Source: UNIVERSITY OF FLORIDA submitted to NRP
UNDERSTANDING AND MANAGING CANDIDATUS LIBERIBACTER ASIATICUS INFECTION AND ITS EFFECTS ON ROOT HEALTH
Sponsoring Institution
National Institute of Food and Agriculture
Project Status
COMPLETE
Funding Source
HATCH
Reporting Frequency
Annual
Accession No.
1003042
Grant No.
(N/A)
Cumulative Award Amt.
(N/A)
Proposal No.
(N/A)
Multistate No.
(N/A)
Project Start Date
Jun 11, 2014
Project End Date
Apr 30, 2019
Grant Year
(N/A)
Program Code
[(N/A)]- (N/A)
Recipient Organization
UNIVERSITY OF FLORIDA
G022 MCCARTY HALL
GAINESVILLE,FL 32611
Performing Department
Citrus Research and Education Center, Lake Alfred
Non Technical Summary
The citrus industry contributes nine billion dollars to the Florida economy. This provides thousands of jobs for Floridians. It also provides a nutritious alternative to sweetened beverages that is part of a healthy balanced diet. Along with these economic and health benefits, it contributes to food security by maintaining a diverse agricultural system in the United States. Recently this $9 billion agricultural industry has been threatened by a devastating systemic disease of citrus trees called Huanglongbing (HLB) caused by the bacterium Candidatus Liberibacter asiaticus (Las). This disease reduces yield and eventually makes citrus trees unproductive. The ideal solution for Huanglongbing is to prevent trees from becoming infected; however, citrus groves in Florida are rapidly approaching 100% infection. Advances in control of the psyllid vector, Diaphorina citri (Asian citrus psyllid; ACP), are continuing to improve the ability of growers to reduce infection in new plantings. This provides hope for the future, but current citrus production is dropping rapidly because of fruit drop caused by HLB in existing mature citrus groves. To maintain current citrus production, needed to keep juicing plants and packing houses open, growers require methods to extend the productive life of existing HLB affected trees.It has recently been discovered that HLB halves the amount of roots on a citrus tree before the grower knows it is infected. This project aims to discover how the disease is causing this root loss, how it interacts with other root diseases, and possible methods for improving the root mass through chemical treatments or rootstock selections. It will also determine which plant tissue works best for early diagnosis of disease. This project will provide valuable information for maintaining citrus health, with an emphasis on root health, in existing HLB infected groves to prolong their productive life. It will also provide additional value by assisting detection and rogueing efforts in new plantings and providing targets for transgenic approaches to disease control.
Animal Health Component
30%
Research Effort Categories
Basic
40%
Applied
30%
Developmental
30%
Classification

Knowledge Area (KA)Subject of Investigation (SOI)Field of Science (FOS)Percent
2120999110060%
2020999110020%
2160999116020%
Knowledge Area
212 - Pathogens and Nematodes Affecting Plants; 216 - Integrated Pest Management Systems; 202 - Plant Genetic Resources;

Subject Of Investigation
0999 - Citrus, general/other;

