**4. Transmission of citrus HLB**

The graft transmitted HLB was due to a viral disease [77]. Soon afterward, similar opinions were put forward in South Africa, strengthened by the results of grafting trials showing that greening was inconsistently transmitted to healthy plants. Lin [21] confirmed that HLB was transmitted through grafting in China, thus establishing the causative agent as a pathogen. McClean and Oberholzer [78] confirmed the graft transmissibility of African greening in 1965. The pathogen is not easily transmitted to progeny trees propagated by buds from infected trees, possibly due to sieve tube necrosis and uneven pathogen distribution, but more transmission occurs if stem pieces are used. No infection could be obtained when material from apparently healthy sectors of diseased trees was used. In 1964, natural spread by exposing seedlings to insects in a HLB-affected orchard developed yellowing symptoms similar to greening [79].

#### **Figure 2.**

*Illustration of* **Candidatus** *Liberibacter virulence mechanisms in the plant.* Candidatus *Liberibacter species associated with HLB (red circles) live in phloem elements. Phloem is mainly liable for the distribution of the carbohydrate from the source to the sink. Nutrients are transmitted to the phloem either through the apoplastic pathway or the symplastic pathway.* Candidatus *Liberibacter species may secrete effector protein (sec-dependent effectors) and virulence factors (orange and blue circle, red triangles) into phloem sieve elements and companion cells to interfere with host target (genes and protein) that can cause cell necrosis, cell death, and phloem malfunction. Effectors or virulence factors may interfere with phloem organelles, such as mitochondria, plastids, or endoplasmic reticulum, to trigger cellular responses. Some effectors (SDEs) may directly or indirectly affect the expression of target genes. In addition,* Candidatus *Liberibacter species may trigger plant immune responses through pathogenassociated molecular patterns leading to cell death and callose accumulation, resulting in inhibition of phloem transportation. The presence of* Candidatus *Liberibacter species and its metabolic activity may interfere with the function of the phloem by interrupting the osmatic gradients and integrity of phloem transportation. Abbreviations: PMPs—Pathogen-associated molecular patterns; RFO—Raffinose family oligosaccharide.*

Two insect vectors are responsible for the rapid transmission of citrus HLB from Las-infected citrus to healthy citrus species, Asian citrus psyllid *D. citri* in Asia and America, and the African citrus psyllid, *Triozaerytreae* in Africa. The acquisition feeding period is 30 min or longer, and the pathogen remains latent for 3–20 days. The inoculation feeding period is 1 hour or more [80].

Asian citrus psyllid is widespread around the world and found in hot and humid conditions and lower-lying areas in China, India, Myanmar, Taiwan, Philippine Islands, Malaysia, Indonesia, Sri Lanka, Pakistan, Thailand, Nepal, Ryukyu Islands (Japan), Afghanistan, Saudi Arabia, Reunion, and Mauritius [81]. Asian citrus psyllid firstly evolved in India [82], then spread in South America in the 1940s, invading Brazil, Argentina, and Venezuela, and then invaded the West Indies (Guadeloupe), Abaco Island, Grand Bahama Island, Cayman Islands, and the USA in the 1990s. In 2001, ACP was found in the Dominican Republic, Cuba, Puerto Rico, and Texas [83, 84]. Asian citrus psyllid has been reported more recently in many new Americas, including Mexico, Costa Rico, Belize, Honduras, and the states of Alabama, Arizona, California, Georgia, Louisiana, Mississippi, and South Carolina, USA [85].

*Devious Phloem Intruder* Candidatus *Liberibacter Species Causing Huanglongbing: History… DOI: http://dx.doi.org/10.5772/intechopen.105089*

African citrus psyllid (AfCP) thrives in cool and moist temperatures, at higher areas about 100 to 500 m above sea level. And it is sensitive to excessive heat and exists in Africa from the islands of the Indian Ocean through east and central Africa to South Africa, Saudi Arabia, Yemen, the northwestern region of the Iberian Peninsula, Cameroon, Kenya, Ethiopia, Zimbabwe, Tanzania, Malawi, Galicia, northern Portugal, Swaziland, Madagascar, Rwanda/Burundi, and Reunion [86]. Psyllid populations in Africa, Saudi Arabia, and Yemen might be able to adapt and settle under a wide range of environmental conditions, such as equatorial, arid, and warm temperate climates with varying temperatures and rainfall [86].
