2008 Florida Citrus Pest Management Guide: Huanglongbing (Citrus Greening)
Click here to view a PDF version of this document.
Home Search What's New Products Survey Help
2008 Florida Citrus Pest Management Guide: Huanglongbing (Citrus Greening)

   

2008 Florida Citrus Pest Management Guide: Huanglongbing (Citrus Greening)1

R.H. Brlansky, K.R. Chung and M.E. Rogers2

Huanglongbing (HLB), commonly called citrus greening disease, is caused by the bacterium, Candidatus Liberibacter spp. The name huanglongbing means "yellow shoot" which describes the symptom of a bright yellow shoot that commonly occurs on a sector of infected trees. HLB is a serious disease of citrus because it affects all citrus cultivars and causes decline of trees. HLB has seriously affected citrus production in a number of countries in Asia, Africa, the Indian subcontinent and the Arabian Peninsula, and was discovered in July 2004 in Brazil. Wherever the disease has appeared, citrus production has been compromised with the loss of millions of trees. HLB has not been reported in Australia or in the Mediterranean Basin. In August 2005, the disease was found in the south Florida areas of Homestead and Florida City. Since that time, multiple residential and commercial citrus sites have been found infected with huanglongbing. The HLB type found is the Asian form which occurs in warm low altitude areas and is transmitted by the Asian citrus psyllid (Diaphorina citri Kuwayama). The Asian citrus psyllid was discovered in Florida in 1998 and now occurs throughout the state wherever citrus is grown.

The initial or early symptoms of HLB on leaves are vein yellowing and asymmetrical chlorosis referred to as a blotchy mottle. The blotchy mottle symptom is the most diagnostic symptom of the disease. Leaves might be small and upright with a variety of chlorotic patterns that often resemble mineral deficiencies such as those of zinc, iron, manganese, calcium, sulfur and/or boron. Often some of the leaves may be totally devoid of green or with only islands of green spots. The blotchy mottle symptom also may be confused with other diseases such as stubborn, severe forms of citrus tristeza virus (CTV), Phytophthora root rot, water logging or citrus blight. Root systems of infected trees are often poorly developed and new root growth may be suppressed. The early symptoms of yellowing may appear on a single shoot or branch. Hence the Chinese names of yellow shoot and yellow dragon are descriptive of this symptom. The yellowing usually spreads throughout the tree and affected trees may show twig dieback and the productivity may decline in a few years. Fruit are often few in number, small, may be lopsided with a curved central core and fail to color properly remaining green at the stylar end. A yellow stain may be present just beneath the peduncle on a cut fruit. The affected fruit often contain aborted seeds and have a salty bitter taste. Symptoms may be enhanced by the presence of other pathogens such as CTV.

The causal bacterium, Ca. Liberibacter asiaticus has not been cultured and diagnosis is by PCR. Detection of the bacterium is usually only possible from symptomatic tissues. Four different types of the HLB bacterium exist. One causes the Asian type, one causes the African disease and another causes a disease of Cape Chestnut in South Africa. A fourth type was found in Brazil where the Asian type also was found and has been named Ca. Liberibacter americanus. The host range includes all citrus species regardless of rootstock. Normally symptoms are severe on sweet orange, mandarins and mandarin hybrids; moderate on grapefruit, lemon and sour orange; lime, pummelo and trifoliate orange are listed as more tolerant. However in south Florida, the symptoms were severe on pummelo, lime and grapefruit.

When psyllids are abundant and conditions are favorable, HLB can spread, destroying existing groves and preventing the commercial production of oranges and other citrus cultivars. Infected mature trees may decline and become non-productive and young trees that become infected never come into full production. In China, the disease was reported to kill young trees in 1-2 years. HLB also can be transmitted with infected budwood. Therefore, use of certified disease-free planting materials is essential to minimize further spread.

Recommended Practices

  1. HLB is difficult to manage and continued production of citrus has proven difficult and expensive in areas where it is widespread. Since HLB is transmitted by the Asian citrus psyllid which is well established in Florida, there is clearly a potential for the continued spread of HLB throughout Florida citrus. The use of clean budwood and certified healthy trees is essential. Budwood sources and nursery stock should be protected from psyllid infestation by screened enclosures and the use of systemic insecticides such as imidacloprid (see ENY-734 Asian Citrus Psyllid and Citrus Leafminer). Some biological control of the psyllid is available but the amount of psyllid control provided by introduced parasitoids has not been sufficient to limit disease spread.

