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United States Department of Agriculture

Agricultural Research Service

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Research Project: Development and Implementation of New Control Strategies for Citrus Huanglongbing (Greening) Phase II

Location: Subtropical Plant Pathology Research

2012 Annual Report


1a.Objectives (from AD-416):
Application of new chemical therapies to eliminate huanglongbinb (HLB) bacterium from HLB-infected citrus plants.


1b.Approach (from AD-416):
Implementation of chemical control strategies by applying new device to deliver effective chemicals into citrus plants. Evaluation of the control efficacy of the treated citrus plants using polymerase chain reaction (PCR)-based detection.


3.Progress Report:

This is related to inhouse project objectives: 1. Characterize ecology, biology, epidemiology, molecular genetics, and vector and host (crop and weed) interactions of domestic, exotic, newly emerging, and re-emerging pathogens and 3. Develop or improve comprehensive integrated disease management strategies.

‘Candidatus Liberibacter asiaticus’ (CLas), the prevalent bacterial pathogen associated with citrus huanglongbing (HLB), harbors at least two prophages named FP1 (homologue to SC1) and FP2 (homologue to SC2). Due to the fastidious nature of Clas, little is known about the response of FP1 and FP2 under stress conditions. In this study, we used real time polymerase chain reaction (PCR) to investigate the variations of FP1 and FP2 in comparison with the copy number of 16S rDNA in the huanglongbing (HLB)-affected periwinkle and citrus under stress conditions. When the two-year old HLB-affected periwinkle plants were exposed to heat stress, more FP1 and FP2 were released at 42°C and 45°C than at 37°C. With a shift from 23°C to 37°C and stayed for 4h, the relative copy number of FP1 and FP2 doubled that of periwinkle samples before heat treatment. As a shift from 23°C to 42°C or 45°C and stayed for 4h, the relative copy number of FP1 and FP2 increased to more than 2.5~5-fold compared to initial samples. Meanwhile, tetracycline was used as antibiotics stress. HLB-affected citrus scions were treated with tetracycline at concentration 500ppm to 2500ppm by soaking, and then grafted to rootstock for further evaluations. When treated by tetracycline for 7h to 9h, the relative copy number of FP1 and FP2 increased to 2~3.7-fold compared to initial samples. The results indicated prophages in CLas were induced from lysogenic cycle to lytic cycle by stress (temperature up shift or antibiotics), which support our results of HLB-infected-periwinkle becoming undetectable 3 months after heat treatment. This is the first report of stress-induced variations of HLB bacterial prophages.


Last Modified: 7/31/2014
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