Understanding Citrus Greening Disease and Its Possible Management Strategies in Nepal

Abstract

Huanglongbing (HLB), also known as citrus greening, is a devastating disease of citrus that has decimated several citrus orchards throughout the world. The disease is associated with three species of unculturable and phloem-limited bacteriae, Candidatus Liberibacter asiaticus, Candidatus Liberibacter africanus and Candidatus Liberibacter americanus. The most common species of bacteria found in Nepal is Candidatus Liberibacter asiaticus which is transmitted by an insect vector, Asian citrus psyllid (Diaphorina citri). This disease has been detected in several economically important citrus production areas of Nepal, which resulted in heavy yield loss. No cure for the disease has been discovered yet and it is essential to practice proper management strategies to maintain citrus health and sustain citrus production under HLB pressure. Several disease management approaches such as pathogen-free nursery establishment, use of disease tolerant rootstock cultivars, proper irrigation and nutrient supply, removal of HLB affected trees, and control of psyllid with frequent insecticide application are widely practiced throughout the world. This review article highlights the characteristics of the citrus greening disease and its insect vector and gives insights into their management techniques. Several technologically advanced options available to minimize the HLB infection might not be feasible currently in Nepal due to economic and topographic constraints. This article also aims to bring into focus the cost-effective methods that growers in Nepal can practice to mitigate the impact of HLB disease in their citrus orchards. Int. J. Appl. Sci. Biotechnol. Vol 9(4): 227-238.

A review on economic impact of citrus greening on Mandarin production in Nepal

“…The bacteria are transmitted by an insect vector Asian citrus psyllid (Diaphorina citri). The vector Asian citrus psyllid (ACP), Diaphorina citri (Hemiptera: psyllidae) is a small plant feeding insect with the size of an adult ranging from 2.7mm to 3.3mm long and ~ 239 ~ identified by distinctive mottled brown wings and abdomen with three color ranges; gray/brown, blue/green and orange/yellow (S. Pokhrel et al, 2021) [22] . Freshly deposited eggs are oval shaped and light-yellow colored while distinctive red eye spots are seen when matured (Hall et al, 2013) [11] .…”

“…According to Chen (1943), the clear symptoms of disease were first observed in 1938 whereas the insect vector Diaphorina citri was first described in 1927 (Husain and Noth, 1927) [12] . The disease was mostly detected and rapidly spreading during nineteenth century and now it has reached over 50 citrus producing countries of Africa, Asia, Europe, North America and South America (S. Pokhrel et al, 2021) [22] . Citrus greening bacterium has been already found in seven of the world's top ten orange producing countries (Zhang et al, 2019) [30] .…”

“…Some native parasitoids of Asia such as Tamarixia radiata and Diaphorencyrtus aligarhensis are found to attack and control ACP (Hall et al, 2013) [11] . Pathogenic fungi like Cladosporium spp, Capnodium Citrix etc have been used for biological control of D. citri since a long time (Hall et al 2013; S. Pokhrel et al, 2021) [11,22] . The first line of defense for the management of ACP and HLB is restricting the movement of infested plants and quarantining areas where the pests are abundant (Hall et al,…”

“…As growers need to rely on various management techniques and integrated management in order to maintain the trees health and longevity, those management strategies have very high cost associated with them (Li et al, 2020) [14] . The disease has been discovered from several economically important citrus growing districts in Nepal resulting significant yield losses (Pokhrel et al, 2020) [22] . Early researchers revealed that HLB has been reported from most of the citrus growing districts in Nepal namely kaski, Tanahun, Gorkha, Syangja, Dhading, Kavre, Dhankuta (FAO, 2011).…”