The dogs can detect the disease weeks to years before it appears on tree leaves and roots, the researchers report.A study on their findings was published recently in the Proceedings of the National Academy of Sciences.The report says using dogs is also faster,less costly and more exact than having people collect hundreds of leaves for lab analysis.

Timothy Gottwald is a researcher with the U.S.Department of Agriculture and a co-writer of the study.He told The Associated Press,"This technology is thousands of years old-the dog's nose.We've just trained dogs to hunt new prey."

Citrus-greening is caused by a bacteria(细菌）that is spread by a tiny insect that feeds on the leaves and stems of citrus trees.Once a tree is infected(感染），there is no cure.The disease has also hurt citrus crops in Central and South America and Asia.In one experiment involving grapefruit trees in Texas,trained dogs were correct 95 percent of the time in telling the difference between newly infected trees and healthy ones.“The earlier you detect a disease,the better chance you have at stopping an epidemic by removing infected trees,"Gottwald said.