Huanglongbing

Huanglongbing (HLB), also known as citrus greening or yellow shoot/dragon disease, is one of the most destructive citrus tree diseases worldwide. It is a bacterial disease that leads to tree decline and death. HLB is one of Australia’s national priority plant pests and there are strict quarantine measures in place to prevent its introduction into Australia.

Three bacterial pathogens can cause this disease: Candidatus Liberibacter asiaticus, Ca. L. americanus and Ca. L. africanus. There is currently no known cure for HLB.

HLB can be found in parts of Asia, the Middle East, Africa, the Americas and Oceania, with all citrus species being susceptible to the disease.

Transmission

HLB is spread by the movement of infected plant material and insect vectors (psyllids).

The most important vectors are the Asian citrus psyllid (Diaphorina citri) and the African citrus psyllid (Trioza erytreae). Both species are exotic to Australia, although the Asian citrus psyllid is found in nearby countries such as Indonesia (including Papua), Timor-Leste and north-western Papua New Guinea.

Host range

HLB affects citrus and other hosts within the family Rutaceae.

For more extensive host information, please check the European and Mediterranean Plant Protection Organization and Centre for Agriculture and Biosciences International host lists.

Symptoms

HLB is often confused with nutrient deficiencies or other diseases, such as Australian citrus dieback, citrus tristeza virus, Phytopthora root rot and citrus blight.

HLB symptoms on trees include:

• branch/twig dieback
• unseasonal and heavy flowering on diseased branches
• out-of-phase flushing.

HLB symptoms on leaves include:

• yellowing of leaves and shoots (Figure 1)
• small upright chlorotic leaves (Figure 1)
• asymmetric, blotchy and mottled leaves that cross the leaf veins (Figure 2 and Figure 3)
• leaf symptoms randomly arranged in the canopy.

HLB symptoms on fruit include:

• poor fruit production
• uneven fruit ripening (Figure 1)
• small, lopsided (Figure 4), bitter-tasting fruit
• excessive and premature fruit drop.

For more photos of HLB symptoms, please check the photo sections of the European and Mediterranean Plant Protection Organization and Centre for Agriculture and Biosciences International web pages.

Detection and prevention measures

Agriculture Victoria does active surveillance for HLB and its vectors under the National Plant Health Surveillance Program (NPHSP), regularly responds to reports of suspect plant diseases and conducts exclusion diagnostics on plants with symptoms.

There is no cure for huanglongbing. To protect your property, maintain best practice on-farm biosecurity standards. Make sure propagation material is purchased from reputable suppliers and regularly check your plants for symptoms. Report suspect symptoms immediately.