Taking Aim at Citrus Greening

Published: May, 19 2009

By Tom Burfield

The dilemma Florida growers face from citrus greening disease is kind of like the plot of a Western shown on TV at 2 a.m., where the cavalry comes to the aid of a wagon train of settlers trying to defend themselves from a predatory enemy.

In the real-life version, the enemy is citrus greening—also known as Huanglongbing or HLB—and the Asian citrus psyllid that vectors it. The role of the cavalry is played by researchers armed with about $20 million in funding from state, federal and industry sources. And the threatened settlers, of course, are citrus growers.

Unlike in the movies, though, there is no guarantee that help will arrive in time.

Growers face the daunting task of staving off the psyllids on their own for an undetermined period of time. The most optimistic researchers say even preliminary solutions likely are years off.

Photo courtesy of the Florida Department of Agriculture and Consumer Services Division of Plant Industry

Fruit from trees infected with citrus greening typically remain green. Leaves from infected trees are mottled yellow.

Photo by Ron Brlansky, University of Florida

An immature orange infected with greening has aborted seeds and appears lopsided.

Citrus greening is caused by Candidatus Liberibacter asiaticus, a bacteria that can live in citrus trees and other hosts for up to three years before symptoms appear. Common symptoms include mottling on leaves, which also take on a yellowish- green color. It was discovered in Florida in 2005 and has since been detected in each of the state’s 32 citrus-growing counties.  

The disease eventually kills a citrus tree, but before that, it retards fruit growth and gives the juice a salty, bitter taste. It’s called greening because the bottom of the fruit remains green.

Three-step program

Growers should do three things to control the disease, says Tim Spann, assistant professor of horticulture at the University of Florida’s Citrus Research and Extension Center in Lake Alfred.

First, be vigilant. Inspect groves frequently for signs of psylla and damaged trees.

Second, apply pesticides regularly to control the psylla.

Third, remove infected trees as soon as they’re detected.

Groves in Southern Florida seem to be hardest hit.

“The infection rate generally declines as you move north,” Spann says.

Peace River Packing Co. in Fort Meade has removed about 1,700 citrus trees since greening was discovered in one of its groves in August 2007, says general manager Larry Black, who oversees 1,600 acres of citrus trees for the company. Black also serves as a member of the board of directors of Lakeland-based Florida Citrus Mutual.

“When we first found it, it was the wakeup call we needed to get serious with greening scouting and spraying,” he says.

Black stepped up efforts to control the psyllid by doubling his spraying frequency from four times per year to eight and adding a psyllid spray.

The best time to spray for psylla is during dormancy, in February or March, says Lukasz Stelinski, assistant professor of entomology at the Citrus Research and Extension Center.

“That will kill off a large portions of the adults,” he says.

He recommends chemicals in the pyrethroid-organophosphate category that kill both adults and nymphs.

Pyrethroids, such as Danitol and Mustang, have been recommended, as well as organophosphates such as dimethoate and chloryriphos, and carbamates such as carbaryl and Vydate.

Growing costs at Peace River Packing have increased 25 percent as a result of efforts to fight the psyllid, Black estimates.

Good news

Jim Snively, vice president of grove operations for Southern Gardens Citrus in Clewiston, says he’s seen some encouraging signs in the form of fewer infected trees during the past couple of inspections. But it will be a year before he knows for sure if real progress has been made, or if the decrease is temporary.

Meantime, he will continue to follow the steps Spann recommends and replant only trees from certified, screened nurseries.

“If you start with a healthy tree and if you can keep the psyllid off the tree, you should produce fruit and income,” he says.

Snively, who serves on the Florida Citrus Industry Research Coordinating Council and the Florida Citrus Research Advisory Council, estimates that about 15 percent of the company’s groves have been infected since the fall of 2005. That’s a fairly high rate because of the groves’ unfortunate location—about 50 miles from where the psyllid was first found.

Fighting the disease has cost at least $450 per acre per year for inspections, tree removal, new trees and psyllid control, he says.

He estimates the company’ three-year loss due to citrus greening at $18 million.

Replanting choices

Once trees are removed, there are two schools of thought on replanting, Spann says.

One is to replant where the original tree was pulled out. The other is to wait until enough trees have been removed so it no longer is economical to manage the block, then replant several acres at once.

Black advocates the first method.

The thought of waiting for a grove to reach a threshold and then replacing the whole grove “is not optimizing our investment,” he says. “You have to keep your production up to keep your costs down.”

Snively, however, leans toward replacing all the trees at once after the infection rate reaches 25 percent to 30 percent. Taking care of young trees is much easier when they are all in one place, not spread throughout the grove, he says.

Meantime, the cost of seedlings has doubled as nurseries implement procedures to produce disease-free trees.

Abandonment issues

Another problem the industry faces is that of citrus groves abandoned by growers who have given up fighting the disease or by developers who snapped up property intending to build commercial or residential structures before the economic downturn hit.

Untended groves can serve as a breeding ground for psylla and the disease, and owners are encouraged to spray or kill the trees.

As an incentive, Florida agriculture commissioner Charles Bronson has interpreted Citrus Health Response Program rules as allowing property owners to continue to claim a lucrative agricultural tax exemption if they spray the groves, says Andrew Meadows, director of communications for Lakeland-based Florida Citrus Mutual. They might lose that exemption if they don’t.

The policy can vary from county to county, however.

As for the future, growers seem optimistic for the long-term, but no one sees a quick solution.

“Research has got to come up with an answer,” Snively says.

Possible solutions are developing disease-resistant trees or making existing trees resistant through genetic modification. Some researchers are working on ways to control the psylla, as well, he says.

“We’re very excited about the future of Florida citrus,” Black says.

The likely solution, he says, will be a combination of improved, adaptable plant varieties and production practices. Short term, he says, new chemistries for longer control “would be very helpful.”

Spann also says the eventual solution may be a disease-resistant tree, but he does not expect any major developments in that area for at least 10 years.