Late-winter herbicide treatments on henbit and white clover may help growers control cotton leafroll dwarf virus (CLRDV), a new culprit that has swept into Southern cotton. Early cotton planting may also minimize the virus’s damage, but don’t expect to manage it by spraying its carrier – cotton aphids.
Unfortunately, common weeds henbit and white clover are key hosts of the virus. Carpetweed and evening primrose are also prime hosts for it. Taking out the weeds removes a source of infection that would be transmitted to cotton by aphids.
“Treat these weed hosts of CLRDV with an inexpensive, broad-spectrum herbicide,” says Austin Hagan, Auburn University plant pathologist. “I think that weed cover can be left to grow until late winter or early spring. That would involve killing the weeds at least four weeks prior to planting cotton.
“That way, the aphid vector won’t move the virus from the weed hosts to cotton.”
A Vicious Circle
In a circulating and persistent manner, aphids spread the CLRDV between both cotton plants and weeds, Hagan says. Think of it as a vicious circle with the two groups of plants in the loop. PCR lab tests from field samplings show CLRDV is present in many commercial cotton fields and test plots across Alabama, Hagan says.
Cotton in the Florida Panhandle, Georgia and Mississippi also has tested positive for the virus, via research by pathology and agronomy personnel from the University of Georgia and Mississippi State University. Hagan consults with those colleagues to learn more about the virus.
“Cotton plants with CLRDV have maroon leaf mid-veins (see photo),” Hagan explains. “Severely damaged plants develop a ‘Christmas tree’ or triangular shape with elongated whips or shoots with many fruiting nodes and squares.
“Nodes and squares shed later, resulting in fewer blooms and bolls set on the main terminal whips and lateral branches.”
Cover Crops Are In The Clear
Upon reading this, growers with cover crops in their conservation programs may be concerned about those plants also harboring the virus.
Don’t panic. Hagan’s tests of widely used covers indicated they are not hosts to the virus.
“We tested crimson clover many times last year using PCR, and all tests came up negative for CLRDV,” Hagan reports. “Grasses are not hosts, so small grains and ryegrass should be fine. Wild radish also tested negative. We still have to check the winter turnips and similar brassica cover crops, but it’s unlikely that they will be hosts.
“So far, we have not sampled either peanuts or soybeans, and they are not listed as hosts of this virus. But other legumes, such as cowpea and white clover, are.”
A Fast-Moving Infection
CLRDV infestation of cotton plants is quick. Aphids spread the disease on a rampage.
“Aphids can transmit the virus in as little as 40 seconds,” Hagan explains. “And they can continue transmitting the virus for up to 12 days.
“However, aggressive chemical control of the aphid vector likely is a waste of time. For 2019, weekly insecticide applications failed to eliminate aphids in cotton as well as reduced CLRDV incidence.
“Insecticides are still recommended when aphid populations exceed treatment thresholds. And notably, whiteflies, a serious pest in Georgia cotton and occasionally in adjacent states, do not transmit CLRDV.”
Plant Early To Further Minimize Effects
Hagan says field observations and results of a single trial suggest that planting early will reduce the CLRDV incidence and minimize disease impact on yield.
“Attempts will be made this growing season to determine whether planting date significantly impacts CLRDV in cotton,” he says. “It’s also possible that tillage practices, recommended plant populations and other agronomic activities may influence CLRDV incidence in cotton.
“We’re still feeling our way to be able to give growers reasonable advice on how they might reduce the risk of having the disease impact cotton fields. Seed breeders are working to develop lines with resistance to the virus.
“But so far, managing weed control and early planting are the best bets.”