Candidatus Liberibacter africanus
(African greening)

Candidatus Liberibacter africanus is not considered as invasive as Candidatus Liberibacter asiaticus, however, the species and its vector appear on several alert lists including the EPPO A1 list of Regulated Quarantine Plant Pests. African greening occurs mostly in cooler, moist, elevated production regions (Schwarz, 1967) and symptom expression is temperature dependant, occurring under relatively cool conditions (20-24°C optimum) (Garnier and Bové, 1993). This suggests that Ca. L. africanus is temperature sensitive. This temperature sensitivity coincides with the temperature sensitivity of its natural vector, Trioza erytreae. Temperatures above 30°C, together with relative humidity (RH) below 25%, are lethal to certain life stages of the vector (Catling, 1969). The combined sensitivity of the pathogen and the vector limit the spread of the disease to climatically favourable regions.

Candidatus L. africanus has not been identified on indigenous rutaceous species. However, Ca. L. africanus subspecies clausenae, was originally identified from Clausena anisata (horsewood) in South Africa (Roberts et al., 2015). C. anisata is widespread in tropical Africa, from Guinea and Sierra Leone, eastwards to Ethiopia and Sudan and southward to the Eastern and Western Cape Provinces of South Africa, but not in arid regions. It also occurs in tropical Asia and South-East Asia and is cultivated in Malaysia and Indonesia (https://uses.plantnet-project.org/en/Clausena_anisata_(PROTA)#). To date, Ca. L. africanus subspecies clausenae has only been reported in C. anisata in South Africa and in citrus in Uganda, Kenya, Tanzania and Ethiopia (Roberts et al., 2015, 2017; Ajene et al., 2020).

All citrus and Fortunella species, as well as Poncirus trifoliata (including hybrids) are susceptible to Ca. L. africanus. The least affected citrus species is the acid lime (Citrus aurantifolia) (da Graça, 1991).

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

Candidatus L. africanus subsp. clausenae was originally identified from Clausena anisata (horsewood) (Roberts et al., 2015) and subsequently from Citrus (Roberts et al., 2017).

The disease is vectored and is graft transmissible (McClean and Oberholzer, 1965; McClean and Schwarz, 1970).

Three Candidatus Liberibacter species have been identified on citrus including Ca. L. asiaticus (Asian origin), Ca. L. americanus (American origin) and Ca. L. africanus (African origin). The African form is less severe than the Asian form and is normally found at higher altitudes, above 600-900 m above sea level. Its distribution is restricted to cooler, moist regions, together with its natural vector, the African citrus triozid, Trioza erytreae (Schwarz, 1967; Catling and Green, 1972; da Graça and Korsten, 2004). Ca. L. africanus, is heat sensitive as symptoms are produced under relatively cool conditions (20-24°C optimum) (Garnier and Bové, 1993). Symptom expression is generally restricted to temperatures below 27°C (da Graça and Korsten, 2004). Extended periods of high temperatures suppress symptom development but do not suppress infection of citrus (USDA, 2012). This temperature sensitivity is similar to that of T. erytreae. Temperatures above 30°C, together with relative humidity (RH) below 25%, are lethal to certain life stages such as eggs and first-instar nymphs. T. erytreae survives short periods of high temperatures or low RH, but these extreme climatic conditions become lethal to young life stages when occurring together (Catling, 1969; Catling and Green, 1972).

The vector, Trioza erytreae, has natural predators and parasites (Catling, 1970; van den Berg et al., 1987). The parasitoid wasp Tamarixia dryi (=Tetrastichus dryi) is widespread in Africa and is the most common and effective parasitoid of T. erytreae (van den Berg and Greenland, 2000).

A diagnostic protocol for the detection of Candidatus Liberibacter africanus, Ca. Liberibacter americanus and Ca. Liberibacter asiaticus in their host species and for detection in their experimental and natural vectors, Diaphorina citri and Trioza erytreae was published by EPPO (2014). The protocol involves detection based on the disease symptoms and molecular tests (PCR), as well as reporting and documentation.

PCR diagnostics were updated to avoid misdiagnosis of subspecies of Ca. Liberibacter africanus (Roberts et al., 2017).

African greening symptoms are difficult to recognize as they often resemble those of other citrus disorders and nutrient deficiencies. If suspected, the presence of the disease should be confirmed by PCR.

Disease symptoms are almost identical to and can be confused with those of the other species of Candidatus Liberibacter. Mixed infections of two of the strains have been reported (Roberts et al., 2017).

Leaf symptoms can resemble nutrient deficiencies, particularly those of zinc and iron (Oberholzer et al., 1965; McClean and Schwarz, 1970). Symptoms may include yellowing and interveinal chlorosis. Nutrient deficiencies however tend to present uniformly across the canopy on similar aged shoots, whereas greening symptoms first appear on single shoots or branches. Symptoms of zinc deficiency are also similar to the early stages of citrus blight, a disease of unconfirmed aetiology (Brlansky, 2000). However, greening is not associated with xylem dysfunction and wilting observed in blighted trees.

Blotchy mottle is the most characteristic symptom of greening, but is not specific to this disease. Stubborn disease (Spiroplasma citri) and Australian citrus dieback, a disease of unknown aetiology, also induce blotchy mottle. Australian citrus dieback is further associated with other typical greening symptoms such as yellowing, reduced fruit size and dieback (Broadbent et al., 1976). Stubborn disease similarly displays symptoms such as small, lop-sided fruit, with colour inversion, aborted seeds and out-of-season flowering which can be confused with greening disease (Bové and Garnier, 2000).

Other diseases that may confuse diagnosis include severe infections of Citrus tristeza virus (CTV) which display twig and branch dieback, sparse foliage and small leaves, with various deficiencies and small sized fruit, as well as root rot, caused by Phytophthora spp., which can cause leaf yellowing, leaf drop and dieback, often sectorially.

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 such as exclusion are essential to prevent the introduction of the Candidatus Liberibacter pathogen or the vector. Exclusion means importation of disease-free propagation material from covered insect-free nurseries or pest free regions/areas/countries. Furthermore, the possibility exists that the vector can spread through natural range expansion and simultaneously introduce the pathogen to unaffected regions. Regular scouting to detect disease outbreaks and monitoring for insect vectors is needed for early implementation of control interventions (da Graca and Korsten, 2004).

Disease-free nursery trees should be used to establish orchards and infected, abandoned or unproductive trees should be removed. Branches can be removed from trees if sectorial infections are observed to reduce the inoculum pressure (Buitendag and von Broembsen, 1993).

Biological Control

In the absence of hyperparasitoids, the parasitic wasp Tamarixia dryi (=Tetrastichus dryi) significantly reduced populations of Trioza erytreae on the Indian Ocean island of Reunion, leaving a greatly 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 control strategy including the supply of certified greening-free nursery trees to commercial growers, the reduction of inoculum by the removal of infected trees or branches, and insecticide applications to control T. erytreae. The use of systemic insecticides with long residual action, applied prior to the spring flush and followed by shorter acting insecticides, is the primary method to effectively control the vector. This approach has brought about a distinct reduction in the incidence of greening-infected trees in commercial plantings in greening affected regions in South Africa.

01/02/2021 Updated by:

Glynnis Cook, Citrus Research International, South Africa
Hans J. Maree, Citrus Research International and Genetics Department, Stellenbosch University, South Africa
M.C. Pretorius, Citrus Research International, South Africa
Wayne Kirkman, Citrus Research International, South Africa
Elma Carstens, Citrus Research International, South Africa

27/03/13 Updated by:

Esther Arengo, National Agricultural Research Laboratories, Uganda