Field Of Science
1160 - Pathology; 1100 - Bacteriology;
Goals / Objectives
The major goal of this project is to develop an understanding of how the disease huanglongbing (HLB) affects citrus root systems and how we can prevent or manage this damage to improve citrus production in the presence of endemic HLB.1. Understand how Candidatus Liberibacter asiaticus causes root loss in citrus.2. Characterize the interactions between HLB and soil-borne pests and pathogens in increasing citrus root loss.3. Develop management strategies for maintaining tree productivity with an emphasis on root health in HLB affected citrus trees.4. Improve early detection and diagnosis of HLB affected trees for rapid rogueing in new citrus plantings
Project Methods
1. Understand how Candidatus Liberibacter asiaticus causes root loss in citrus. The cause of early HLB associated root loss remains unknown. Root growth and root dieback will be quantified throughout the growing season in the field through root cages and soil cores adapted from previously described methods (Graham, 1995). Complementary greenhouse studies will also be performed where gene expression and phytohormone analysis can be monitored in response to root growth and dieback in a controlled setting using rhizotrons. Phytohormone and gene expression will be assayed from root tips to structural roots to determine in which part(s) of the root system the major changes are occurring. Based on root densities and root growth measured in rhizotrons and the field, we will target our mechanistic studies to the most likely causes by examining different signalling pathways for programmed cell death and necrosis or phytohormones. Once the mechanism of loss is identified, the bacterial causes of the mechanism will be pursued with transgenic approaches due to the inability to culture Las.This information will be published and presented at scientific conferences to cause a change in knowledge among the plant pathology research community. The results will also be used to guide research in objective 2-4, which will provide more direct outputs to the citrus grower communities.2. Characterize the interactions between HLB and soil-borne pests and pathogens in increasing citrus root loss. The ability of Las infection to increase the damage and infection by other root infesting pests such as Phytophthora and nematodes will be studied in controlled greenhouse trials. The effects of preexisting infestations of these pests on the movement and early damage caused by Las will also be investigated. Where appropriate, fungicide or nematicide control measures to reduce any synergistic damage between these organisms and Las will be tested in greenhouse and/or field trials. Successful control will be assessed through root density and yield responses to treatments.Results of this work will be communicated to growers at extension events through presentations, handouts, and posters.3. Develop management strategies for maintaining tree productivity with an emphasis on root health in HLB affected citrus trees. Based on the results from objective 1 and 2, management strategies will be developed to prolong productivity of HLB-affected trees. The effects of traditional treatments for Phytophthora and other citrus pests on managing the interactions with HLB will first be studied in the greenhouse. Those treatments that show promise will then be field tested in the grove. The quantity and quality of yield will be assayed along with root health through these treatments to assess the value and cost effectiveness of the management scheme. The results of seasonal changes in root flushing and root dieback will be correlated with environmental factors to identify possible cultural and horticultural measures that may improve HLB-affected root health. As other promising HLB management strategies are developed, their effects on root health will be studied. Citrus breeders at UF have developed many rootstocks that may show promise in improving yield and productive life of citrus trees affected by HLB. The ability of these rootstocks to mitigate HLB associated early root loss will be investigated in both field and greenhouse assays. The most effective methods will likely be tolerant rootstocks or transgenic rootstocks. Transgenic rootstocks would either need to be resistant to Las induced damage or resistant to infection by Las. Target genes for transgenic tolerance will be identified in objective 1 and known antimicrobial genes tested in scions will be tested for root infection tolerance or resistance based on higher root density and/or lifespan. Because canopy infection plays a key role in later stage root loss, both grafted and whole tree experiments will be conducted. The results of rootstock trials will be provided to citrus growers through extension meetings via presentations, handouts, and posters.4. Improve early detection and diagnosis of HLB affected trees for rapid rogueing in new citrus plantings. To properly detect Las in trees shortly after initial infection it is important to identify Las is distributed through the tree early after infection and the seasonal variations in this distribution. The most effective method currently available for tracking and quantifying Las is qPCR. However, qPCR is only as effective as the DNA extraction methods used on the tissue. Leaf tissue is rapidly and easily extracted with kits that remove PCR inhibiting compounds. However, other tissues have significant inhibitory compounds that are not removed using standard DNA extraction methods. Modifications to specialty DNA extraction kit protocols will be made to create consistent and efficient methods of extracted Las DNA from various citrus tissues. Where possible these methods will be designed to allow high throughput automation of the extraction method. These DNA extraction methods will be used with existing protocols to determine the tissues that provide the earliest, most consistent detection of Las in citrus orchards and to characterize the degree of infection sectoring in these tissues to develop recommended sampling for early detection in suspect trees. These methods will be communicated to growers and diagnostic labs through extension meeting presentations, handouts and posters. We will provide training to diagnostic groups that request to come to the lab and learn the techniques.

Progress 10/01/17 to 09/30/18

Outputs
Target Audience:The main audience of this project is citrus growers. Results and outcomes of the research were translated into 3 extension talks and updates to 2 citrus production guide chapters. The scientific community was reached through 3 peer reviewed journal articles and 1 conference presentation. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided?One visiting PhD. student from Brazil completed a research project in my lab and has presented this work at a national conference after returning as a postdoctoral associate to continue her work in my lab. Two posdoctoral associates have learned new techniques in pursuance of this research and have continued to improve their written English skills. How have the results been disseminated to communities of interest?The results of this project have been presented in 3 grower talks and updates to 2 citrus production guide chapters to provide citrus growers with the most up to date scientific information. 3 scientific papers and 1 scientific presentation at a national conference have helped share findings with the research community. What do you plan to do during the next reporting period to accomplish the goals?We will repeat greenhouse experiments to confirm the results on systemic effects of Liberibacter on the root system and improve our analysis of root dynamics obtained from rhizotrons.

