Citrus greening (HLB) — blotchy mottle and lopsided fruit Photo: Jeevan Jose, Kerala, India · CC BY-SA 4.0 · source ↗

Citrus greening (HLB, Candidatus Liberibacter)

Huanglongbing (HLB), also called citrus greening, is the most destructive citrus disease worldwide and is caused by the phloem-limited bacterium Candidatus Liberibacter asiaticus (CLas) in Asia, transmitted by the Asian citrus psylla Diaphorina citri. HLB is the principal long-term threat to sweet-orange production across the YSR Kadapa Sathgudi belt (Sathgudi Sweet Orange) and the Vidarbha-Marathwada Mosambi belt (Citrus Mosambi Sweet Lime) and is the single most important reason why orchard decline in YSR Kadapa has accelerated since the 2000s.

Identification and symptoms

Early symptoms are an asymmetric, blotchy yellow mottle on individual leaves and shoots — the pattern crosses the leaf midrib, unlike the symmetric chlorosis of zinc or iron deficiency. As the disease progresses, leaves become small, narrow and upright with a "rabbit-ear" appearance; twigs die back from the tip; flowering becomes off-season and excessive; fruit becomes small, lopsided, hard and bitter, with aborted seeds. The classic diagnostic feature is fruit that fail to ripen evenly — the stylar (blossom) end stays green while the rest of the fruit colours up, hence the name "greening". Confirmation is by PCR or qPCR for CLas on midrib tissue.

Hosts and lifecycle

CLas infects all commercial citrus species and is moved by Diaphorina citri (Asian citrus psylla), which feeds on tender citrus flush. Nymphs acquire the bacterium during feeding on infected flush and retain it for life; transmission to a new tree occurs in 30-60 minutes of feeding. Long-distance spread is by movement of infected budwood and budded plants — a far more important pathway than psylla flight for crossing district and state borders. The pathogen colonises the phloem, blocking sugar transport and causing the systemic symptoms above.

Damage and economic impact

A single infected tree continues to bear poor-quality fruit for several years before dying, so HLB silently undermines orchard productivity for a decade after introduction. The disease is the dominant cause of the long-term decline of mandarin orchards in Vidarbha and is increasingly cited by ICAR-CCRI Nagpur and YSR Horticultural University as a major emerging constraint in YSR Kadapa Sathgudi. Globally, HLB has reduced Florida sweet-orange production by more than 70% since 2005 and is the case study for what unmanaged HLB does to a citrus economy.

Management

There is no cure. ICAR-CCRI Nagpur and NHB recommend a three-pillar management approach:

Indexed planting material: source budded plants only from accredited insect-proof screenhouse nurseries that test mother trees by PCR for CLas. This is the single most cost-effective intervention and the one most often skipped by farmers.
Vector control: monitor for Diaphorina citri nymphs on tender flush; spray imidacloprid (0.005%) or thiamethoxam (0.025%) as a soil drench on young plants and as foliar spray at each flush; rotate insecticides to delay resistance.
Removal of infected trees: identify and remove symptomatic trees as soon as PCR confirms infection — leaving them in place sustains an inoculum reservoir that infects neighbouring trees. Replacement with indexed budded plants on tolerant rootstocks (Rangpur lime (Rangpur Lime Rootstock)) is the recommended replanting practice.

Open-canopy training and wider spacing (Citrus Spacing Canopy Collapse) make psylla scouting and insecticide coverage more effective. Antibiotic injections (oxytetracycline, ampicillin) reduce symptoms in heavily infected trees but are not recommended in India because of antibiotic-resistance concerns.

Sources

Huanglongbing (Citrus Greening). ICAR-Central Citrus Research Institute, Nagpur.
Citrus Greening Disease. National Horticulture Board.
Huanglongbing: Candidatus Liberibacter asiaticus. CABI Compendium.