Citrus greening disease, already a major problem in much of the globe's citrus crop, has found its way to south Louisiana. Also known as “yellow shoot” disease or huanglongbing, the disease is spread by the Asian citrus psyllid and can lead to quarantine and the need to destroy trees or groves.
An invasive species, the Asian citrus psyllid — not the disease it vectors — was first reported in Florida in 1998. Shortly after that, it was found in Texas' citrus production area.
There is currently no treatment for the disease vectored by the psyllid, which causes a gradual decline in trees. One of the symptoms is the tree will push out yellow shoots in new, flushing tissue. The disease also doesn't allow the fruit to ripen at an even rate.
“That's why it's called greening disease — the fruit will be half-green,” says Natalie Hummel, LSU AgCenter entomologist. “That means a decline in fruit quality. Sometimes the fruit will also be bitter.”
The greening disease wasn't found in the United States until 2005 when it was located in Florida. By then, however, the psyllid — which picks up greening disease bacteria when feeding on an infected tree for as little as 30 minutes — had spread throughout the entire citrus production area of Florida with populations higher in the southern area of the state.
As a result, the USDA quarantined Florida citrus. “That's why other citrus-producing states can't get fresh oranges out of Florida any longer. USDA doesn't want the disease to spread into other citrus-producing states.”
Then, last spring, an odd insect was reported by an alert homeowner in Algiers City, New Orleans. She found it on a backyard lime tree and, intrigued, went to the LSU AgCenter Web site, where Hummel had posted a PowerPoint presentation.
“At meetings I like to tell homeowners about invasive species they might be on the lookout for. So, she contacted me by e-mail with some pictures of the insect attached. Based on that, I contacted the USDA, our state representatives and the Louisiana Department of Agriculture and Forestry.”
Shortly after, the insect was confirmed as an Asian citrus psyllid and the lime tree was found to harbor greening disease. Like Florida, Orleans Parish citrus was put under quarantine.
The discovery also caused great alarm in Louisiana's citrus industry. The state has only around 500 acres of commercial production. However, Louisiana also has a sizeable nursery industry and there were implications for movement of trees into other states.
Following the Orleans Parish find, the Asian citrus psyllid has been found in six other parishes, all in the southeastern part of Louisiana. The pest hasn't yet been found west of the Atchafalaya basin.
Since the Asian citrus psyllid was found in Algiers City, states between Louisiana and Florida have looked more intensively for the pest. While the psyllid has been found in Alabama, South Carolina, Georgia and Mississippi, no other cases of the greening disease have been discovered. The pest has also been found in southern California and northern Mexico.
“All of that has happened since June 2008. I believe Arizona is the only citrus-producing state not to have found the psyllid.”
The federal government takes greening disease very seriously as, until recently, it was on the National Select Agent Registry. Readers may remember the list because Asian soybean rust was on it. The FBI can get involved when such registry diseases occur because the organisms or pathogens could be introduced as an act of bioterrorism.
Like Asian soybean rust, greening disease was recently taken off the list because it was determined to be widespread, not a result of terrorism. One consequence of the list: limits on what can be done to research the named disease. In such cases, only government labs, or those that are certified, are allowed to work with the organisms.
Now that greening disease is off the list, “hopefully, a lot more research will be happening. We need a solution and/or cure. From the management perspective, the only thing that can be done now is to keep the vector population as low as possible. The insects will carry the disease from one tree to another. It can also be carried from one infested area.”
One of the complications with greening disease — which can be confirmed only by a genetic test — is often infected trees won't show symptoms for six months to two years.
That may be what happened in Florida, says Hummel. “There, they had a huge jump in the detection of the disease in groves in a matter of only months. In some cases, groves went from having five infected trees to thousands. Most likely, it didn't actually spread that quickly. But because of the latency period, it wasn't detected immediately.”
California officials are extremely worried about greening disease harming their vast citrus groves. “I believe USDA has put millions of dollars into dealing with the situation very quickly. They don't want what's happening in Florida repeated in California.”
How best to control the insect?
In commercial production, it can be controlled with a number of insecticides. “Control is very different if you have the insect and the disease versus just the insect. There's less tolerance for the insect if the disease is present.
“It can be controlled using area-wide measures, which is what we typically encourage. That's where there's a coordinated effort in the treatments occurring at the same time. Otherwise, the psyllid can move from treated areas into untreated very quickly.”
In Louisiana, Plaquemines Parish funded and coordinated an area-wide treatment by helicopter of citrus trees with “formulations of imidacloprid (Pravado) and a pyrethroid (Danitol), both of which are labeled for use in Louisiana citrus.
For the backyard producer, Louisiana has emergency labels for CoreTect tablets and Merit 2F.
“The thing about the Asian citrus psyllid is it only reproduces when there's new flush on the tree. They will attack all citrus and orange jasmine, an ornamental plant that's popular in the South.”
Orange jasmine, if fertilized, will often produce a flush nearly year-around. “That's why it's suspected to be such an important source of the psyllids. And that's why we discourage having orange jasmine around any citrus groves.”
One way to combat the insect is to manage the flush. As a grower, “you need to know when your trees will flush. You want to put a treatment out before the flush comes out, to kill the adults before they have a chance to lay eggs in that flushing tissue. “
If you have a tree with the disease, “it's encouraged that it be removed. After the infected tree in Algiers, two more with greening disease were found in Washington Parish, although no psyllids have been found. At this point, we've yet to find any greening disease in commercial citrus operations in the state.”
It's unknown how the Asian citrus psyllid came to Louisiana — from Florida or Texas or somewhere else. “The silver lining to this is we only found out because a homeowner reported it. That's a positive because homeowners are out there looking proactively. That's why we try to keep the public informed about what to watch for.”