Impacts
What was accomplished under these goals? Prior to this research project, there was little focus or knowledge on the effects of huanglongbing (HLB or citrus greening) on the root system. This project has demonstrated that it is one of the major contributors to tree decline and crop loss. Curent work is aimed at understanding the mechanism of root loss and how the bacteria moves within the tree to spread infection and act as reservoirs for reinfection of treated tissues. This will allow for knowledge based development of citrus root health management techniques on HLB affected trees to economically improve tree health and productivity. Objective 1. This year we fully utilized the split root rhizotron system to study the local and systemic effects of CLas on citrus roots. Based on past work showing rapid lateral movement we developed aninoculation system that allows us to keep the infection isolated in half the root system for about 8-9 months (until the spring flush pulls bacteria up through the phloem into the canopy and the rest of the roots. This allowed us to document through biweekly images of the roots that both infected and uninfected roots on the same tree dieback as early as 6 weeks after inoculation, which is about the time the bacteria is detectable after inoculation. This suggests a systemic signal is causing root dieback. Additionally, root growth was stimulated in the non-inoculated roots less than 2 weeks after inoculation of the other half of the root system. This shows rapid systemic signalling in HLB plants leading to systemic root system damage, unlike the sectored symptoms observed in the canopy. Objective 2. Much of the work on the HLB-Phytophthora interaction was published this year in 2 peer-reviewed journal articles. Additional greenhouse experiments suggest that chemical management of Phytophthora is less effective on HLB-affected trees. This probably results from reduced pesticide uptake by the root system. Objective 3. The focus on managing root loss shifted to maximizing function of the limited root system based on results in objective 1. This has led to spoon feeding approaches of irrigation and fertilizer management where benefit has been observed by multiple growers in tree response. Objective 4. No additional findings were made this year in early detection. Infection is more consistently detected in roots than leaves of symptomless trees, but is labor intensive. Past work has confirmed that a key piece of missing information is how long after a psyllid inoculation in a single flush, pathogen can be detected in the roots. If this is more than 1-2 months then it would be more effective to use other methods that depend on the systemic signalling that causes root dieback instead of the bacteria itself. This will require substantially more mechanistic understanding of root loss.

Publications

  • Type: Journal Articles Status: Published Year Published: 2018 Citation: J. Wu, E. G. Johnson, K. M. Gerberich, D. B. Bright, J. H. Graham. 2018, Contrasting canopy and fibrous root damage on Swingle citrumelo caused by Candidatus Liberibacter asiaticus and Phytophthora nicotianae. Plant Pathology. 67(1):202209.
  • Type: Journal Articles Status: Published Year Published: 2018 Citation: M. Young, A. Ozcan, M. E. Myers, E. G. Johnson, J. H. Graham, S. Santra. 2018. Multimodel generally recognized as safe ZnO/nanocopper composite: A novel antimicrobial material for the management of citrus phytopathogens. Journal of Agricultural and Food Chemistry. 66(26):6604-6608
  • Type: Journal Articles Status: Published Year Published: 2018 Citation: J. Wu, F. M. Alferez, E. G. Johnson, J. H. Graham. 2018. Up regulation of PR1 and less disruption of hormone and sucrose metabolism in roots is associated with lower susceptibility to Candidatus Liberibacter asiaticus of the citrus rootstock Swingle citrumelo than Cleopatra mandarin. Plant Pathology. 67(6):1426-1435
  • Type: Journal Articles Status: Published Year Published: 2017 Citation: E. G. Johnson, M. M. Dewdney, S. Santra, J. H. Graham, M. Danyluk. 2017. Zinkicide: A nanotherapeutic for HLB. MISA (Materials Innovation for Sustainable Agriculture) Symposium, University of Central Florida, Orlando, Florida, November 6-7, 2017.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2018 Citation: J. D. Veloso Dos Santos, M. M. Murata, K. Gerberich, D. B. Bright. E. G. Johnson. 2018. Limited movement of Candidatus Liberibacter asiaticus in split-root citrus provides a model system for local and systemic effects of Huanlgongbing. Phytopathology. 108(10): S.127


