作者：J.M. Bové
作者单位：Université de Bordeaux Ségalen. INRA Center, Bordeaux-Aquitaine (France). E-mail: joseph.bove@wanadoo.fr
摘要：The University of Florida estimates that in the State, HLB has resulted in the loss of more than 6,600 jobs, $1,3billion in revenue to growers and $3,6 billion in economic activity. Can HLB be controlled, how and when?
Answering these questions is the goal of the workshop! Two forms of HLB have to be considered: (i) hightemperature-sensitive African-HLB, with Candidatus (Ca.) Liberibacter (L.) africanus and the African CitrusPsyllid, Trioza erytreae, and (ii) high temperature-tolerant Asian-HLB, with Ca. L. asiaticus and the Asian CitrusPsyllid, Diaphorina citri (2). African-HLB is presumed to be less aggressive than Asian-HLB.
There is little or no evidence of genetic resistance to HLB in citrus. Apparently, citrus has had only a recentassociation with Liberibacter species, an association too short to have built up resistance to the bacterium.
Hence, according to a general consensus, resistance to HLB will probably be obtained by engineering, intocitrus, genes with anti-Liberibacter and/or anti-psyllid activity (8). However, such HLB-resistant cultivars willprobably not become available to the growers for several years and, in the meantime, solutions must bedeveloped to control HLB and save the existing citrus industry from destruction (1, 3, 9). Thus, for HLB control,short-term systems for “today” and long-term systems for “tomorrow” must be discussed.
Short-term systems. Even though they are called “short-term”, these systems, when efficient, will probablyhave to be used for several years. Various systems are known and listed. (i) The classic, Three-ProngedPackage (TPP) has been recommended in China since the 1950s and is comprised by (a) insecticide treatmentsagainst psyllids, (b) detection and removal of symptomatic trees, and (c) resets with disease-free trees frominsect-proof nurseries. TPP is a “preventive” control system; as it “prevents” trees from becoming infected.
Application of the TPP in S?o Paulo State, Brazil, since 2004 when HLB was first identified, has resulted in twohundred thousand hectares of citrus with less than 1% of HLB-affected trees, even in areas where the initialHLB incidence was high. In Florida, growers were reluctant to remove symptomatic trees and the TPP systemhas not gained much acceptance, except in some large citrus farms (6). Factors having a significant influenceon the efficiency of the TPP system, including restructuring, will be highlighted (1). For instance, HLB controlwas found to be easier in large farms than in smaller ones. “Regional” management overcomes this problemin Brazil. In Florida, “Area Wide Control of Psyllids” has been developed and “Citrus Health ManagementAreas” have been established (8). Ways to improve the detection of symptomatic trees (rapid field detectionmethods, hyperspectral imaging, biomarkers, etc) and the treatments against the Asian and African citruspsyllid (biological control, insecticides, application methods, prevention of insecticide resistance, flush control,overwintering sites, etc) should be discussed. (ii) Foliar sprays of micronutrients, with or without inducers ofSystemic Acquired Resistance (SAR), are also believed to give immediate protection against HLB. However,this system in which no tree removal is practiced, rapidly results in a high percentage of HLB-infected trees, inspite of good psyllid control (5, 9). The effect of nutritional treatments on HLB-control, fruit-quality, bacteriatiterand disease-progress has been challenged recently and found inconsequential (5). (iii) In the Mekongdelta of Vietnam, farmers have found that the presence of guava trees surrounding citrus trees (interplanting)prevents or at least retards HLB. Volatiles from guava repel the Asian citrus psyllid, thus explaining the guavaeffect. However, recent experiments in Vietnam have shown that efficacy of guava interplanting on HLB islimited, as the effectiveness of guava against disease invasion breaks down after one year, probably becausecitrus trees outgrow guava trees (7). (iv) Advanced Citrus Production System (ACPS). The aim of the ACPSis to shorten and enhance the citrus production cycle through high density plantings, as well as intensivecomputerized daily fertirrigation (Open Hydroponics System) for automated irrigation and intensive nutrientmanagement. (v) Additional short-term systems might be brought up from the floor and discussed.
