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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

2013 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 research relates to inhouse project objective: 1. Characterize ecology, biology, epidemiology, molecular genetics, and vector and host (crop and weed) interactions of domestic, exotic, newly emerging, and re-emerging pathogens.

We have investigated the mechanism that may be controlling how the tree and the bacteria respond to the heat and chemicals. At this time, two prophages have been identified in the Candidatus Liberibacter asiaticus (Las) genome. We have investigated if stress conditions such as heat and antibiotics can cause the conversion of the FP1 and FP2 prophages to phage (activation of the lytic cyle) by comparing the 16S rDNA copy number in HLB-affected periwinkle and citrus with the copy numbers of the two prophages. When HLB-affected periwinkle was exposed to heat stress for 4.0 hours, more FP1 and FP2 phage particles were released at 42°C and 45°C than at 37°C. A temperature increase from 23°C to 37°C caused the relative copy numbers of FP1 and FP2 to increase six-fold, while a shift from 23°C to 42°C or 45°C caused the relative copy numbers of FP1 and FP2 to increase between 7.5 and 15-fold compared to the initial samples. Meanwhile, similar results were found when HLB-affected citrus scions were treated with tetracycline at concentrations of 500 ppm to 2000 ppm by soaking for three days. These results indicate a shift from the lysogenic to the lytic cycle. Understanding this mechanism may have applications in terms of modulating HLB populations in naturally occurring infections and may also lead to potential control methods that exploit this conversion. A manuscript is in preparation.


Last Modified: 10/23/2014
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