Progress 10/01/16 to 09/30/17

Outputs
Target Audience:The main audience of this project is citrus growers. Results and outcomes of the research obtained so far were translated into 2 extension posters and 8 presentations at grower events. Additionally, experiments and information related to this project was shown and described to 1 adult outreach tour group. The scientific community is also an important audience, to share findings and receive feedback that both serve to expedite research and develop solutions for growers. Research associated with this project was presented at regional (1 presentation), national (3), and international (2) scientific conferences. There was also 1 research journal article and 1 Masters student thesis published based on research associated with this project. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided?One Master's student completed his degree ?with research associated with this project. The PhD student and a postdoc had the opportunity to develop written and oral communication skills through the writing of their dissertation and thesis, the publication of one journal article and presentation at national scientific conferences. A visiting PhD student from Brazil was introduced to new techniques to study pathogen effects on citrus and provided the opportunity to improve their written and spoken English skills. Two postdoctoral associates working on this project in my lab had the opportunity to develop expertise with new techniques, especially microscopy, that has broadened their understanding of important research techniques and how the limitations of the techniques need to be considered in experimental design. How have the results been disseminated to communities of interest?The results of this project have been presented at 8 grower meetings with management recommendations based on our current knowledge, 2 posters at UF-IFAS extension displays. Results from this project have been incorporated into appropriate chapters of the Florida Citrus Pest Management Guide that is updated yearly. One scientific paper and multiple scientific presentations at national and regional conferences have helped share findings with the research community. What do you plan to do during the next reporting period to accomplish the goals?New greenhouse experiments have been initiated to identify key timings in root loss related to infection to develop optimal sampling times to investigate root loss mechanism. Much fo the current work has been limited by untargeted root sampling, this will hopefully be solved by the new experiment with the updated root tracing protocol.

Impacts
What was accomplished under these goals? Prior to this research project, there was little focus or knowledge on the effects of huanglongbing (HLB or citrus greening) on the root system. This project has demonstrated that it is one of the major contributors to tree decline and crop loss. Curent work is aimed at understanding the mechanism of root loss and how the bacteria moves within the tree to spread infection and act as reservoirs for reinfection of treated tissues. This will allow for knowledge based development of citrus root health management techniques on HLB affected trees to economically improve tree health and productivity. 1. We confirmed that the baceteria moves rapidly around the trunk making systemic infection very rapid in the direction of phloem sap flow. This finding emphasizes the need to minimize tree stress to reduce the reversals of phloem flow to feed new leaf flushes, flowering, or fruit set. Through microscopy of fibrous roots with fluorescent stains specific to certain organelles involved in programmed cell death, we identified HLB specific changes in root physiology and anatomy that suggest the roots of HLB-affected trees are aging faster than healthy trees. However, this suggests that there is no specific triggering event of root dieback. Attempts to detect differences in phytohormones important for root development have not detected disease specific differences, suggesting that the root growth stimulation and dieback is not directly related to phytohormone changes. 2. The initial findings on the interaction of Phytophthora and HLB were published in the Journal of Citrus pathology that demonstrated that HLB-affected roots exude more sugars into the soil environment attracting Phytophthora zoospores. However, in favorable environments, it was found that the damage caused by HLB or Phytophthora alone was the same as the damage of the two together. There is some indication that in the perennial field setting with cycling of favorable and unfavorable environments for Phytophthora that this may speed the decline of the root system each year compared to either disease alone. Ongoing experiments to study the underlying gene expression responsible for the root exudation was performed and is being prepared for publication. 3. Results from objective 1 have confirmed that HLB root loss will be difficult to manage without understanding the molecular events leading to dieback and developing gene edited or transgenic rootstocks. This reinforced the need to adjust grove management practices to provide fertilizer and water in regular small doses to match the uptake capacity of the trees rather than occasional large doses that the tree cannot absorb at one time and will likely leach into ground and surface water without being used by the crop. 4. No significant findings in early detection ofCandidatus? Liberibacter asiaticus (CLas) in infected trees were found this year although the Masters student work on lateral movement showed that detection of CLas in any given root sample is not dependent on the location in the canopy where initial infection occured.

