By Clint Thompson
Aphids may be vectors of cotton leafroll dwarf virus (CLDV), which causes cotton blue disease, but Auburn University plant pathologist Austin Hagan believes farmers shouldn’t waste time trying to kill them.
He said that through preliminary work done last year, research showed that aphids can’t be killed very well. Even if a farmer sprays every week with an insecticide, the aphids are still there.
“You may kill 95% of them but 5%, it’s enough to transmit the virus,” Hagan said. “Growers shouldn’t be out there trying to control aphids to manage this disease. All they’re going to end up with is resistance of the aphid to that particular insecticide. If they have aphid issues, populations are growing at a time when they’re a concern, then fine, they need to go out there and spray for them,” Hagan said. “(But) They’re still there in August and September and October. In partially defoliated cotton (last year), there were aphids up in the top of it. They’re just going to be there. They’re background pests. Really, we don’t need to be overspraying in trying to control the aphid vector.”
Symptoms from early season infections of the virus include stunting or dwarfing of the plants, compacted terminal growth, upward-cupped leaves, red discoloration of petioles and stems, distorted growth with yellowing around leaf edges and crinkled leaves. Research specialists in Alabama have been monitoring the aphid transmitted disease since its appearance in cotton fields during the fall of 2017.
“As far as we know, once it shows up, there’s really nothing we can do to stop it. You really can’t go out there and spray for it that we know of. We don’t think that works at all,” Hagan said. “You make all your decisions before the seed goes in the ground. Once it’s in the ground, you got what you’ve got, and you just hope it doesn’t show up early. If it shows up around cut out, it’s not going to hurt anything. But if it gets in there, end of July or early August and starts stunting plants, then you might run into situations where you’re having some yield loss.”
The virus was seen throughout Alabama last year, but its impact was felt more in the southern and central portions of the state.
“In every cotton producing county it was found. Now, was it in every field? In north Alabama, maybe not or maybe it shows up so late it has no impact on yield,” Hagan said. “The risk seemed to be higher in southwest Alabama, and it drops off a little bit there further we get from that area. (But) we were still showing yield loss in Tallassee, central Alabama.”
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