US: Goodbye to oranges?

The orange has become one of the world's favorite fruits: In 2005, Americans ate nine million metric tons of them, and Brazilians consumed another five million. According to the United Nations Food and Agriculture Organization, the top producers of oranges worldwide in 2005 were Brazil with 17.8 million metric tons, and the United States with 8.4 million. However, we may be about to kiss goodbye to orange juice, marmalade, and duck á l'orange. Oranges worldwide are under attack from a whole spectrum of diseases, and in one serious case, growers are to blame.The citrus industry in Florida is being decimated by a disease that renders the fruit inedible. Called citrus greening, it is caused by the bacterium Candidatus Liberibacter asiaticus . Greening has already destroyed the citrus industry in China and Thailand, and it is spreading in Brazil. In the Philippines, more than 1,200 hectares (2,965 acres) are affected by greening in the Kasibu region alone. Detection is a problem, because initial symptoms resemble nutrient deficiency and can take years to develop. Control is with pesticides to kill the small, grayish moth-like insects called citrus psyllids that spread the disease.Florida is also reeling from the effects of a disease known as citrus canker, which has been spreading since 1995. Canker, caused by the bacterium Xanthomonas axonopodis pathovar citri , dries out juice oranges, which make up 90% of the state's orange crop. In 2004, wind-driven rain from three hurricanes blew the bacteria all over Florida, causing an explosion of the disease in 2005. There is no known chemical that will destroy the bacteria within the plant tissue, so all infected and exposed trees must be destroyed.Almost 12 million commercial and 800,000 residential citrus trees have been cut down. The US Department of Agriculture has compensated growers millions of dollars, and experts predict canker will create losses of $35 million per year. According to the biennial Commercial Citrus Inventory carried out in 2006 by the Florida Agricultural Statistics Service, the state's citrus industry has decreased by 16.4% since 2004, in part due to disease.Turning to the other coast, California farmers are dealing with "quick decline," a disease caused by citrus tristeza virus (CTV). It is spread by aphids and grafting, and is lethal to sweet orange grown on sour orange rootstocks, a major rootstock worldwide because it provides resistance to the dangerous Phytophthora root rot. CTV can cost citrus growers with sour orange rootstocks 25% or more trees per year. Sour orange is the sole rootstock in Mexico, the world's third biggest producer, where all the trees will die as the virus advances. (Thanks to Dagmar Hanold at the University of Adelaide, and J. Allan Dodds of the University of California, Riverside, who provided technical details.)This is a frustrating situation, as some of this could have been prevented. Growers in Tulare County, Calif., voted a few years ago not to participate in the CTV suppression effort, because their rootstock is not sour orange, so CTV had only a minor effect on their trees. But their groves served as a reservoir, and the virus spread to other groves, including the Lindcove Research Center in Exeter. This spring, the center found CTV in 44 of its trees, and most troubling, in four trees in the Citrus Cloning Protection Program that supplies new buds to citrus nurseries. As a result, it could not certify the buds as disease-free. So to save themselves the expense of replanting, Tulare County growers literally condemned citrus to death in the rest of the state. The only way to fight the disease is to cut infected trees down and replant with virus-free stock, and now the source of that has disappeared.Even replanting would probably be futile if (or when) a more efficient transmitter, the brown citrus aphid, arrives in California; it has already reached Florida. It takes decades to breed new resistant trees, assuming resistance genes are even available, so that is no alternative.The selfishness of California growers in refusing to destroy their infected trees is amazing. And neighboring growers are to blame, as well - they should convince growers with infected trees to cut them down and replant, by cooperating to provide compensation when the government does not, for example from a levy on their production. That would be in their own self-interest, and benefit the entire region.Let's hope a solution appears quickly, before American orange orchards go the way of those in China, Thailand, and soon, Mexico.
Source: the-scientist.com
Publication date: 8/23/2007