Citrus greening, also called huanglongbing or Yellow Shoot/Dragon Disease, is a plant disease caused by a bacterium. The bacterium, vectored by an insect called a psyllid, is named Candidatus Liberibacter asiaticus. The Candidatus part of the bacterium's name indicates that it cannot be cultured. Two other related forms of the disease are known, of African, and South American origin; both are also vectored by the Citrus Psyllid. The bacterium is carried by the psyllid from host plant to host plant, where it resides exclusively in the phloem tissues. As the bacterium multiplies, it chokes off the supply of nutrients moving throughout the plant, weakening the plant and eventually killing it.
Citrus greening is transferred from plant to plant primarily by piercing-sucking insects called psyllids. Dodder, a parasitic plant, has also been shown to transmit the bacterium, but this rarely occurs in nurseries, groves, or homeowner yards. Pruning and other mechanical transmission methods do not appear to be important in transmission, but human movement of host material contributes to spread of the vector and the disease.
Hosts for the citrus greening pathogen include nearly all citrus species and hybrids, as well as citrus relatives in the Family Rutaceae. Economically-important hosts include Citrus spp., Fortunella spp., Murraya spp., and Severinia buxifolia (you can find these and many more listed in the January 2008 Federal Quarantine Order). Additional hosts for the psyllid vector are listed in the Federal Order; not all of these serve as hosts for the disease, but movement of the vector is important in the spread of the disease.
Citrus greening can be difficult to detect since the host may remain symptomless for up to three years before showing outward signs of infection. Some symptoms include yellow shoots on single random branches (not to be confused with nutrient disorders, which would show uniform distribution), deformed fruit that does not ripen or produce seeds, and a mottled appearance that crosses leaf veins (again, not to be confused with nutrient disorders, which are often vein-limited).
As noted above, detection of the disease is extremely difficult by symptoms alone. NPDN diagnostic laboratories utilize PCR assays to detect the DNA of the bacterium in plant hosts or insect vectors. However, sampling is difficult when there are no symptoms. Generally, samples are collected from new flushes of tip growth, and the veins and petioles are cut from the leaves and processed to maximize the chance of finding the bacterium. As of June 2008, the disease has been confirmed in 30 counties in Florida  and a single parish in Louisiana .
Chemical management tools are not utilized for controlling the disease, but insecticides are used in nurseries and groves to manage the vector population. Current efforts are underway to quarantine movement within and out of Florida, and to eradicate the disease in Louisiana.
References and Resources
- USDA-APHIS Citrus Greening Plant Pest Information Page
- USDA-APHIS January Federal Quarantine Order
- University of Florida Citrus Greening EDIS Publication