Publications

  • Type: Conference Papers and Presentations Status: Published Year Published: 2017 Citation: K. M. Gerberich, S. L. Commerford, R. Ehsani, M. M. Dewdney, E. G. Johnson. 2017. Root response of Huanglongbing-affected citrus trees to steam thermotherapy. Phytopathology S5:49
  • Type: Conference Papers and Presentations Status: Published Year Published: 2017 Citation: J. Wu, F. Alferez, E. G. Johnson, J. H. Graham. 2017. Hormone cross-talk in the interaction between Candidatus Liberibacter asiaticus; and citrus roostock. Phytopathology S5.85.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2017 Citation: N. Thapa, S. L. Commerford, R. Ehsani E. G. Johnson, M. M. Dewdney. 2017. Canopy and root response of HLB-affected citrus trees to steam-generated thermotherapy. Phytopathology S5.51.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2017 Citation: J. Orrock, H. Davis, E. G. Johnson. 2017. Rapid lateral movement between sieve tubes by Candidatus Liberibacter asiaticus in split root trees. International Research Congress on Huanglongbing. Orlando, FL.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2017 Citation: N. Thapa, S. L. Commerford, R. Ehsani, E. G. Johnson, M. M. Dewdney. 2017. Canopy and root response of HLB-affected citrus trees to steam-generated thermotherapy. International Research Congress on Huanglongbing. Orlando, FL.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2017 Citation: Johnson, E. G. 2017 Citrus Huanglongbing in the roots: the hidden half of a systemic disease. Georgia Association of Plant Pathology. March 8, 2017
  • Type: Journal Articles Status: Published Year Published: 2017 Citation: J. Wu, E. G. Johnson, D. B. Bright, K. M. Gerberich, J. H. Graham. 2017. Interaction between Phytophthora nicotianae and Candidatus Liberibacter asiaticus damage to citrus fibrous roots. Journal of Citrus Pathology, 4(1):1-7.
  • Type: Theses/Dissertations Status: Published Year Published: 2016 Citation: J. Orrock. 2016. SYSTEMIC INFECTION OF SPLIT ROOT TREES BY CANDIDATUS LIBERIBACTER ASIATICUS SUGGESTS RAPID LATERAL MOVEMENT BETWEEN PHLOEM SIEVE TUBES. Thesis (M.S.) University of Florida.


Progress 10/01/15 to 09/30/16

Outputs
Target Audience:The main audience of this project is citrus growers. Results and outcomes of the research obtained so far were translated into 3 extension posters, a short video, and 6 presentations at grower events. Two trade journal articles describing results from this project were published. Additionally, experiments and information related to this project was shown and described to 1 high school and 1 elementary school tour group. The scientific community is also an important audience, to share findings and receive feedback that both serve to expedite research and develop solutions for growers. Research associated with this project was presented at regional (2 presentation), national (5), and international (2) scientific conferences. Changes/Problems:In our split root study of lateral movement of Liberibacter around the citrus tree for systemic spread, we left out important controls and realized the importance of seasonality even in greenhouse studies. In future studies we will take more care to make sure seasonality is accounted for and that the important controls will not be left out due to space limitations. What opportunities for training and professional development has the project provided?One PhD student, one Master's student, and 1 undergraduate had the opportunity to present their research and regional or national conferences training them in professional writing and presentation skills. All three received one on one training in multiple techniques and experimental design as they pursued their disstertation, thesis, and undergraduate thesis projects on HLB-Phytophthora interactions, pathogen movement into the root system, and pathogen populations within different tree phloem/bark tissues. How have the results been disseminated to communities of interest?The results of this project have been presented at 6 grower meetings with management recommendations based on our current knowledge, 3 posters and a video at UF-IFAS extension displays. Additionally results from this projecte have been incorporated into appropriate chapters of the Florida Citrus Pest Management Guide that is updated yearly. Multiple scientific presentations at international, national, and regional scientific conferences have helped share the knowledge gained with the citrus research community. What do you plan to do during the next reporting period to accomplish the goals?As ongoing field and greenhouse experiments are completed, we plan to design and implement new HLB-root rhizotron projects based on the findings of the current projects to expand our knowledge of Huanglongbing and root health management in HLB endemic conditions.

