Relation between plant nutrition, hormones, insecticide applications, bacterial endophytes, and Candidatus Liberibacter Ct values in citrus trees infected with Huanglongbing
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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.
KeywordsBoyd’s nutritional program Ca. Liberibacter asiaticus Cycle threshold (Ct) value Induced systemic resistance (ISR) Huanglongbing (HLB) Systemic acquired resistance (SAR)
The authors would like to thank Stephanie Shea Teems for her work with real-time PCR analyses of leaf samples at Southwest Florida Research and Education Center, University of Florida. We also like to thank Debbie Jones of the Division of Plant Industries (DPI) for providing negative and positive control samples of citrus leaves and for teaching some of us the real-time qPCR techniques used at the DPI in Gainesville. We are grateful to Ganyu Gu, Hongling Er and Christinah Chiyaka for their help with sampling. We thank Ellen Dickstein for organizing soil and plant nutrient analyses at the Soil Analysis lab of the University of Florida. Funding for this research was provided by the Emerging Pathogens Institute and the Smallwood Foundation.
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