Citrus greening is caused by the bacterium Candidatus Liberobacter spp.

It invades conducting tissues, causing a decline of citrus trees and rendering them unproductive.

Leaves are smaller. Advanced or chronically infected trees show yellowing of the entire canopy with sparse foliage and severe twig dieback. Fruit are small, lopsided, tend to remain mostly green even when mature and are bitter.

Common zinc deficiency and several other conditions can cause symptoms similar to citrus greening. Any suspicious symptoms of the disease will need to be confirmed by using one of several laboratory detection methods. See the reporting section for what to do if you notice suspicious symptoms.

The disease affects all citrus cultivars, species and hybrids. Sweet orange and mandarin are severely affected; grapefruit, lemon and pummelo less so. Mexican lime and trifoliate orange are the least affected.

Two forms of the disease have been identified: an African form expressing symptoms in relatively cool conditions (20-25ºC); and an Asian form expressing symptoms in cool and warm conditions (up to 35ºC). Host range and symptoms are similar for both.

The disease can be spread by grafting, but transmission rates can be variable due to the irregular distribution of the bacterium in the tree. It is possible to propagate a disease-free plant from an infected one.

Two psyllid insects are important vectors (agents of spread) of the disease. Diaphorina citri is found in Asia and Trioza erytreae is found in Africa. D. citri, commonly called the Asian citrus psyllid, is the insect of most concern to Australia due to its proximity and its tolerance to warm temperatures.

Adult Asian citrus psyllids are small, about the size of aphids (2.5 mm), resemble small cicadas and prefer to feed on leaves and stems of citrus and closely related Rutaceae. They have mottled brown wings, characteristically sit at an angle to the shoot or leaf on which they feed and will jump when disturbed. Eggs are bright yellow and deposited on young leaves. The Asian citrus psyllid is most likely to be found on new shoots, and the population increases during periods of active plant growth. It is capable of building to large numbers on one of its preferred hosts, the ornamental plant Orange Jessamine Murraya paniculata, which is common in Queensland.

Although of primary importance as a vector of citrus greening, high numbers of Asian citrus psyllid can themselves damage citrus trees, causing stunting and twisting of new shoots and build up of black sooty mould, leading to defoliation and a reduction in fruit quality.

Once a tree has been infected, it cannot be 'cured' of citrus greening. Where the disease exists, management strategies rely on preventing its spread into uninfected areas. A variety of measures are employed, including regulation of the movement of propagating material, destruction of infected trees, and control of insect vectors.

Citrus greening or its insect vectors could be introduced into Australia through illegal importation of host plants. The Australian Quarantine and Inspection Service closely regulates any legal host plant import.

• Garnier M and Bové JM (2000) Huanglongbing (greening). In Timmer LW, Garnsey SM and Graham JH (ed.) Compendium of citrus diseases. St. Paul, APS Press. p.46-48
• Photographs courtesy of Dr Patricia Barkley, Elizabeth Macarthur Agricultural Institute, NSW Agriculture.