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A ProMED-mail post
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International Society for Infectious Diseases

Date: Fri 15 Mar 2019
Source: FreshPlaza [edited]

Researchers expect the imminent arrival of the Asian greening disease [huanglongbing, HLB] vector or the bacteria in South Africa. The South African [citrus] industry should be very concerned, [said] Dr Vaughan Hattingh, Citrus Research International. Arrival of HLB is not a case of if, but rather when.

The vector, the Asian citrus psyllid (ACP; _Diaphorina citri_), was in 2015 detected in Tanzania, 2016 in Kenya, and is known to occur in Reunion and Mauritius. However, in Kenya and Tanzania the psyllids were free of the bacteria. The bacterium itself has been detected in Ethiopia in 2000 and 2014. "What's worrying is a detection of the psyllid just north of Mozambique a couple of weeks back. ACP is on the move and [will] enter South Africa at some point in time."

South Africa aims to emulate the Brazilian experience. The psyllid had been present in Brazil for 62 years when the 1st incidence of the disease was reported in 2004. The country [did not] follow the gloomy Florida narrative [e.g. see ProMED-mail post 20190320.6377319] and has since managed, despite a loss of over 52 million trees and a quarter of its citrus hectares, to keep HLB under check through hypervigilance applied in a collective manner -- something with which the South African citrus industry is well acquainted.

HLB differs from African greening in its higher heat tolerance and its fatality to trees. It is a lot more severe than African greening which is present in South Africa, and traps are not very attractive for monitoring of the vector.

[Byline: Carolize Jansen]
Communicated by:

[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, HLB; main vector _Diaphorina citri_), africanus (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.

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. 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, and 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).

Several phytoplasma species have been reported to cause similar symptoms in citrus and coinfections of phytoplasmas with CaLas have also been reported (for example, see ProMED-mail post 20180214.5629251). Further research is needed on symptomatology, epidemiology, and host impact of both single and mixed infections of these pathogens.

In Brazil, both CaLas and CaLam are known to occur, with CaLas having outcompeted CaLam over time and millions of HLB affected trees having been eradicated (see ProMED-mail post 20121114.1408468). The report above underlines the success of the Brazilian approach which uses integrated disease management strategies. In contrast, greening control in the US seems largely to have failed. Wide scale spraying of antibiotics has now, incomprehensibly, been approved there, resulting 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).

South Africa: and
Africa, overview:

Citrus greening symptoms, leaves and branches:,, and
Citrus greening, fruit symptoms: and
Greening symptoms and vector photo galleries: (African) and (Asian)
CaLas psyllid vector _Diaphorina citri_:

Citrus greening information: (with pictures),,, and
Asian greening, information and distribution: and
African greening, information and distribution:
Taxonomy of Liberibacter species via:
Taxonomy and information for psyllid vectors (with pictures) via: and

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