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.
How to identify Huanglongbing (Candidatus Liberibacter asiaticus)
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.
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
Fruit from infected trees can be misshapen or lopsided, and when cut lengthwise, the arrangement of internal tissues may be irregular Source: DAWR