Citrus Huanglongbing, aka greening disease, has been the subject of several reviews in recent years. In this article, the author presents a concise compilation of the main features and symptoms of the disease, the causal organisms and the vectors including impacts for Florida and Brazil, two relatively new areas where the disease has been discovered, as well as implications for California, Texas, and Arizona citrus industries, which are threatened by the close proximity to the new Florida epidemic, and the rest of the western hemisphere. Accepted for publication 19 June 2007. Published 6 September 2007.

Citrus huanglongbing (HLB) has become a major disease and limiting factor of production in citrus areas that have become infected. The destruction to the affected citrus industries has resulted in a tremendous increase to support research that in return has resulted in significant information on both applied and basic knowledge concerning this important disease to the global citrus industry. Recent research indicates the relationship between citrus and the causal agent of HLB is shaped by multiple elements, in which host defense responses may also play an important role. This review is intended to provide an overview of the importance of HLB to a wider audience of plant biologists. Recent advances on host-pathogen interactions, population genetics and vectoring of the causal agent are discussed.

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In March 2020, following the annual International Committee on Taxonomy of Viruses (ICTV) ratification vote on newly proposed taxa, the phylum Negarnaviricota was amended and emended. At the genus rank, 20 new genera were added, two were deleted, one was moved, and three were renamed. At the species rank, 160 species were added, four were deleted, ten were moved and renamed, and 30 species were renamed. This article presents the updated taxonomy of Negarnaviricota as now accepted by the ICTV.

Citrus huanglongbing (HLB), formerly known as greening, is a highly destructive disease of citrus, especially on sweet orange and mandarin varieties. A range of primary and secondary leaf symptoms are associated with HLB, making field diagnosis difficult, unless the typical lop-sided greened fruit are present. To date, the causal organism has not been cultured on artificial media and a diagnostic polymerase chain reaction technique has been developed to confirm the presence of the pathogen. The disease is caused by two closely related phloemlimited bacterial species, "Candidatus Liberibacter asiaticus" and "Candidatus L. africanus". The former, which causes the more severe Asian form, is found in Asia from southern Japan in the east through southeast and south Asia to eastern Iran in the west, as well as Saudi Arabia, Mauritius and Reunion. It is transmitted by the psylla, Diaphorina citri. The milder, yet still serious, African form is less heat tolerant, and is transmitted by another psylla species, Trioza erytreae. This form is found in Yemen, throughout eastern and southern Africa, as well as in Mauritius and Reunion. Of concern is the fact that both psylla species are capable of transmitting both bacterial species under experimental conditions. In Reunion, propagating healthy trees and releasing hyperparasites for vector control achieved almost complete control. Elsewhere, HLB is best controlled through integrated disease management involving the use of healthy nursery material, removal of infected trees or branches, and integrated vector control. The recent arrival of the vectors in citrus producing areas previously regarded as HLB-free highlights the potential threat of one of the most serious diseases of citrus, thus emphasizing the need for effective quarantine services.

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The Asian citrus psyllid, Diaphorina citri Kuwayama, a vector of citrus huanglongbing, is now present in all citrus-producing states in the USA and Mexico. In addition to citrus, the insect can reproduce on several other plant species in the Rutaceae family; orange jasmine (Murraya spp.) and curry leaf (Bergera koenigii) are among its preferred hosts. There are several indigenous Rutaceae species in North America, and some are popular ornamentals. A study was therefore initiated to determine the suitability of some of these plants for feeding and development of the psyllid in choice and no-choice experiments. D. citri was found to reproduce successfully on Choisya ternata, C. arizonica and Helietta parvifolia in no-choice tests, but preferentially selected orange jasmine and curry leaf for feeding and reproduction, in choice tests. On Amyris madrensis, A. texana and Zanthoxylum fagara, adult psyllids laid eggs which hatched, but no successful nymphal development was recorded beyond the first instars. No oviposition was recorded on Esenbeckia berlandieri, Ptelea trifoliata and Casimiroa tetrameria, although adult psyllids were able to survive on these species for several days. Results showed that C. ternata, C. arizonica and H. parvifolia can serve host plants of D. citri and this constitutes the first report of these plants serving as host for D. citri. The findings of the present study suggest that native rutaceous host plants can serve as host plants and thus affect D. citri population dynamics and the epidemiology of Huanglongbing, the deadly citrus greening disease whose pathogen is vectored by D. citri. Thus, area-wide management of this pest also should target these riparian habitats where these host plants are present with D. citri biological control agents for sustainable management of this pest.