Huanglongbing

Primefact number Edition Published Author
1184 Second Apr 2017 Plant Biosecurity and Product Integrity

Grapefruit leaves that have yellowed especially along leaf veins

Halved grapefruit showing misshapen half is much smaller and poorly coloured compared to other half

Citrus fruit and leaves showing how the fruit is yellow towards stem but remains green at bottom

Huanglongbing (Candidatus Liberibacter asiaticus) is an exotic plant pest not present in Australia. This plant disease is a serious threat to Australia’s citrus industry.

Notifiable status

Huanglongbing (Candidatus Liberibacter asiaticus) is a notifiable plant disease in NSW.

All notifiable plant pests and diseases must be reported within 1 working day.  You can report notifiable plant pests and diseases by one of the following methods:

A full list of notifiable plant pests and diseases can be found in Schedule 2 of the NSW Biosecurity Act 2015.

Description

Huanglongbing is a bacterial disease affecting citrus and other Rutaceae. The disease is also called citrus greening.

All growth stages of a citrus tree and all plant parts are affected by huanglongbing. Seedling, vegetative, flowering, and fruiting growth stages are susceptible to the disease.

Affected plant parts are leaves, stems, flowers, fruit and roots. Symptoms include yellowing of leaves, dieback of branches and stems and distorted and discoloured fruit.

Symptoms of huanglongbing can be confused with citrus tristeza virus infection, Phytophthora root rotinfection, citrus blight and nutrient deficiencies.

The pathogen

Three regional types of “Candidatus Liberibacter” which cause symptoms of huanglongbing in citrus have been identified.

The Asian type is called “Candidatus Liberibacter asiaticus”; The African type is called “Candidatus Liberibacter africanus” and a third type, “Candidatus Liberibacter americanus” occurs with “Candidatus Liberibacter asiaticus” in Brazil.

Symptoms

The first symptom of huanglongbing is usually a yellow shoot in the tree canopy. Progressive yellowing of the rest of the canopy follows. Affected leaves appear blotchy and mottled or are small, upright thickened and chlorotic (Figure 1).

The small, upright chlorotic leaf symptoms of huanglongbing are also symptoms of mineral deficiencies such as zinc. Huanglongbing and mineral deficiencies may be present in the same tree. If both are present the huanglongbing mottling becomes less distinct than the mineral deficiency symptoms over time.

Severely infected trees show twig dieback and are sparsely foliated. Unseasonal flowering, heavy flowering and flushing out of phase with healthy trees occurs on diseased branches.

Fruit production is poor on affected trees and affected fruit is small, lopsided in shape (Figure 2) and bitter tasting. If seeds are present they are small dark coloured and infertile.

Development of fruit colour is uneven particularly in sweet oranges and mandarins in temperate and subtropical areas. The bottom end of the fruit remains green while the stalk end turns orange (Figure 3).

Fruit falls prematurely.

Huanglongbing is a plant disease. It is not harmful to people or animals. Affected fruit is safe to eat although it may taste bitter.

Vectors

Huanglongbing is vectored by psyllids. Asiatic citrus psyllid (Diaphorina citri) is the main vector of “Candidatus Liberibacter asiaticus”. Asiatic citrus psyllid is an exotic plant pest.

Hosts

All species and cultivars of commercial citrus can be affected by huanglongbing. Many plants in the Rutaceae family, including Australian natives, are susceptible to infection.

Spread and movement

Long distance spread of huanglongbing occurs through the movement of infected citrus plants or plant parts infested with huanglongbing-infected citrus psyllids.

World distribution

Candidatus Liberibacter asiaticus” is found in many countries throughout Asia including Australia’s close neighbours Indonesia, East Timor and New Guinea.

“Candidatus Liberibacter asiaticus” has spread to the Middle East, South and Central America and the United States.

Actions to minimise risks

Put in place biosecurity best practice actions to prevent entry, establishment and spread of pests and disease:

  • practice “Come clean, Go clean”
  • ensure all staff and visitors are instructed in and adhere to your business management hygiene requirements
  • source propagation material of a known high health status from reputable suppliers
  • keep records