Relation between plant nutrition, hormones, insecticide applications, bacterial endophytes, and Candidatus Liberibacter Ct values in citrus trees infected with Huanglongbing

Intensive insecticide and nutrient management 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 treatment, ‘nutrition’ treatment (including systemic resistance 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 insecticides, 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 applications 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 positively 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 significant 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.