Huanglongbing (Candidatus ​Liberibacter asiaticus)

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​​​​​​​​​​​​​​​​​​​​​​​​​​​ Infected host species of huanglongbing showing blotchy yellowing on the leaves that is not symmetrical or mirrored on both sides of the leaf.

PLANT PEST

Huanglongbing

Exotic to Australia

Life form: Bacterium
Origin: Asia and Africa
Distribution: Varies, depending on the species
Features: Yellowing, blotchy mottling and unseasonal leaf
flushing, leaf drop, dieback of branches
Pathways: Imported plant propagative material, infected insects
At risk: Commercial citrus varieties & relatives


Huanglongbing (yellow dragon disease), previously known as citrus greening disease, is one of the worst diseases of citrus trees worldwide. It is caused by the bacterial disease Candidatus Liberibacter asiaticus that spreads through the tree canopy, causing decline and then death of the tree.

There is no cure – the only way to stop the disease is to destroy all infected trees and replace them.

The disease huanglongbing originated from China, with its vectors from Asia (Asiatic citrus psyllid) and Africa (African citrus psyllid). Depending on the species, the disease and its vectors can now be found throughout:

  • North, Central and South America
  • South East Asia, including Indonesia and East Timor
  • Papua New Guinea.

The islands of Torres Strait provide a potential pathway for the movement of serious pests into Australia, such as huanglongbing and the Asian citrus psyllid, present in countries to our north.

See if you can identify the pest

Everyone needs to keep an eye out for symptoms of huanglongbing.

Huanglongbing is spread by the movement of infected plants and plant propagative material and by sap sucking insects. These insects – the Asiatic citrus psyllid (Diaphorina citri) and African citrus psyllid (Trioza erytreae) – are not present in Australia and are of major concern due to their ability to spread huanglongbing.

  • Adults of the Asiatic citrus psyllid are 3-4 millimeters long with brown markings on the wings. When feeding on the veins of the young leaves, they adopt a ‘head-down, tail-up’ position.
  • Juvenile psyllids are yellow and commonly found feeding on young, soft shoots.

The African citrus psyllid is similar but larger with a light brown-grey body and black head, and large transparent forewings.

Psyllids
Tiny Asiatic citrus psyllid on host plant species. The winged insect is a vector for transmitting the disease.

Huanglongbing causes yellowing of citrus plant leaves and in some instances deformed, sour and bitter fruit.

  • Symptoms on leaves are subtle and hard to pick but one key sign is a blotchy yellowing that is not symmetrical or mirrored on both sides of the leaf.
  • Later, new young leaves are small, upright and yellow, with green bands around the veins.

In well-managed orchards, a yellowing that spreads slowly over the tree and through an orchard is an easily seen sign. The spreading yellowing effect can be especially hard to see in neglected backyard citrus trees growing in poor soils.

Infected trees have a blotchy yellowing that is not symmetrical or mirrored on both sides of the leaf Source: DAWR
Symptoms of huanglongbing are evident on the leaves of the citrus tree. The leaves in this picture show subtle yellowing.
Fruit from infected trees can be misshapen or lopsided, and when cut lengthwise, the arrangement of internal tissues may be irregular Source: DAWR
When cut lengthwise the lime is showing irregular arrangement of internal tissue. Misshapen fruit is a sign of infected trees from huanglongbing.

Importers

If you work around imported plant material, including citrus fruit, cut flowers and nursery stock you need to look for disease symptoms and insect vectors.

Growers and home gardeners

Ensure that that propagation material is purchased from suppliers that source their budwood from Auscitrus.

If huanglongbing established in Australia it could infect all commercially grown citrus varieties, some Australian native and citrus relatives, including Murraya (native and ornamental varieties of orange jessamine and curry tree). Orange, mandarins and tangelos are known to be the most susceptible, whilst grapefruit and lemons have moderate susceptibility. Limes are the least susceptible to the disease.

Any psyllid on citrus should be considered suspect. They can be confused with aphids. However, when compared with aphids, they are more active and jump readily when disturbed. They are most commonly found on new, tender growth of any commercially grown citrus, as well as some Australian native citrus.

Keep a watch for sick citrus trees in your area. A general decline accompanied by fruit becoming deformed, sour and bitter, or a spreading yellowing in orchards, are signs of the disease.

Dieback evident on Pomelo tree Source: Hilda Gomez, USDA
Pomelo tree starting to show dieback from being infected with huanglongbing. Leaves and fruits on the tree have started to die display bare branches.

Check what can legally come into Australia

All Australians and international tourists have a role to keep out exotic pests and diseases.

Huanglongbing and its insect vectors are present in a number of countries. Australia remains free of both the exotic vectors and the disease-causing bacterium. We need your help to keep it this way.

Check what you can and cannot bring into Australia, whether you are a:

Keep the dragon at bay

The only way to rid citrus orchards of the disease is to replace infected trees and control insect vectors, so the cost of an outbreak to Australia’s citrus industry would be enormous. If huanglongbing established in Australia, most backyard citrus trees would not survive for more than a few years.

Import restrictions and biosecurity measures

Some items, by law, are subject to certain import conditions to be allowed into Australia. Please check the Biosecurity Import Condition System (BICON).

Be aware of any relevant biosecurity measures that may be in place for incoming goods and conveyances. Industry advice notices are reviewed regularly and could change.

Secure any suspect specimens

Containment is critical. This could be as easy as bagging a suspect plant specimen that has symptoms or stopping an insect escaping by closing the doors on a shipping container.

Controlling vectors and removing infected plant material are the only control options, in conjunction with movement restrictions on infected plant material, to limit its spread. Identification and use of resistant plant stock may provide a long term management option, as the disease would be nearly impossible to eradicate.

Report detections of exotic pests

Any detections of huanglongbing symptoms or the psyllid must be reported to the authorities.

Import community

If you receive or work around goods imported from overseas, including mail, you need to be vigilant to huanglongbing symptoms and other exotic pests. If you see an unusual pest, secure the goods to limit the movement of the pest and immediately report it to the Department of Agriculture and Water Resources SEE. SECURE. REPORT. Hotline 1800 798 636 or by using the online form.

Growers and home gardeners

If you see the huanglongbing symptoms or anything unusual, report it to the Exotic Plant Pest Hotline 1800 084 881. This will put you in touch with the department of primary industries or agriculture in your state or territory.

When reporting your concern, you will be given advice on handling the specimen and what to do next until an officer can investigate.

Additional information

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