About this Research Topic
Citrus greening or huanglongbing (HLB) is a disease caused by the unculturable, fastidious, phloem-restrictive, Gram-negative bacteria Candidatus Liberibacter spp. There are three forms of HLB disease. The Asian form associated with Candidatus Liberibacter asiaticus is heat-tolerant and can survive up to 35°C. The African (Candidatus Liberibacter africanus) and American forms are heat-sensitive and develop between 20-25°C (Candidatus Liberibacter americanus). HLB infection is vector-transmitted by the African citrus psyllid (Trioza erytreae) and the Asian citrus psyllid (Diaphorina citri). The disease was first described in 1929, and reported in China in 1943. The African variation was reported in South Africa in 1947. The disease was reported in Brazil (São Paulo) in 2004 and the United States (Florida) in 2005. More than 17% of citrus trees in Brazil and 90% in Florida are infected, with symptomatic trees present in Texas and California. In Europe, the Mediterranean Basin the area where citrus is grown, is the only HLB free zone in the World. However, the threat in Europe is constant due to the proximity of the disease and its vectors and the unstoppable increase in international trade.
Candidatus Liberibacter asiaticus is able to infect most citrus species, cultivars and hybrids. Leaves of infected trees develop a blotchy mottle symptom (yellowing vein and an asymmetrical chlorosis). Infected branches suffer substantial leaf drop, resulting in canopy thinning. Fibrous root density decreases nearly 30%, directly affecting water and nutrient uptake, with severe impact on fruit yield and demanding more often irrigation and improved nutrition. Fruit from HLB-affected trees are often lopsided, poorly colored, and contain aborted seeds, with low commercial value due to small size and quality. The juice from affected fruit present low soluble solids content, high acidity and bitterness.
There is no cure for the disease. Current management strategies focus on either delay of infection or on management of infected trees. Delay of infection methods include removal of symptomatic trees, planting with uninfected nursery trees, protection of grove edges, and intensive monitoring and pesticide control of the vector. Management of infected trees includes soil pH, nutrition, and irrigation management based on altered tree capacities and needs when affected by HLB. Research has evolved rapidly to address this devastating challenging, and several recent alternatives based on psyllid control, bactericides, cultural practices (thermotherapy and vector exclusion in young trees by protected citrus cultivation to stop contamination), and genetic transformation have been tested. While most attempts at management have focused on a single component of the disease pyramid, most do not explicitly consider multiple elements at the same time.
This Research Topic article collection aims to present the last advances in managing this pathosystem, focusing on assessments of near-term feasible practices in the context of the vector, pathogen, host plant, and environment. We also want to hear what are the prospects and potential short-term solutions to address this threat to the worldwide citrus industry.
Keywords: Huanglongbing disease, Citrus, Candidatus Liberibacter asiaticus, Infection, Disease Menagement
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