**1. Introduction**

Citrus fruits are the most predominantly produced fruits worldwide. The citrus species, Rutaceae family, is one of the major fruit crops in the world, which has provided an immune-enhancing source of vitamin C, nutrients, and medicinal value since ancient times [1]. Citrus crops are cultivated in more than 135 countries worldwide [2]. Worldwide citrus production is estimated at over 124.3 million tons annually [3]. Cultivated commercial citrus plants, consisting of rootstock and scion varieties, have a significant impact on scion growth, fruit quality, yield, and tolerance to biotic and abiotic stresses [4, 5]. Therefore, the selection of rootstock may make a significant contribution to the success or failure of the planting process [2]. However, various biotic and abiotic stresses impede citrus production worldwide, among which Huanglongbing is one of the significant pernicious diseases devastating the citriculture industry in the last few decades. Citriculture industries in Asia, Africa, and America have suffered massive economic losses due to the devastating Huanglongbing (HLB) malady [6].

Citrus HLB (Yellow dragon disease or citrus greening) is one of the highly ruinous diseases in citrus species caused by proteobacteria *Candidatus* Liberibacter species. The casual organisms of HLB have not been successfully cultured on axenic culture to date, and the prevalence of the HLB pathogen in citrus plants was evaluated using a diagnostic polymerase chain reaction (PCR) technique. *Diaphorina citri Kuwayama* (Asian citrus psyllid (ACP)) and *Triozaerytreae* (African citrus psyllid (AfCP)) transmit HLB disease from one citrus plant to another and also feed on many other species of the *Rutaceae* family [7]. The ACP resides in warm and humid zones and is most prevalent in Asia, the Indian subcontinent, Saudi Arabia, Reunion, and Mauritius. Now, ACP has also spread to South and Central America, such as Brazil, USA, and Mexico [8]. AfCP thrives in cold weather and is sensitive to the sweltering climate. AfCP resides in Africa, Cameroon, South Africa, Yemen, Madagascar, and Madeira Island [9]. HLB was first identified as a significant issue of unknown disease in citrus by farmers in southern China at the end of the nineteenth century [10]. HLB was first known a century ago as Citrus "Dieback" in India and "Yellow Dragon shoot Disease" in China, with a clear impact on citrus production in many countries, followed by South Africa, the Philippines, Indonesia, Thailand, Brazil, and the United States [11].

Citrus are susceptible to HLB, that is, nearly all commercial citrus and some citrus relatives. *Poncirus trifoliate* citrus, some *P. trifoliate* hybrids, and a few lemon varieties are considered more HLB tolerant [12]. The most efficient and sustainable strategy against citrus HLB is breeding resistant citrus cultivars. However, conventional citrus breeding is a long-term process that takes about 20 years to develop a new variety. Further, breeding efficiency is affected by gametophytic cross-incompatibility, heterozygosis, pollen-ovule sterility, apomixis, seedlessness, graft incompatibilities, polyembryony, and unstable characteristics [13]. Genetically engineered resistant citrus varieties are yet to be available for commercial cultivation due to the lack of acceptance of GMOs from farmers and consumers. It will, therefore, take many years to develop a promising resistant cultivar against HLB [14].

Many strategies to combat HLB were initiated, such as thermotherapy, antibiotics, plant defense initiators, pesticide, vector control management, chemotherapy, nanotechnology, and a transgenic approach [15, 16]. Beta-lactams, tetracyclines, and silver nanoparticles have obtained better results against HLB malady [17, 18]; however, the emergence of antibiotic resistance to microorganisms and indirect effects on human health and the environment is a significant and increasing risk that certainly restricts the use of antibiotics at the field level [18]. However, no effective strategies to eliminate or repress the HLB pathogens have been identified. This review attempts to provide an overall picture of HLB disease, distribution, casual organism and its pathogenic mechanism, and vector control management, and post the current and possible strategies to mitigate/combat HLB malady in the field.
