U.S. Department of Agriculture: Agricultural Research Service, Lincoln, Nebraska


Date of this Version


Document Type



Agricultural Research January 2013.


Finding the solution to some crop diseases is like solving a jigsaw puzzle. The solution has many pieces, and fitting them together can be a challenge. Citrus greening, also known as “Huanglongbing” (HLB), is just such a disease. HLB is costing Florida citrus growers millions of dollars each year in lost revenue from unproductive trees. It continues to spread and is the most serious threat to the U.S. citrus industry in history. The disease was recently confirmed in commercial groves in Texas and in a homeowner’s yard in California. Both states are important citrus producers, and if HLB spreads in these states the way it has throughout Florida, the economic consequences will be devastating.

The Agricultural Research Service is working aggressively with university and private-industry partners to help mitigate the threat posed by HLB. The goal is to keep the U.S. citrus industry sustainable and profitable and to preserve the nutritious and delicious array of fruits and juices that people have come to treasure.

Solving the HLB puzzle depends on developing a deeper understanding of its complicated nature. HLB is presumed to be caused by nonculturable, Gram-negative bacteria belonging to the genus Candidatus Liberibacter. So far, the species of bacteria associated with HLB in the United States is Candidatus Liberibacter asiaticus, which is transmitted by the Asian citrus psyllid, an insect.

The puzzle pieces are questions that constitute gaps in our knowledge: Do some types of citrus have a natural resistance that can be bred into commercial varieties? How can psyllid populations be effectively suppressed? Are there molecular tools for tapping into the genomes of the psyllid or bacterium in ways that would “disarm” them? How do we replant young, healthy trees and keep them free of HLB? Is there a cure for infected trees?