Wednesday, March 20, 2013

HLB, short for huanglongbing, Chinese for “yellow dragon disease”

Huanglongbing, also called HLB for short, is a disease of citrus plants [1]. The Chinese name “huanglongbing” means “yellow dragon disease.” Another term for this devastating disease is citrus greening disease, or simply citrus greening, due to the growth of green and bitter citrus fruits that drop prematurely [2]. Other names in use also refer to symptoms of this pest: citrus vein phloem degeneration (CVPD), yellow shoot disease, leaf mottle yellows and citrus dieback [3].

Huanglongbing is caused by gram-negative bacteria in the genus Candidatus Liberibacter that live in the salivary glands of the Asian citrus psyllid [1-6]. Psyllids and disease have now spread from China over India, South Africa and Brazil all the way to the Caribbean and into Florida and California, affecting commercial orange and mandarin groves around the world—except the Mediterranean basin.

A salivary toxin emitted by the gnat-sized psillids can deform leaves. The bacteria are transmitted into a citrus tree while psyllids drink sap from its leaves. The bacteria infect the circulatory system that transports sugars from the tree canopy to the roots. The result is  root blockage: the starving roots slow down in absorbing micronutrients and other substances from the soil and sending them to the canopy [2]. Weakened citrus plants will be more susceptible to stressors such as a cold spell or drought.

To contain huanglongbing, extra nutrients are fed to weakened citrus plants. In selected areas the Asian parasitic wasp Tamarixia radiata has been introduced to prey on psyllid nymphs and kill psyllids by drinking their blood. Also, nursery citrus trees are grown inside psyllid-proof screens. Are we going to witness a return of orangery greenhouses?

Keywords: biology, entomology, Hemiptera, plant pathology, epidemiomology, invasive species, insects, citrus industry, lemon, lime, pomelo.

References and more to explore
[1] Tim R. Gottwald: Current Epidemiological Understanding of Citrus Huanglongbing. Annual Review of Phytopathology 2010, 48, pp. 119-139. DOI: 10.1146/annurev-phyto-073009-114418.
[2] Anna Kuchment: The End of Orange Juice. Sci. Am. March 2013, 308 (3), pp. 52-59.
[3] Wikipedia > Huanglongbing [].
[4] Citrus Diseases, Huanglongbing (HLB) [].
[5] Citrus Pest & Disease Prevention Program [].
[6] USDA > Plant Health > Citrus Greening [].

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