ORIGINAL RESEARCH Relation between plant nutrition, hormones, insecticide applications, bacterial endophytes, and Candidatus Liberibacter Ct values in citrus trees infected with Huanglongbing Weishou Shen & Juan M. Cevallos-Cevallos & Ulisses Nunes da Rocha & Hector A. Arevalo & Philip A. Stansly & Pamela D. Roberts & Ariena H. C. van Bruggen Accepted: 19 August 2013 / Published online: 7 September 2013 # KNPV 2013 Abstract Intensive insecticide and nutrient manage- ment have been attempted worldwide to reduce citrus huanglongbing (HLB) symptom development and yield loss. However, effects of insecticide and nutrient applications on HLB have been poorly understood. Leaf nutrients, jasmonic and salicylic acid contents, cycle threshold (Ct) values of Ca. Liberibacter asiaticus (Las), and community structure of endophytic α-proteobacteria were evaluated after insecticide treat- ment, ‘nutrition’ treatment (including systemic resis- tance inducing agents), or both in comparison with a control in a two-factor field experiment in 2008–2012. Leaf N, Mn, Zn and B significantly increased whilst Cu decreased after nutrient applications. Salicylic acid significantly increased in old leaves treated with insec- ticides, nutrients or both, and in young leaves treated with nutrients only. The jasmonic acid concentration was highest after the nutrition treatment in both old and young leaves. Ct values of Las and leaf area and weight significantly increased after long-term nutrient appli- cations in 2011 and/or 2012. Redundancy analysis of the endophytic α-proteobacteria community structure indicated that the communities were mainly separated according to nutrient applications, which were posi- tively associated with Ct values of Las and Ca, Mn, Zn, B, Mg, and Fe contents in leaf samples collected in 2012. Thus, effects of insecticides on HLB were sig- nificant in the early 2-year period whilst nutrients had significant effects on Las content and leaf size and weight after at least 3 years of application. Keywords Boyd’ s nutritional program . Ca. Liberibacter asiaticus . Cycle threshold (Ct) value . Induced systemic resistance (ISR) . Huanglongbing (HLB) . Systemic acquired resistance (SAR) Eur J Plant Pathol (2013) 137:727–742 DOI 10.1007/s10658-013-0283-7 Electronic supplementary material The online version of this article (doi:10.1007/s10658-013-0283-7) contains supplementary material, which is available to authorized users. W. Shen : J. M. Cevallos-Cevallos : U. Nunes da Rocha : A. H. C. van Bruggen (*) Emerging Pathogens Institute and Department of Plant Pathology, University of Florida, Gainesville, FL 32611, USA e-mail: ahcvanbruggen@ufl.edu W. Shen Department of Environmental Science and Engineering, Nanjing Normal University, Nanjing 210023, China J. M. Cevallos-Cevallos Centro de Investigaciones Biotecnológicas del Ecuador (CIBE), Escuela Superior Politécnica del Litoral (ESPOL), Guayaquil 090112, Ecuador U. Nunes da Rocha Lawrence Berkeley National Laboratory, Earth Sciences Division, Berkeley, CA 94720, USA H. A. Arevalo : P. A. Stansly : P. D. Roberts Southwest Florida Research and Education Center, University of Florida, Immokalee, FL 34142, USA