Liberibacter africanus
(African greening)

The African form of Huanglongbing is not considered as invasive as the Asian form; however, the species and its vector are on several alert lists including the EPPO A1 Regulated Quarantine Plant Pests.

Calodendrum capense, the host of L. africanus subsp. capense, is native to the southern African region. The description of L. africanus subsp. capense was from two locations, one on an ornamental tree bordering an orchard, which displayed the blotchy mottle symptom characteristic of HLB (Garnier et al., 2000).

All citrus species appear to be susceptible to L. africanus; however, it is primarily a disease of sweet orange (Citrus sinensis) with Valencia showing more pronounced symptoms than navels. It is also particularly severe on mandarins (C. reticulata) and tangelos (C. reticulata x sinensis) but less so on lemon (C. limon). The least affected species is the acid lime (C. aurantifolia) (da Graca, 1991).

Transmission to Catharanthus roseus, which shows distinct yellowing symptoms, is via the parasitic plant dodder, not the insect vector (Garnier and Bové, 1978).

To date, the African form of the disease has only been detected in Citrus species for L. africanus and Calodendrum capense for L. africanus subsp. capensis.

Four forms of greening now exist worldwide. The South African form is least severe and is normally found at higher altitudes, above 900 m above sea level. Its distribution is more restricted than the other forms and it is transmitted by Trioza erytreae (da Graca and Korsten, 2004). It is temperature sensitive and is restricted to temperatures below 27°C (da Graca and Korsten, 2004). Extended periods of high temperatures suppress symptom development but do not suppress infection of citrus (USDA, 2012).

The vector Trioza erytreae is parasitized by Tetrastichus dryi in Africa, Reunion and Mauritius.

A diagnostic protocol for Liberibacter africanus, Liberibacter americanus and Liberibacter asiaticus and for their detection in their psyllid vectors Diaphorina citri and Trioza erytreae has been published by EPPO (2014). The protocol involves detection based on the disease symptoms and molecular tests (PCR), and reporting and documentation.

For a description of the diagnostic tests for this species, see the datasheet on Citrus huanglongbing (greening) disease. 

Huanglongbing is difficult to recognize due to symptoms of the disease resembling those of other citrus disorders (see Symptoms). If suspected, the presence of the disease should be confirmed by identifying the bacterium by PCR or electron microscopy.

Disease symptoms are almost identical and can be confused with those of the other strains of Liberibacter causing huanglongbing. Mixed infections of two of the strains have been reported (Coletta-Filho et al., 2005). Leaf symptoms also resemble nutrient deficiencies, particularly in zinc, calcium and nitrogen.

Blotchy mottle is the most characteristic symptom of huanglongbing, but is not specific to it. Stubborn disease (Spiroplasma citri), severe forms of Citrus tristeza virus (CTV), species of Phytophthora, waterlogging and the use of marcots can produce similar blotchy mottle patterns. Symptoms of zinc deficiency are also associated with the early stages of citrus blight (a disease of unconfirmed aetiology). However, huanglongbing bacteria do not induce the xylem dysfunction and wilting observed in blighted trees.
 

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Phytosanitary Methods

In areas where the disease is not present, effective quarantine measures are essential to prevent the introduction of the huanglongbing (HLB) organism or the vector. Furthermore, the possibility exists that the vector could be introduced 'naturally' or through alternative hosts (da Graca and Korsten, 2004).

Biological Control

In the absence of hyperparasitic wasps, the parasitic wasp Tetrastichus dryi significantly reduced populations of Trioza erytreae, the vector of HLB, on the Indian Ocean island of Reunion, leaving a strongly limited population of the vector (Aubert and Quilici, 1984).

Chemical Control

There are no chemical controls that specifically target the bacterium. Several antibiotics have been trialled to treat the tree via trunk injection methods. However, this was not sustained as a commercial treatment because the method proved expensive, remission was temporary, treated trees were inclined to produce small fruit, there were phytotoxic effects at the injection site and high levels of residues were found in the fruit of treated trees. Treatment then turned to control of the vector (Buitendag and von Broembsen, 1993).

IPM Programmes

Buitendag and von Broembsen (1993) recommended a three-pronged strategy of the provision of certified greening-free nursery trees to commercial growers, a reduction of inoculum through an ongoing programme of removing plant parts showing greening symptoms and the implementation of effective measures to control the psylla vector, T. erytreae. This has brought about a distinct reduction in the incidence of greening-infected trees in commercial plantings in South Africa.

27/03/13 Updated by:

Esther Arengo, National Agricultural Research Laboratories, Uganda