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Treating citrus greening with antimicrobials

citrus-greening-uf-ifas-lorcaWhat is Citrus Greening Disease?

Citrus greening disease (also known as Huanglongbing or HLB) is the major cause of crop and tree loss in many parts of Asia, Africa and, now in the United States of America. Although it has been known in China since 1890 as the yellow shoot disease, no cure for this devastating disease has been found.

The gold standard in finding antimicrobials agents to fight pathogens is the ability to culture the microorganism under controlled laboratory conditions. One of the main obstacles in advancing the search for antimicrobials against the citrus greening bacterium, Candidatus Liberibacter asiaticus (CLas), is the inability to culture CLas to perform laboratory tests. Consequently, scientific progress in understanding the physiology of this bacterium has been limited. An important step toward understanding the biology of CLas was the sequencing of its genome, which took place here at the University of Florida. Unfortunately, bioinformatic analyses of the genome sequence did not reveal any relevant answers.

USDA-Funded Project Summary

In previous experiments, a biochemical approach was used to identify chemicals that inhibit critical proteins in the bacterium CLas. Since these proteins are involved in biological processes required by the bacteria to survive in the citrus host, its inactivation resulted in effective treatment of the disease. Using this approach, new chemicals were identified capable of curing CLas in infected seedlings maintained in a greenhouse.

The goal of this research is to develop an antimicrobial treatment to cure citrus greening in the field. We have designed a plan to efficiently carry out treatments, thoroughly evaluate results on a laboratory scale, and quickly translate treatment discoveries to groves in production. We will achieve this through three objectives:

  1. Optimization of antimicrobial treatment in infected citrus seedlings. Infected citrus seedlings will be used to determine the optimal dosage, dosing intervals, and the elimination rate for each compound, in a controlled environment. Seedlings will also be monitored for symptoms of plant toxicity throughout the treatment period.
  2. Field trials: application of the antimicrobial treatment and evaluation of environmental impact. Mature HLB-infected trees will be treated in actively producing groves. Each compound will be analyzed for effectiveness in bacterial clearance and potential environmental impacts.
  3. Identify and evaluate the antimicrobial activity of natural compounds that are chemically similar to the two effective compounds we have already been identified. The significance is twofold: it may allow a faster translation of chemicals into field trials and offer a plausible treatment option for organic growers.

Economical Assessment

An economic analysis will include two primary activities: First, a traditional cost-benefit analysis of the compounds will be conducted to determine which compound has the highest net benefit per tree. The second activity will involve the construction of a dynamic bioeconomic model to determine the optimal (profit-maximizing) use of each treatment in the field.

Potential Impacts of the Project

An effective antimicrobial treatment will have an immediate positive impact on the economic and social aspects of the citrus industry. The citrus industry, directly or indirectly, involves a large network of workers and companies currently operating in the state of Florida. Because the citrus industry represents a large portion of the Florida economy, the recovery of infected groves will have a substantial, positive impact on the economy in Florida as a whole.

Related Articles and Links

Pagliai F.A., Gonzalez C.F., Lorca G.L. 2015. Identification of a Ligand Binding Pocket in LdtR from Liberibacter asiaticus. Frontiers in Microbiology. 6:1314. doi: 10.3389/fmicb.2015.01314. eCollection 2015.

Pagliai, F.A., Gardner Ch., Bojilova, L., Sarnegrim, A., Tamayo, Ch., Potts, A.H., Teplitski, M., Folimonova, S.Y., Gonzalez, C.F., Lorca, L. (2014) The transcriptional activator LdtR from ‘Candidatus Liberibacter asiaticus’ mediates osmotic stress tolerance. Plos Pathogens. 24;10(4):e1004101. PMID: 24763829