  2. The Asian citrus psyllid feeds on many rutaceous plant species. The psyllid has a preference for the landscape ornamental, orange jessamine (Murraya paniculata). It has recently been found to be a host of the HLB bacterium, and can serve as a host plant for the psyllid. Another rutaceous ornamental, Severinia buxifolia or orange boxwood, is also a host for the bacterium as well as the psyllid. Movement of these ornamentals is restricted under state compliance agreements and should not be moved from areas where the disease occurs.

  3. Scouting for greening infected trees should be done routinely so that infected trees can be removed. It has been suggested that scouting be conducted four times per year. The frequency of scouting may be higher in areas previously determined to have positive greening trees. Symptoms are the easiest to find from October to March. However, symptoms may be present at other times of the year. The current methods used to scout are walking, using all-terrain vehicles and on platforms mounted on vehicles. Symptomatic tree numbers and the rows in which they are found should be marked with colored flagging tape and GPS coordinates taken or the sites marked on a map to facilitate relocation and removal of these trees. In some cases a greening PCR diagnosis test may be necessary to confirm the disease (see diagnosis below).

  4. Diagnosis of citrus greening may be difficult since some nutrient deficiency symptoms and other problems are often confused with some of the symptoms associated with greening. Greening affected leaves may accumulate starch. An iodine based starch test can be used to assist in determining what leaves should be sent for PCR diagnosis. The iodine test alone is not used for greening diagnosis; however, it is useful in deciding which leaves should be sent for diagnosis. The procedure for the test can be found on the Citrus Research and Education Center greening website at the address listed below. Samples of suspected greening infected trees may be sent in for PCR diagnosis to the Southern Gardens Diagnostic Laboratory or to the Southwest Florida REC in Immokalee beginning in the late Spring of 2008. The procedures for submission of suspect samples for PCR testing is available at the following website: http://www.flcitrusmutual.com/content/docs/issues/canker/sg_samplingform.pdf

  5. Removal of infected trees is the only way to ensure that they will not serve as a source of the bacteria for psyllid acquisition and subsequent transmission. Prior to removal, the infected tree should be treated with a foliar insecticide (such as Danitol, fenpropathrin) to kill all adult psyllids feeding on that tree. Failure to control these psyllids will result in the infected psyllids dispersing to new plants once the diseased tree is removed. Pruning of symptomatic limbs has been attempted in many countries to reduce the inoculum available to the psyllids. However, because the disease is systemic, pruning has not been successful since other parts of the tree may already be infected but not yet symptomatic. Additionally, if a tree is still infected after pruning, the new flush produced will serve as a feeding site for adult psyllids to acquire the greening bacteria. The infected psyllids may then disperse to uninfected trees once the new flush hardens off.

  6. Integrated pest management strategies should focus on the following: use of disease-free nursery trees, reduction of the inoculum by frequent disease surveys, removal of symptomatic trees, and suppression of Asian citrus psyllid populations. Refer to ENY-734, Asian Citrus Psyllid and Citrus Leafminer , in the Florida Citrus Pest Management Guide for more information on management of Asian citrus psyllid.

Additional Information:

Links to websites on citrus greening disease can be accessed through the Citrus Research and Education website at the following address: http://www.crec.ifas.ufl.edu/extension/greening/index.htm

Footnotes

1. This document is PP-225, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Date printed: November 2005. Date revised: December 2007. This publication is included in SP-43, 2008 Florida Citrus Pest Management Guide. A copy of this publication may be found at http://edis.ifas.ufl.edu/CG086. Please visit the EDIS Web site at http://edis.ifas.ufl.edu. For a copy of this handbook, request information on its purchase at your county extension office.

2. R.H. Brlansky, professor, and K.R. Chung, associate professor, Plant Pathology Department; and M. E. Rogers, assistant professor, Entomology and Nematology Department; Citrus REC, Lake Alfred, Florida; Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.


Copyright Information

This document is copyrighted by the University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS) for the people of the State of Florida. UF/IFAS retains all rights under all conventions, but permits free reproduction by all agents and offices of the Cooperative Extension Service and the people of the State of Florida. Permission is granted to others to use these materials in part or in full for educational purposes, provided that full credit is given to the UF/IFAS, citing the publication, its source, and date of publication.