The color of greening: The leaf on the left from a minneola tangelo (honeybell) tree shows signs
of the citrus greening disease, which kills trees. Notice the yellowing and the darker spine. The leaf on
the right is not showing signs yet. Greening was detected in Homestead in 2005, and now the disease has
been found virtually everywhere citrus grows in the state.

http://www.palmbeachpost.com/news/content/business/epaper/2009/01/12/a1f_greening_0112.html

Saving citrus: Greening threatens groves, but a cure may lurk in disease-resistant trees from China, and spinach

By SUSAN SALISBURY

Palm Beach Post Staff Writer
Monday, January 12, 2009

University of Florida scientist Fred Gmitter was walking through abandoned, diseased groves in Southern China when he stumbled onto something strange.

Still thriving amid trees killed by the lethal "greening" disease was a lone, healthy citrus tree. Searches of other groves in China have found fewer than 10 such trees, still growing while surrounding relatives are long gone, victims of what is now considered the world's most serious citrus malady.

Call them survivors. Call them lucky trees.

Gmitter hopes to call them a first step toward a cure.

Along with colleagues and collaborators, the geneticist and plant breeder at the Citrus Research and Education Center in Lake Alfred is deliberately inoculating the trees with greening bacterium.

"We are trying to find out if they are truly resistant or just lucky," Gmitter said.

Also known as Huanglongbing, greening has been in Asia and Africa for decades. It was detected in Brazil in 2004, then in Homestead in 2005, and in Cuba in 2006.

Now the disease has been found in 32 Florida counties, virtually everywhere citrus grows in the state. Only one other state, Louisiana, has groves infected with greening, but other states are worried, especially California.

Worried may be an understatement. Florida growers may have struggled with canker, which blemishes fruit, but they talk about greening in apocalyptic terms: Unlike canker, greening kills trees. Many believe it is capable of wiping out this state's citrus groves.

"The citrus industry in Florida is facing collapse," Peter McClure, chairman of the Florida Citrus Producers Research Advisory Council, said at an international conference on greening held in December in Orlando. Organized by Florida Citrus Mutual, the conference attracted more than 400 people from 26 countries.

Glimmers of hope are emerging as research advances on everything from those seemingly naturally resistant trees in China to inserting disease-resistant spinach genes into citrus chromosomes. Others are developing high-speed orchards that produce trees that bear fruit earlier, before greening can kill them off.

In December, the Florida Citrus Commission approved 83 greening and canker-related projects recommended by the council and the National Academy of Sciences that could be funded for as much as $20 million. Researchers also are obtaining grants and private funding from sources such as Southern Gardens Citrus west of Clewiston, a subsidiary of U.S. Sugar Corp.

This spring, Southern Gardens will be the site of the first field test of citrus trees containing spinach genes. Weslaco, Texas-based Texas A&M plant pathologist and microbiologist Erik Mirkov said he started working with a gene from spinach for canker control in 2000. Proteins called defensins fend off bacteria, and the defensins spinach makes have "broad spectrum anti-microbial properties," Mirkov said.

In the lab, the spinach gene was inserted into citrus chromosomes that were then grown into tiny trees — tiny trees with apparent resistance to greening and canker, at least in the lab.

"When the (greening) bacterium sees the spinach defensin, it has never seen that before in citrus and doesn't know how to get around it," Mirkov said.

Gmitter is working to sequence the citrus genome, then use it as a tool to figure out how to thwart the disease. "People are trying everything," he said.

As to how China's citrus industry is still going strong if greening has been there for decades, UF agricultural economist Tom Spreen said the disease is confined mostly to China's eastern production zone.

"Citrus production in China is scattered across many provinces, separated by mountains which probably have helped keep the disease isolated," Spreen said.

Gmitter said the Chinese citrus growers have developed a hands-on approach and know exactly when to spray to kill psyllids, tiny insects that transport the greening pathogen from infected trees to healthy trees, based on the condition and behavior of the trees.

They also conduct area-wide spraying, he said. Psyllid control is the focus of many of the research projects approved by the citrus commission.

Until greening-resistant trees are proven in the field and regulatory agencies give the OK, the industry is counting on advanced production systems.

The idea is to produce fruit as quickly as possible before greening kills the tree. Mani Skaria, a Texas A&M plant pathologist, has developed a system he calls micro-budding, which spurs trees to generate fruit earlier.

"It will allow people to plant trees for a short-term cycle," he said. "That is probably what Florida will end up doing until they come up with guaranteed resistant trees."

But the battle is wearing down growers.

"We can't continue to do what we are doing," said Southern Gardens chief plant scientist Mike Irey: scouting for and removing infected trees, spraying to control psyllids and planting new disease-free trees. "The price of fruit and juice is down. Production costs are up by 40 to 50 percent."

Some have thrown in the towel and sold their groves.

Like others still in the business, "We are hoping to hang on until the solution comes." Irey said.