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Tree Sentinels Help Monitor Citrus Canker


  Asiatic citrus canker (ACC) is thought to have arrived in Florida in 1992 or 1993 and has already claimed more than a million commercial and residential citrus trees. A joint federal-state Citrus Canker Eradication Program removes and destroys infected trees while scientists search for new ways to stop this bacterial threat to Florida's $8.5-billion citrus industry. Florida produces 75 percent of U.S. citrus. Worldwide, the United States is second only to Brazil in citrus fruit production.


ARS researchers at the U.S. Horticultural Research Laboratory in Fort Pierce, Florida, led by plant pathologist Timothy R. Gottwald, have developed an early-warning system for spotting new outbreaks. It uses a sentinel tree grid to detect and help prevent the further spread of ACC into major citrus production areas along the state's eastern central coast.


"The grid is formed by dividing each square mile into a 12-by-12 grid of 144 subsections. A sentinel tree is selected for repeated survey in each subsection," explains Gottwald.


ACC moves primarily by wind-driven rain and raises brown blemishes surrounded by an oily, yellow margin on citrus leaves and fruit. It causes fruit to drop prematurely and lowers yields and quality. It can lead to a loss of markets due to quarantines on the transport, sale, and export of fruit from affected areas.


The lime industry in south Florida—the only place in the United States where limes are grown commercially—has been hardest hit. Early this year, 1,539 of the 3,000 acres of lime groves surveyed tested positive for ACC or were exposed to the disease, according to the Florida Department of Agriculture and Consumer Services. About 4,500 total citrus acres have been destroyed throughout Florida so far.


"The sentinel tree grid is currently being implemented across the state in high-risk residential areas such as Miami, where the most recent outbreaks have occurred," says Gottwald.


By visually surveying the 1-mile-square gridded areas every 30 days, researchers can identify new outbreaks and destroy infected trees quickly. "The tree grid has helped us detect a number of new infections that otherwise wouldn't have been found as early," says Gottwald.—By Jesús García,Agricultural Research Service Information Staff.


Timothy R. Gottwald is at the USDA-ARS U.S. Horticultural Research Laboratory, 2001 South Rock Rd., Fort Pierce, FL 34945; phone (561) 462-5883, fax (561) 462-5986.

"Tree Sentinels Help Monitor Citrus Canker " was published in the December 2000 issue of Agricultural Research magazine.

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