Source: Kathmandu Post [abridged, edited]
Orange farmers [in Salyan district, province of] Karnali are deeply distressed because harvest was slashed by half due to a pest attack during the flowering period. The winter was slightly warmer, which attracted pests to the crops, experts said. A majority of orange growers are in trouble as production has been dropping annually.
The bacterial scourge known as citrus greening has devastated orange orchards in Salyan. According to the Agriculture Development Office [ADO], 700 orange farmers have been impacted by the pest problem in [the area]. Farmers cultivated the fruit on 1520 hectares this year [2020/21]. According to the office, 18 000 trees did not bear fruit. Output has been shrinking by 6% annually due to pest problems.
[A farmer] who planted 500 orange trees said that half of his trees did not bear fruit. "The leaves and fruits turn yellow and start falling from the trees." [He] chopped down 50 trees to keep the infestation from spreading, but that did not help.
[The ADO] said that the citrus greening disease had spread to all municipalities in the district [and] that the disease and climate change led to a 40% drop in production. "Regardless of the amount of fertiliser or care given to the plants, the trees look malnourished. The pesticides we distribute to the farmers did not work." According to the office, orange production is good in [some parts of the area].
The Plant Quarantine and Pesticide Management Centre [said] recently that new pests had been increasing and damaging crops due to climate change. "New pests have emerged, and they are more resilient. It's a worrying sign. The use of pesticides is likely to increase as farmers will choose to spray more chemicals to protect their crops."
[Byline: Biplav Maharjan]
[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, and 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 disease 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 Venezuela, a recent survey reported that citrus in colder areas has been less affected by CG (ProMED post 20201207.7999673), possibly due to vector insects being less active in colder temperatures 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 overall temperatures increasing there. This reflects similar effects observed for other pathogens and pests (ProMED posts 20160902.4459660, 20160622.4302098, 20160509.4211696, and multiple other posts in the archives) migrating to new areas in many regions due to warming climates.
Nepal provinces and districts:
Citrus greening symptoms:
Citrus greening, symptoms and vector photo galleries:
http://www.invasive.org/browse/subinfo.cfm?sub=4695 (Asian) and
Citrus greening information:
http://www.pestnet.org/fact_sheets/citrus_huanglongbing_greening_230.htm (with pictures),
Asian greening, information & distribution:
African greening, information, and distribution:
Taxonomy of Liberibacter species via:
Taxonomy and information for psyllid vectors (with pictures) via: