Page Banner

United States Department of Agriculture

Agricultural Research Service

Related Topics

Research Project: Emerging Diseases of Citrus, Vegetables, and Ornamentals

Location: Subtropical Plant Pathology Research

2013 Annual Report


1a.Objectives (from AD-416):
1. Characterize ecology, biology, epidemiology, molecular genetics, and vector and host (crop and weed) interactions of domestic, exotic, newly emerging, and re-emerging pathogens.

1a. Characterize the etiology, molecular biology and genetics of ‘Candidatus Liberibacter asiaticus (Las),’ the bacterium associated with citrus huanglongbing (HLB).

1b. Molecular characterization, vector interactions and/or epidemiology of Groundnut ringspot virus (GRSV), Squash vein yellowing virus (SqVYV) and other viruses of vegetables, ornamentals, and weeds, and Xanthomonas fragariae [causing angular leaf spot (ALS) on strawberry].

1c. Characterize meteorological components affecting the epidemiology of Asiatic citrus canker (ACC), the interaction of the Asian leaf miner with ACC, the Asian citrus psyllid with HLB, the interaction of whiteflies with SqVYV, and the interaction of thrips with GRSV.

2. Develop/refine rapid, sensitive reliable detection/sampling methods for pathogens.

2a. Develop improved detection methods for GRSV, Las and Xanthomonads on citrus and strawberry.

2b. Develop new and augment existing surveillance methods and protocols for HLB, Xanthomonas citri subsp. citri on citrus, and the other newly introduced citrus diseases such as citrus black spot (CBS) and sweet orange scab (SOS). 3. Develop or improve comprehensive integrated disease management strategies.

3a. Develop and use stochastic models to test various disease control strategies for HLB, ACC, CBS and diseases caused by other exotic pathogens.

3b. Develop and implement the most efficacious strategies for disease management of HLB, Xanthomonads of citrus and strawberry, CBS, Plum pox virus (PPV), and viruses of vegetables and ornamentals.


1b.Approach (from AD-416):
The overall approach is to thoroughly characterize new exotic and emerging plant pathogens at multiple levels: epidemiologically epidemics will be followed and modeled by traditional and newer stochastic methods at the regional, and plantation levels, biologically the pathosystems will be characterized at the level of host-pathogen-vector interaction, as well as at the cellular, molecular and/or biochemical levels. New pathogens will be identified and characterized by molecular biological and traditional cultural methods. Recombinant DNA and genomics technologies will be applied to study host/pathogen interactions and to investigate virulence differences between strains of a pathogen.


3.Progress Report:
Progress made on all three objectives. Under sub-objective 1a: Progress on culture of Candidatus Liberibacter asiaticus (Las) in vitro was made. Genetic diversity of Las was demonstrated with molecular markers to differentiate pathogenic vs. non-pathogenic and psyllid- vs. non-transmissible isolates. Two novel autotransporters and a flagellin of Las and Ca. L. solanacerum were identified and further characterized. Studies on HLB effects on seedling growth and vitality are ongoing. Psyllid transmissions of Las to different citrus varieties are nearly complete. Resistance studies in Murraya paniculata were initiated. Purification of Las from plant and insect tissues is ongoing. Metabolomic analyses to identify compounds associated with early Las infection are ongoing. Under sub-objective 1b: Transmission of tospoviruses and Squash vein yellowing virus (SqVYV) is being elucidated. Host range and genetics of tospoviruses were investigated. GFP-transformed X. fragariae was developed and being used in crown-infection studies. Under sub-objective 1c: Plots to test the interaction of citrus canker (ACC), leafminer, and wind breaks were established in Brazil in 2010 and data collection continues. Packinghouse studies have demonstrated ACC survival is poor and declines in mature fruit. Meteorological, insect count, and virus incidence data continue to be collected in cucurbit/solanaceous crops. Under sub-objective 2a: A new detection kit for Las was developed and optimized, reducing diagnostic time for Las, and is being commercialized for field use. Sensitivity/specificity of multiple virus detection assays are being evaluated. PMA-PCR protocol was improved through adoption of long-amplicon qPCR. Under sub-objective 2b: Surveillance methods were adapted for statewide sweeps for HLB and its vector and CBS for Florida and deployed. Risk-based residential survey methods for ACP and HLB were completed, deployed to California, Texas and Arizona, and validation begun. Surveys used by regulatory agencies and commodity groups to target disease/vector hotspots for existing HLB and predict new outbreaks locations. Under sub-objective 3a: Both ACC and HLB stochastic models were developed, validated and publications submitted or in draft. A user-friendly front end to both canker and HLB models was completed for use by regulatory agencies and commodity groups. AgScouter interface was improved. Smartphone applications for iPhone and Android were made publicly available. Subsequent development will focus on field-specific, pest management recommendations. Under sub-objective 3b: Tests using guava as an intercrop between citrus trees as a mitigation strategy for HLB are completed and a publication is in draft. A CBS probabilistic risk model is in the 2nd year of development and will be important to citrus international trade to determine if fruit are a pathway for disease establishment in new locations. Several compounds were effective for control of HLB in greenhouse trials. Heat treatment cured citrus plants in greenhouse settings, providing a simple, effective method for Las control; promising results were obtained in field trials.


Review Publications
Hoffman, M.T., Doud, M.S., Williams, L., Zhang, M., Ding, F., Stover, E., Hall, D., Zhang, S., Jones, L., Gooch, M., Fleites, L., Dixon, W., Gabriel, D., Duan, Y. 2013. Heat treatment eliminates ‘Candidatus Liberibacter asiaticus’ from infected citrus trees under controlled conditions. Phytopathology. 103:15-22.

Adkins, S.T., Mccollum, T.G., Albano, J.P., Kousik, C.S., Baker, C.A., Webster, C.G., Roberts, P.D., Webb, S.E., Turechek, W. 2013. Physiological effects of Squash vein yellowing virus infection on watermelon. Plant Disease. doi:10.1094/PDIS-01-13-0075-RE.

Acevedo, V., Rodriques, J.C., Estevez De Jensen, C., Webster, C.G., Adkins, S.T., Beaver-Wessel, L. 2013. Squash vein yellowing virus affecting watermelon in Puerto Rico. Plant Disease. doi:10.1094/PDIS-03-13-0322-PDN.

Webster, C.G., Kousik, C.S., Turechek, W., Webb, S.E., Roberts, P.D., Adkins, S.T. 2013. Squash vein yellowing virus infection of vining cucurbits and the vine decline response. Plant Disease. doi:10.1094/PDIS-01-13-0076-RE.

Last Modified: 9/2/2014
Footer Content Back to Top of Page