Source: European Plant Protection Organisation (EPPO) Reporting Service 03/2020/053 [edited]
Distribution of _Candidatus_ Liberibacter species associated with citrus greening in Eastern Africa and first report of '_Candidatus_ Liberibacter asiaticus' in Kenya
Surveys were conducted in Uganda (300 sites), Ethiopia (170 sites) and Kenya (9 sites) to assess the status of citrus greening in these countries, identify the associated Liberibacter species in citrus plants and their psyllid vectors.
In Ethiopia, citrus greening symptoms were not seen at low altitudes (<1000 m; Oromia region) but were found at high altitudes (1876 to 2116 m; Gondar region). Symptoms were found in 26% of the Ugandan sites, 20.6% of the Ethiopian sites and 66.6% of the sites surveyed in Kenya. _Trioza erytreae_ was found at 10 sites in Uganda and 7 sites in Ethiopia, but not in any of the sites in Kenya. Conversely, _Diaphorina citri_ was found at all sites in Kenya at the time of the survey, but not in Uganda or Ethiopia.
Sequencing showed that '_Ca._ L. africanus' and '_Ca._ L. africanus subsp. capensis' occurred together in the western region in Uganda, and that '_Ca._ L. africanus subsp. clausenae' is the only species found in the eastern region. In Ethiopia, '_Ca._ L. africanus subsp. clausenae' (75%), '_Ca._ L. asiaticus' (25%) and '_Ca._ L. africanus' (4%) occurred in the Gondar region while only '_Ca._ L. asiaticus' was found in Tigray and Wollo regions. In Kenya, '_Ca._ L. asiaticus' was present at the coastal region, while '_Ca._ L. africanus subsp. clausenae'was found in the western region.
This is the 1st report of '_Ca._ Liberibacter asiaticus' in Kenya.
[Source paper: https://www.nature.com/articles/s41598-020-60712-0]
[Greening is one of the most damaging diseases of citrus crops affecting leaves and fruit. It is caused by fastidious phloem-inhabiting bacteria classified as _Candidatus_ Liberibacter asiaticus (CaLas; Asian greening, huanglongbing; main vector _Diaphorina citri_), africanus (including several subspecies, e.g. capensis; African greening; main vector _Trioza erytreae_) or americanus (South American greening). The 3 pathogens can only be distinguished by molecular methods. Several phytoplasma species have been reported to cause symptoms similar to greening disease in citrus and coinfections of phytoplasmas with CaLas have also been recorded (see ProMED-mail posts 20180214.5629251 and 20190329.6392077). Further research is needed on symptomatology, epidemiology and host impact of both single and mixed infections of these pathogens.
Symptoms may include blotchy mottling and yellowing of leaves, as well as small, irregularly shaped fruits with a thick, pale peel and bad taste. Early symptoms may be confused with nutrient deficiencies. Affected trees become stunted, bear multiple off-season flowers and may live for only a few years without ever bearing usable fruit. The diseases are restricted to _Citrus_ and close relatives because of the narrow host range of their psyllid vectors. The pathogens can also be spread by grafting and possibly by seed from infected plants or transovarially in the vectors. Both pathogens and vectors can be spread with plant material.
Disease management requires an integrated approach including use of clean planting and grafting stock, elimination of inoculum, use of pesticides for vector control in orchards, as well as chemical or biological control of vectors in non-crop reservoirs. Control using cultural methods, such as interplanting with non-host crops, is being trialled. In areas where a pathogen has not yet been detected, biological control of vectors has been used successfully to reduce insect numbers and, therefore, the risk of greening outbreaks (for example, see ProMED-mail post 20090601.2034).
Antibiotics as leaf sprays, seed treatments or trunk injections are being used occasionally to treat citrus greening (see for example, ProMED-mail posts 20181119.6154764 and 20190320.6377319), but are subject to strict regulations in most countries, with residues potentially leading to rejection of produce. Wide scale spraying of antibiotics results in serious risks of emergence of antibiotic resistances in other crop, animal and human pathogens, as well as the potential to impact plant and soil microbiomes which are essential for plant and environmental health (see ProMED-mail post 20190320.6377319).
In South America, both CaLas and CaLam are known to occur, with CaLas having outcompeted CaLam over time and millions of affected trees having been eradicated (see ProMED-mail post 20121114.1408468). It remains to be seen if a similar situation will develop in subsaharan Africa for CaLas and CaLafr. The region is on high alert, since CaLas has higher heat tolerance, disease severity and fatality to trees than CaLafr (ProMED-mail post 20190326.6387052). The survey above provides data on the current status of the species needed for establishing management strategies in the region.
Ethiopia (with regions):
Citrus greening symptoms, leaves, and branches:
Citrus greening, fruit symptoms:
Asian and African greening, symptoms and vector photo galleries:
https://gd.eppo.int/taxon/LIBEAF/photos (African) and
Citrus greening information:
http://www.pestnet.org/fact_sheets/citrus_huanglongbing_greening_230.htm (with pictures),
Asian greening, information, and distribution:
African greening, information, and distribution:
Taxonomy of Liberibacter species via:
Taxonomy and information for psyllid vectors (with pictures) via:
EPPO A1 quarantine list: