ProMED
https://promedmail.org/

Source: The Ghana Report [summ. Mod.DHA, edited]
https://www.theghanareport.com/ghana-to-carry-out-nationwide-survey-on-two-emerging-plant-diseases-dr-appiah/

A nationwide survey will be conducted on banana bunchy top disease (BBTD) and citrus greening. The exercise has become necessary due to the proximity and threat the 2 emerging diseases pose to the country's agriculture.

A vector of citrus greening had been identified in the Volta Region, making the chances of the emergence of the disease in the country highly possible. Banana bunchy top disease had been identified recently in Benin, which is not far from Ghana.

It is feared that many plant diseases remain unidentified and uncharacterised, making it difficult to implement control measures. The survey would help to establish presence or absence of the diseases in the country.

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Communicated by:
ProMED

[[1] Banana bunchy top:
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_Banana bunchy top virus_ (BBTV) is the type member of the genus _Babuvirus_ (family Nanoviridae). It affects only _Musa_ species and hybrids. BBTV causes one of the most serious diseases of these crops with symptoms of chlorosis, stunting and death of the host plant. The virus is spreading in Africa, Australasia and the Pacific Islands.

BBTV is transmitted by the banana aphid _Pentalonia nigronervosa_ in a persistent manner. Infectious insects may be spread on plant material or by air currents. The virus is also spread with infected suckers but cannot be transmitted by mechanical means (such as cutting tools). Disease management is extremely difficult and may include vector control, removal of inoculum, use of clean planting material, as well as frequent scouting for new outbreaks. Eradication of established BBTV from a planting or natural cluster of plants is considered impossible; the only option is to kill off all plants in the group. Tolerant varieties can be used to enable some crop production in infected areas, but these cultivars may still support virus replication and thus serve as pathogen reservoirs.

There are 2 groups of BBTV isolates from different regions: the South Pacific group (including Australia, Burundi, Egypt, Fiji, India, Tonga, Western Samoa) and the Asian group (including Philippines, Taiwan, Viet Nam). The mean sequence difference between the 2 groups has been reported as approximately 10% (see link below). This is thought to suggest that the virus has spread after the initial movement of its hosts from the Indo-Malayan region, where edible _Musa_ species originated, to Africa and the Americas.

BBTV is closely related to _Abaca bunchy top virus_ (ABTV) reported from some areas of SE Asia (ProMED post 20191215.6842101). ABTV and BBTV cause the same disease symptoms in both banana and abaca (_M. textilis_).

In Africa, BBTV is thought to be present currently in around 16 countries (e.g. ProMED post 20211117.8699734). The virus is also endangering the diversity of banana varieties and landraces which are predominantly grown by farmers because they are well adapted to local conditions.

[2] Citrus greening:
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Citrus greening (CG) is one of the most damaging diseases of the 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 (CaLaf including a subsp. capensis; African greening; main vector _Trioza erytreae_) or americanus (CaLam; 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; coinfections of phytoplasmas with CaLas have also been recorded (e.g. ProMED posts 20180214.5629251, 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. CG is restricted to _Citrus_ and close relatives because of the narrow host range of its psyllid vectors. The pathogens can also be spread by grafting and possibly by seed from infected plants or transovarially in the vector insects. 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 ProMED post 20090601.2034). Antibiotics as leaf sprays, seed treatments or trunk injections are being used occasionally to treat CG (e.g. ProMED posts 20181119.6154764, 20190320.6377319), but are subject to strict regulations in most countries due to their associated risks of facilitating the emergence of antibiotic resistances in other crop, animal and human pathogens. Furthermore, beneficial soil microbes may be killed off as collateral damage, making the plants weaker and more susceptible to other diseases. Residues of antibiotics may also lead to rejection of exported produce by some countries.

In South America, citrus in colder areas has been found less affected by CG (ProMED post 20201207.7999673), possibly due to vector insects in colder temperatures being less active. However, elsewhere citrus psyllids have been found at increasing altitudes (ProMED post 20161129.4660906), potentially due to increasing overall temperatures there. This reflects similar effects observed for other pathogens and pests (e.g. ProMED posts 20160902.4459660, 20160622.4302098, 20160509.4211696) migrating to new areas in many regions due to warming climates.

Maps
Africa (overview):
https://www.worldatlas.com/r/w960-q80/webimage/countrys/africa/maps/africa.jpg

Pictures
Bunchy top symptoms on banana:
https://feww.files.wordpress.com/2009/08/infected_plant.jpg?w=468&h=311,
https://www.iucngisd.org/gisd/image/ecology/infected_fruits5.jpg and
http://callisto.ggsrv.com/imgsrv/FastFetch/UBER1/ZI-5YXF-2015-JAN00-IDSI-61-1
Bunchy top photo galleries:
https://www.ctahr.hawaii.edu/banana/leaves_bunchy.asp (whole plants),
https://www.ctahr.hawaii.edu/banana/morse_code.asp (leaves),
https://www.ctahr.hawaii.edu/banana/deformed_fruits.asp (fruits) and
https://www.ctahr.hawaii.edu/banana/closeup_photos.asp (banana aphids)
Citrus greening symptoms:
http://www.citrusalert.com/wp-content/uploads/2012/10/GreenIslandsOfColor.jpg and
https://www.abc.net.au/reslib/200904/r362894_1677317.jpg
Asian greening, symptoms and vector photo galleries via:
https://www.invasive.org/browse/subinfo.cfm?sub=4695

Links
Information on BBTV:
https://apps.lucidcentral.org/pppw_v10/text/web_full/entities/banana_bunchy_top_121.htm,
https://gd.eppo.int/taxon/BBTV00 (with distribution),
https://doi.org/10.1079/cabicompendium.8161 and
http://www.ctahr.hawaii.edu/banana/downloads/brochure1.pdf
BBTV description and genetic groups:
https://doi.org/10.1099/0022-1317-75-12-3541
ABTV description and characterisation:
https://doi.org/10.1007/s00705-007-1077-z
Information on genus _Babuvirus_:
https://viralzone.expasy.org/564
Virus taxonomy via:
https://ictv.global/taxonomy
Information on the banana aphid:
http://www.extento.hawaii.edu/kbase/crop/Type/pentalon.htm
BBTD Alliance:
https://www.iita.org/news-item/international-multi-stakeholder-alliance-continue-to-tackle-banana-bunchy-top-disease-in-africa/ and via
https://www.bbtvalliance.org/
Citrus greening information:
https://cisr.ucr.edu/invasive-species/huanglongbing-hlb-or-citrus-greening and
https://www.aphis.usda.gov/aphis/resources/pests-diseases/hungry-pests/the-threat/citrus-greening/citrus-greening-hp
Asian greening, information & distribution:
https://gd.eppo.int/taxon/LIBEAS (with pictures) and
https://doi.org/10.1079/cabicompendium.16565
African greening, information & distribution:
https://gd.eppo.int/taxon/LIBEAF (with pictures) and
https://doi.org/10.1079/cabicompendium.16564
American greening, information & distribution:
https://gd.eppo.int/taxon/LIBEAM
Taxonomy of Liberibacter species via:
https://www.uniprot.org/taxonomy/34019
Taxonomy and information for psyllid vectors (with pictures) via:
http://www.psyllids.org/index.htm
- Mod.DHA