A virus that is previously known to be vectored by aphids into cotton has been recently identified as the primary suspect virus from limited samples of cotton in Alabama. Similar symptomology has been reported in the coastal counties of Alabama, Georgia and the Florida Panhandle.
“The cotton blue disease (CBD) symptomology was observed at the end of 2016 by one of my former graduate students, Drew Schrimsher, in his grower cotton variety trials,” says Auburn University plant pathologist Dr. Kathy Lawrence.
“He observed it again at the end of 2017 and it was much worse; symptomology was observed in areas beyond the area where it was first observed. CBD is a big problem in Brazil, and we hypothesize it may have come to the U.S. by a hurricane, like soybean rust did with Hurricane Katrina.”
Symptoms include mosaic cupping and thickening of the dark blue/green leaves, yellowed leaf veins, and dwarfing of the plant. Other symptoms include no boll set on new growth, swollen and brittle stems, and decreased yields; fields with symptoms in early bloom had fewer bolls per plant.
“Once the virus starts showing its symptoms, the plant stops producing any more cotton,” Lawrence adds. “There’s not a top crop, which many growers depend on for income.
“We seldom spray for aphids in cotton, and we don’t recommend spraying for them to prevent this suspect disease, which would take out beneficials and flare other insect pest problems. We do encourage growers and consultants to watch for the CBD virus symptomology, and if they find it, to call their state plant pathologist to help us keep up with it.
“We also recommend keeping cotton fields and surrounding areas weed-free, especially of legume and malvaceae weeds including pigweed and sida as the literature shows they harbor the virus. If the virus is in the weeds, aphids can pick it up and transmit it to cotton. So management might come down to taking out weed host plants.”
Schrimsher, who is now an agronomist with AGRI AFC, observed mild leaf crumpling symptoms in his cotton variety trials that he was conducting in growers’ fields in south Alabama and the Florida Panhandle in late summer to early fall 2016. He observed extensive severe leaf crumpling in 2017.
Lawrence says, “The virus was much worse by that time; CBD had progressed beyond the area where it was found in 2016. However, infected areas were patchy like aphid infestations are patchy along the outer edges of a field, and close to areas with other plants and trees. It didn’t take over the whole field.
“Schrimsher told me about the symptoms in August 2017. We took samples, and found it’s a virus. We normally don’t have viruses in Alabama, so to get an identification, leaves, petioles and stems were collected from the newest terminal of plants expressing leaf crumpling symptoms and sent to University of Arizona plant pathologist Dr. Judy Brown, who researches the viruses in her state. She tested the samples and ruled out leaf crumple or leaf curl virus; instead, she found a virus associated with aphids that matches the one in Brazil.”
It appears from Schrimsher’s variety trials that the U.S. cotton varieties that were in the trials and are grown in the Southeast region all demonstrated the symptomology. “He saw the virus’ symptoms across all company varieties in his tests,” Lawrence says. “CBD is a big problem in Brazil, but they do have cotton varieties that are tolerant to the disease. The U.S. seed companies have gene markers in their breeding program. It’ll take time to develop resistant varieties for the U.S., but it’s not like starting from scratch.
“We will observe CBD closely this year. We’ve seen it for two years and hope it’s not here to stay. We hope that it will have a limited economic impact like soybean rust did.”
Official confirmation of the suspect virus will require additional sampling and verification by APHIS.