Long-term systems. Long-term systems are still at the research and/or the experimental level, even thoughsome are already under evaluation in the greenhouse and/or the field. These systems involve the productionof genetically modified citrus plants expressing anti-Liberibacter and/or anti-psyllid genes (8). In one system,these genes are inserted into the citrus genome through classic Agrobacterium-mediated transformation.
Transformation of mature citrus tissue ensures that the resulting transgenic cultivar is true to type, butseems to be more difficult to achieve than transformation of juvenile tissue. In a second, more recent system,meant for regions where citrus tristeza virus (CTV) is endemic, the anti-HLB gene is first incorporated into thegenome of CTV, thus using the virus as a gene vector (4). Inoculation into citrus of the CTV strain carrying theant-HLB gene results in viral replication and expression of the viral proteins as well as those of the anti-HLBprotein from the anti-HLB gene.
Genes used for resistance to Liberibacter spp. have essentially been anti-bacterial peptide genes from varioussources (including plants) and genes involved in systemic acquired resistance. Recently, single-chain-fragmentvariable(SCFV) antibodies have been produced. For the first time, SCFV antibodies are now available againstalmost any Liberibacter protein. Candidate SCFV antibodies for resistance to HLB are those directed againstproteins found to be essential for Liberibacter growth and replication. Combination of the CTV gene vectorand the SCFV anti-HLB antibodies might become a most efficient anti-HLB system. Liberibacter phage forpotential control of HLB has also been proposed.
Anti-psyllid transgenic cultivars are developed to express volatile chemicals repelling the psyllids. DoublestrandedRNA corresponding to an essential psyllid gene can also be used to transform citrus cultivars,hopefully resulting in mortality and suppression of the feeding psyllids through RNA interferenceHowever, all these genetically modified plants (GMP), would have to be accepted by the consumers (fromEurope in particular) before products from such GMPs can be marketed.
References
(1) Belasque Jr. et al. 2010. Lessons from Huanglongbing management in S?o Paulo State, Brazil. Journal of Plant Protection 92: 285-302.
(2) Bové, J. M. 2009. www.ivia.es/iocv Citrus diseases, Huanglongbing, text and image gallery.
(3) Chamberlain, H.L. 2010. Importance of awareness to growers, nursery growers and residents for the control of HLB and itsvector. 2nd international workshop on Huanglongbing and Citrus Asian Psyllid, Merida, Yuc., July 2010,
(4) Folimonov, A.S. et al. 2007. A stable RNA virus-based vector for citrus trees. Virology, 368:.205-216.
(5) Gottwald, T. R. et al. 2012. Inconsequential effect of nutritional treatments on Huanglongbing control, fruit quality, bacterial titerand disease progress. Crop Protection 36:73-82.
(6) Irey, M.S. 2010. Experiences of one Florida grower with the management of Huanglongbing—Success over time. 2nd internationalworkshop on Huanglongbing and citrus Asian psyllid, Merida, Yuc., July 2010,
(7) Katsuya, Ichinose et al. 2012. Limited efficacy of guava interplanting on citrus greening disease: Effectiveness of protectionagainst disease invasion breaks down after one year. Crop Protection 34:119-126.
(8) National Research Council. 2010. Strategic Planning for the Florida Citrus Industry: Addressing Citrus Greening Disease.
Washington, DC: The National Academies Press.
(9) Timmer, L. W. et al. 2011. It’s not too late – yet. Citrus Industry, January 2011: 6, 7.
文献注录：J.M. Bové. HLB control [A]. ICC2012 Abstract [C] . 2012: 83-84.
报/刊名：《ICC2012 Abstract》，发表于2012
文献类型： [A]
所属论文集：ICC2012 Abstract
页码：第 83-84 页 / 共2 页