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Citrus Huanglongbing (HLB)
Citrus huanglongbing (HLB) or the commonly called citrus greening is a recently introduced disease in Florida (2005) and Brazil (2004), whereas, it is believed to be a century-old in Asia. The pathogen is a bacterium which is difficult to culture; however, based on molecular methodology, it is named Candidatus Liberibacter. The term Candidatus represents a bacterium that is scientifically less-defined. Three distinct types have been described around the world – Candidatus L. asiaticus in Florida, Candidatus L. africanus in Africa and a recently described Candidatus L. americanus from Brazil. It lives in the phloem tissue that conducts synthesized food materials in plants. Like the human blood vessels, a blockage of the conducting tissue by any foreign substance can produce deleterious effect.
The symptoms of infected citrus trees are temperature dependent – the africanus symptoms are produced under cool conditions and the asiaticus symptoms are produced under both cool and warm conditions. Candidatus L. americanus is heat sensitive. All citrus species and their hybrids are known to be affected by HLB. Infected trees first produce leaf symptoms – typically leaf yellowing on one or more branches, leaves develop a blotchy (=irregular spots) appearance with grades of color. Infected trees may also show micronutrient deficiency, especially, zinc and manganese. Fruit set will be thin, fruit appear lopsided, may contain aborted seeds, and unacceptable juice quality.
The bacterium is transmitted by a psyllid, Diaphorina citri in Asia, in the U.S and Brazil, whereas, the African form of the bacterium is transmitted by another psyllid, Trioza erytreae. HLB survives and multiplies in the vector throughout the psyllid life.
Symptomatic trees can be identified in the field by trained people. Though different laboratory procedures are available to detect, the most reliable one is quantitative real-time PCR that allows both detection and quantification. HLB has a long latency period, therefore, infected trees may not appear 'diseased' to human eye, always and especially in the early stages. Therefore, all suspected samples must be confirmed by a PCR test. An uneven distribution of the bacteria calls for multiple, repeated sample testing from a tree – especially trees that are used for propagation of nursery trees.
HLB is truly a limiting factor for citrus production. Eradication of infected trees is highly desirable in places where infection is very new. Vector control by efficient and effective chemical sprays, the use of natural enemies to the vector, and the use of HLB-free budwood for plant propagation are extremely important. Cultural practices for improved plant health and higher tree density planting are possible strategies when forced to live with HLB. Eradication of HLB-infected trees is a must for citrus producing states such as Texas, Arizona and California. Strategies for early and improved fruit production must be explored to counter the impact of this tiny bacterium/vector complex that threatens a $13 billion U.S citrus industry.
The severity of HLB far exceeds that of previously known citrus diseases such as tristeza virus and bacterial canker. The Asian citrus psyllid was introduced into Brazil over 60 years ago, but was not a problem because greening was not present.
Outside of Florida, HLB has been found in Louisiana, Georgia and South Carolina. Recent neighboring countries that are added to the HLB-affected list include: Belize, Jamaica, Mexico – all in 2009 and Cuba, 2007. HLB has played a key role, along with hurricanes and an economic downturn, resulted in the loss of about 100,000 citrus acres in Florida. The Citrus Health Response Program, disease awareness, state and federal regulations and an active grower participation to manage a mighty pathogen and vector are keys for success in controlling HLB.
HLB is a prime example of how an unintentional, but illegal, plant introduction into a country can cause multi-billion dollar crop loss, unemployment and financial strain of many families. States like Texas, Arizona and California that are thus far apparently HLB-free should use all available resources to stop all illegal entries via border check points and citizen awareness.