2019 was a confusing year for determining symptomology in cotton associated with the cotton leafroll dwarf virus (CLRDV). Some farmers noticed that what seemed like thrips damage turned out to be CLRDV; however, the plants recovered and there was no yield loss associated.
Angus Catchot, Extension professor at Mississippi State University specializing in cotton and soybean, discussed the confusing nature of CLRDV symptoms at the National Conservation Systems Cotton and Rice Conference in Memphis, Tenn.
He discussed options to control thrips.
"There is about a 30% reduction in thrips just by having a ground cover, so cover crops or no-till are a good option for further reducing impact from thrips, especially if seed treatments begin to decline in control," Catchot said. "However, we need to quit spraying thrips when the crop gets to about four to five weeks. I'd say 99.9% of the time there isn't a benefit from treating thrips after that."
According to Catchot, thrips damage and CLRDV are often confused because of node stacking and ragged leaves are common with both thrips and early season infections of CLRDV.
"I started getting calls for three to four weeks about cotton with six to seven stacked nodes and ragged leaves that appeared to be thrips in 2019," he said. "I would ask, 'What have you sprayed?' Farmers would say they did what I had recommended, either acephate sprayed in-furrow on top of imidacloprid or overtreated the imidacloprid seed with acephate and applied two Orthene applications on top. I would tell them it is unlikely to be thrips then."
Thrips are always in a cotton field, but Catchot was skeptical of the level of apparent thrips damage and symptomology for that time in the season.
"This wasn't always the case, but a lot of plants from Tennessee at six to seven nodes that were tested had the cotton leafroll dwarf virus, which has a lot of different symptom expressions," he said. "One of those symptoms is identical to thrips injury. I think we might have over-sprayed a lot of fields for thrips unnecessarily."
The good news is the virus didn't show yield reductions, and, in some cases, symptomology disappeared later in the season.
"We also had some fields that were severely infected with CLRDV later in the season, confirmed by the pathologists, and they grew out of the symptoms and yielded fine," Catchot said. "While we're still concerned about the virus because a similar strain has shown problems in other countries over the years, I'm not as worried about it as I was originally."
Aphids and CLRDV
The virus is transmitted by aphids, and it is said aphids can transmit the disease in 40 seconds after feeding on a plant.
"We had the worst aphids this past year I've seen in my whole career," Catchot said. "It lasted for about two and a half months.
"Spraying for aphids in hopes of managing this virus is not economical and does not eliminate the threat of the virus because you cannot eliminate all aphids even with the best products. You can't spray your way out of this, so in 2020, I would recommend treating aphids as normal as not to reduce yield or delay the crop but not from the standpoint of trying to manage the virus. If we have winged aphids moving into a field, it's not going to do much good. Instead, it will waste a tremendous amount of money on aphid control.
"Phillip Roberts in Georgia did a test this past year where he treated for aphids seven weeks in a row. Roberts sprayed high rate Transform to beat the aphids down, but he and his team could always find some level of aphids.
"There is still much to learn about aphids and this virus moving forward, but now, it does not appear to be as yield-limiting as we once thought it may be. We will be monitoring the situation closely moving forward, and if anything changes, we will get information out timely."