ProMED

Source: Kuensel Online [summ. Mod.DHA, edited]
https://kuenselonline.com/sarpang-farmers-worry-about-drying-orange-trees/

Farmers in Sarpang [Geylegphug] are cutting down orange trees and abandoning the crop. Oranges were widely grown until citrus greening emerged, wiping out entire orchards. Trees started to die in large numbers in 2014, but farmers say the problem became worse in the past 3 years. Changes in weather patterns could have also worsened the problem.

Farmers said that there was no fruiting at all. A study was done, and farmers were trained in managing orchards. However, the disease couldn't be wiped out, as not all infected trees could be destroyed due to farmer resistance. They tried to control the disease by spraying insecticides, but it is difficult to procure them.

An extension officer said that it is a nationwide problem. Citrus canopy management that focuses on improving orchard practices and enhancing soil nutrients proved successful in some areas. Since the disease could not be wiped out in the absence of an advanced rehabilitation programme, the farmers were asked to replace oranges with other fruit trees.

--
communicated by:
ProMED
<promed@promedmail.org>

[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), africanus (including a subsp. capensis; African greening), 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; coinfections of phytoplasmas with CaLas have also been recorded (see 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 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 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 post 20090601.2034).

Antibiotics as leaf sprays, seed treatments, or trunk injections are being used occasionally to treat CG (see for example, 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 neighbouring India, CaLas was shown to be present in most states and widespread in all commercial citrus species and hybrids (ProMED post 20150409.3285806). While molecular diagnosis is often not obtained for local outbreaks, like the one reported above, CaLas seems to be the most likely CG pathogen involved in the region.

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 or their numbers remaining lower. On the other hand, in Nepal, 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 (for example, ProMED posts 20160902.4459660, 20160622.4302098, 20160509.4211696) migrating to new areas in many regions due to warming climates.

Maps
Bhutan:
https://i.infopls.com/images/mbhutan.gif
Bhutan districts:
http://www.maps-of-the-world.net/maps/maps-of-asia/maps-of-bhutan/color-administrative-map-of-bhutan.jpg

Pictures
Citrus greening symptoms:
http://www.citrusalert.com/wp-content/uploads/2012/10/GreenIslandsOfColor.jpg,
https://geneticliteracyproject.org/wp-content/uploads/2016/10/citrus_greening.jpg and
http://www.abc.net.au/reslib/200904/r362894_1677317.jpg
Citrus greening, symptoms and vector photo galleries:
http://www.invasive.org/browse/subinfo.cfm?sub=4695 (Asian) and
https://gd.eppo.int/taxon/LIBEAF/photos (African)

Links
Citrus greening information:
http://www.pestnet.org/fact_sheets/citrus_huanglongbing_greening_230.htm (with pictures),
https://www.aphis.usda.gov/aphis/resources/pests-diseases/hungry-pests/the-threat/citrus-greening/citrus-greening-hp,
http://cisr.ucr.edu/citrus_greening.html and
http://ecoport.org/ep?SearchType=slideshowViewSlide&slideshowId=197
Asian greening, information and distribution:
http://www.cabi.org/isc/datasheet/16565 and
http://www.planthealthaustralia.com.au/pests/huanglongbing-or-citrus-greening-asiatic-strain/
African greening, information and distribution:
http://www.cabi.org/isc/datasheet/16564
Taxonomy of Liberibacter species via:
http://www.uniprot.org/taxonomy/34019
Taxonomy and information for psyllid vectors (with pictures) via:
http://www.psyllids.org/index.htm
- Mod.DHA]