Bissonnette for sharing their advice, gleaned from many years of wheat work with me. Here's what can be pieced together. The fields have not had the same wheat variety; they have sound agronomic practices (nothing odd about N application, planting, etc.); some had herbicide applications, some didn't. Again, nothing questionable there. They are all in the same region of the state. The fields also exhibit characteristic, but nontested, symptoms of BYDV and WSMV.

Virus symptoms. (Photo courtesy of G. Barnard.)

So, in evidence, are many leaves with pinkish yellow discoloration, some stunting, and leaves with mottling and vertical (like drought-stressed corn leaves) rolling, as well as the wood shaving curl. Please refer to issue no. 3 of this year's Bulletin for more information on wheat virus symptomatology. No wheat curl mites were found in these fields. Many of the leaves with the wood shaving curl have whitish stripes. A few plants are showing some frost injury, with leaves looking like a rubber band had been stuck around them. I think we can safely rule out drought. So that's the picture.

What facts can we bring to the table to try to solve the question? First, what causes leaf curl in wheat? A few possibilities: WSMV can exhibit a vertical leaf roll; herbicide damage can cause some crinkling and bending at nodes; some varieties have a genetic tendency to exhibit the wood shaving-type curl; and an aphid, not known to be in Illinois, the "Russian wheat aphid" (RWA), can cause the wood shaving curl. Then, for those of you who have taken one of my troubleshooting workshops, you know "environmental factors" must be added to the list.

Now we try to draw some conclusions. It was recommended that the fields be tested for BYDV and WSMV. Neither, of course, is known to exhibit the odd curling symptom, but very obvious symptoms of them are present; so the problem is not solved, but the whole picture can be documented. Next, the genetic tendency of some varieties to exhibit a wood shaving-type curl is a very real possibility, but it loses strength as a solution as the number of fields and varieties expands. Next, what about Russian wheat aphid? Well, the leaf curl closely resembles RWA-affected leaves, but so far no RWAs have been found; and to the best of our knowledge, if RWA aphids fed in the fall and were absent now, what we likely would see would be significant stunting and winter kill instead of curling. The RWA is quite distinctive, though, so if you see any lime-greenish aphids with stubby antennae I would like to know. You may notice by now that about our only conclusion left is environmental factors. Not a very palatable conclusion, and I try to rely on it only when no other possibility exists. As this situation develops, I would be interested to receive your observations so that we can try to come to a more substantial explanation of the unusual leaf curl.--Suzanne Bissonnette

Author: Suzanne Bissonnette

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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