A perspective of citrus Huanglongbing in the context of the Mediterranean Basin

Abstract

Huanglongbing (HLB) is one of the most catastrophic citrus diseases. HLB pathogens Candidatus Liberibacter asiaticus (Las), Ca. L. africanus (Laf), and Ca. L. americanus (Lam) and their insect vectors Asian citrus psyllids (ACP, Diaphorina citri) and African citrus psyllids (AfCP, Trioza erytreae) are invading citrus producing regions where HLB was absent previously, including the Mediterranean basin. Importantly, the Mediterranean region is one of the two major citrus producing areas without HLB. Here, I provide a short perspective regarding a) information related to the distribution of the HLB pathogens and psyllid vectors in this region and neighboring countries, b) predicted distribution of the HLB for this region, c) the possible evolution of Liberibacters and how they could have established their relationship with different hosts, and d) approaches to fend off HLB. Specifically, I emphasized the following measurements: quarantine measures against AfCP, ACP, Las, Laf, and Lam, early detection and diagnosis of HLB and removal of inoculum, surveillance of AfCP and ACP and eradication of citrus psyllids once identified. Keywords Citrus. HLB. Huanglongbing. Liberibacter. Phloem. Psyllids Citrus Huanglongbing (HLB), also known as citrus greening, is the most devastating disease in many citrus producing areas worldwide. HLB is caused by Candidatus Liberibacter asiaticus (Las), Ca. L. africanus (Laf), and Ca. L. americanus (Lam) that are yet to be cultured (Merfa et al. 2019). Among them, Las is the most widely distributed and has been reported in Asia, North America, South America, and Africa, Laf is only present in Africa, and Lam, originally identified in Brazil, is diminishing and being taken over by Las. Las and Lam are naturally transmitted by Asian citrus psyllids (ACP, Diaphorina citri) and Laf by African citrus psyllids (AfCP, Trioza erytreae). The HLB pathogens and insect vectors have been spreading to new territories including Brazil (Coletta-Filho et al. 2004; Texeira et al. 2005), multiple states of the United States (Kumagai et al. 2013); Belize (Manjunath et al. 2010); countries in the Caribbean including Cuba (Martinez et al. 2009) and the Dominican Republic (Matos et al. 2009);

Evaluation of the control effect of SAR inducers against citrus Huanglongbing applied by foliar spray, soil drench or trunk injection

“…L. africanus' (Laf) in Africa and limited areas of Asia, and 'Ca. L. americanus' (Lam) in Brazil (Bové 2006;Gottwald et al 2007;Wang 2020). Among them, Las is the most prevalent species worldwide (Bové 2006;Gottwald 2010;Wang et al 2017b;Andrade et al 2020).…”

Engineered Orange Ectopically Expressing the Arabidopsis β-Caryophyllene Synthase Is Not Attractive to Diaphorina citri, the Vector of the Bacterial Pathogen Associated to Huanglongbing

“…Genetic modification of citrus opens the possibility of, once genes of resistance to the disease are identified (from sexual compatible but also unrelated genera), introducing them into elite cultivar genetic backgrounds without modification of the rest of their excellent agronomic traits. Thus, genetic modification has been pointed out as the only solution to stop this devastating disease ( Bové, 2012 ; Duran-Vila et al, 2014 ; Yan et al, 2015 ; Mccollum and Baldwin, 2016 ; National Academies of Sciences, Engineering, and Medicine, 2018 ; Cochrane and Shade, 2019 ; Wang, 2020 ). In parallel to the identification and evaluation of traits that confer resistance—through introduction into desired genotypes and field testing—alternative environmentally safe strategies need to be developed to avoid HLB spread or to limit it as much as possible.…”

Citrus CsACD2 Is a Target of Candidatus Liberibacter Asiaticus in Huanglongbing Disease

“…Las and its insect vector Asian citrus psyllid (Diaphorina citri) have been spreading from Asia to other continents, with 53 countries reporting HLB so far (https://www.cabi.org/isc/ datasheet/16567). Billions of dollars of annual economic loss is related to yield, quality and tree loss, and costs for control and regulation measures of HLB (Bove, 2006;Gottwald, 2010;Wang 2020).…”