Government seeks new solutions after giving up on eradication of bacterial disease
By Anne Harding | April 27, 2006
State and government officials are gearing up to release a Citrus Health Response Plan (CHRP) for Florida, after calling a halt to efforts to eradicate the bacterial disease citrus canker. Meanwhile, Florida's citrus growers are facing a new, potentially more devastating threat -- a bacterium called citrus greening, which has wiped out the citrus industry in many parts of Asia.
After 2005's hurricane Wilma, which helped transmit citrus canker, officials estimated that it would be necessary to rip out roughly a third of Florida's citrus-growing acreage to continue with eradication efforts, which require removing all trees within 1,900 feet of an infected tree. The cost: $1.7 billion. Eradication "just wasn't practical or feasible any longer," Philip Berger, national science program leader at the USDA's Center For Plant Health Science and Technology, told The Scientist.
Now, Florida farmers are dealing with another pathogen, citrus greening. Also known as huanglongbing (Chinese for "yellow shoot disease"), the bacterium was identified for the first time in Florida in September 2005 and is now present across the southern half of the state. Spread by the citrus psyllid insect, the disease has wiped out the citrus industry in many parts of Asia, including China and Thailand, and now threatens Brazil, which has the world's largest citrus industry.
Fruit from canker-infected trees isn't pretty, but it can still be juiced and thus remains commercially valuable. But citrus greening dramatically shortens the lifespan of citrus trees, making fruit inedible as the tree sickens. "Not too many pathogens kill plants -- this one does," says Allan Dodds of the University of California at Riverside, who moderates plant diseases on ProMed Mail.
And while canker is fairly easy to detect, greening is tougher to spot. Symptoms can mimic those of nutrient deficiencies. The bacterium that causes the disease, Candidatus Liberibacter asiaticus, invades the phloem, but it is not clear where it goes after that, and it has never been cultured in the lab. PCR tests have been developed to identify the bacterium, but are only capable of detecting it in symptomatic trees, Ron Brlansky of the University of Florida's Institute for Food and Agricultural Sciences' Citrus Research & Education Center in Lake Alfred told The Scientist.
Measures spelled out in the draft CHRP, written by USDA/APHIS and Florida Department of Agriculture and Consumer Services' Division of Plant Industry, focus largely on ensuring that new citrus plant stock is free of both diseases, and require that citrus stock or budwood sources be grown in insect-proof structures featuring double entry doors with positive pressure air displacement. Nursery industry officials have estimated that building such a greenhouse for 175,000 young trees would cost $2.5 million.
In the meantime, scientists like Brlansky are working hard on getting the word out to growers on how to manage citrus greening, which he calls "our number one priority."
With "judicious spraying," the disease can be controlled, Chester Roistacher, an emeritus professor at the University of California-Riverside who has worked internationally on citrus greening and other citrus diseases, told The Scientist. "Where you do not have a good control program, your trees will live anywhere from five to eight years and die." But because the disease is new to Florida, he noted, it's not yet clear how frequently groves will have to be sprayed.
Yet another danger is waiting in the wings -- citrus tristeza virus (CTV). The virus has been present in Florida for years without causing serious damage. But the brown citrus aphid (Toxoptera citricida), which can spread the virus more aggressively than its standard Florida vector, Aphis gossypii, recently arrived in Florida. "It's a much more effective vector," said Dodds. He predicts that the strains of CTV endemic to Florida will gradually become more virulent as Toxoptera spreads. Such CTV strains can cause stem pitting, which reduces the commercial value of citrus fruit.
Links within this article
Citrus Health Response Plan
"USDA determines citrus canker eradication not feasible," Florida Department of Agriculture and Consumer Services, January 11, 2006.
J. Woodall, "Our food is dying," The Scientist, March 2006.
Philip H. Berger
Center for Plant Health Science and Technology
Citrus Research & Education Center
Brown citrus aphid
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