Impacts
What was accomplished under these goals? Prior to this research project, there was little focus or knowledge on the effects of huanglongbing (HLB or citrus greening) on the root system. This project has demonstrated that it is one of the major contributors to tree decline and crop loss. Current work is aimed at understanding the mechanism the bacteria uses to cause this root loss and the movement of the bacteria within the tree to develop and implement appropriate citrus root health management techniques. This will improve tree health and productivity while reducing unnecessary costs to citrus growers by targeting the most effective management strategies. 1. We confirmed in field based studies that root growth is stimulated by Candidatus Liberibactyer asiaticus infection. This means the root loss occurs from reduced lifespan. We worked to develop in field root monitoring techniques for citrus using minirhizotron tubes installed under citrus trees for monthly monitoring of root growth and dieback, allowing the tracking of individual roots throughout their effective life. This will improve seasonal management of root health. We also identified that as decline continues, root loss increases to >70%, which has implications for fertilization and water management of HLB-affected citrus trees. 2. A PhD student continued work on understanding how HLB increases Phytophthora root rot to provide better management tools for growers. The reduced efficacy of oomycete fungicides against Phytophthora in HLB-affected plants has led to a warning about benefit-cost of Phytophthora management in grove settings. 3. Results from objective 1 have demonstrated that the nutrient and water uptake capacity of trees in Florida groves are severely reduced and that frequent applications of small doses is required to improve total nutrient and water uptake by the trees. This message has been shared with growers at extension events and in trade journal publications. Evidence continues to build that pH management to optimize soil conditions to rootstock preference reduces the root damage that HLB causes. 4. Seasonal sampling of roots and leaves has demonstrated that Liberibacter asiaticus follows carbohydrate flow during flushes and that sampling for qPCR should be done with a close eye on the phenology of tissue growth to optimize the chances of sampling infected tissues. We also characterized the bacterial distribution in the bark phloem from the top of the canopy through the branches and trunk down into the roots.

Publications

  • Type: Conference Papers and Presentations Status: Published Year Published: 2016 Citation: J. Graham, E. Johnson, K. Gerberich, D. Bright. 2016. Fertigation and soil acidification sustain root density of huanglongbing-infected trees in Florida citrus groves. Phytopathology 106:S12 p. 77
  • Type: Conference Papers and Presentations Status: Published Year Published: 2016 Citation: S. Commerford, K. Gerberich, P. Rajasekaran, M. Young, S. Das, J. Graham, S. Santra, E. Johnson. 2016. Citrus canker as a bioassay for systemic bactericidal activity of zinc nanoparticles. Phytopathology 106:S12 p.62.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2016 Citation: N. Thapa, S. Commerford, R. Ehsani, E. Johnson, M. Dewdney. 2016. Canopy and root response of HLB-affected citrus trees to steam-generated thermotherapy. Phytopathology 106:S12 p.77.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2016 Citation: H. Davis, J. Orrock, E. Johnson. 2016. Distribution of Candidatus Liberibacter asiaticus within bark tissue of mature infected citrus trees. Phytopathology 106:S2 p. 8
  • Type: Conference Papers and Presentations Status: Published Year Published: 2016 Citation: J. Orrock, E. Johnson. 2016. Systemic infection of split roots by Candidatus Liberibacter asiaticus suggests rapid movement between phloem sieve tubes. Phytopathology 106:S2. p. 12.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2016 Citation: J. Orrock, E. Johnson. 2016. Systemic infection of split root trees by Candidatus Liberibacter asiaticus suggests rapid movement between phloem sieve tubes. Phytopathology: 106:S12 p. 19-20.
  • Type: Other Status: Published Year Published: 2015 Citation: E. Johnson, J. Grosser, J. Graham. Oct. 2015. Rootstocks and HLB: Whats happening below ground? Citrus Industry. 96:10
  • Type: Other Status: Published Year Published: 2016 Citation: E. Johnson, R. Bassanezi. 2016. HLB in Brazil: Whats working and what Florida can use. Citrus Industry. 97:6 p. 14-16.
  • Type: Other Status: Published Year Published: 2016 Citation: R. Ehsani, M. Dewdney, E. Johnson. 2016 Controlling HLB with thermotherapy: What have we learned so far? Citrus Industry. 97:9 p. 26-28.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2016 Citation: E. G. Johnson, J. Orrock, S. L. Commerford, N. Thapag, M. M. Dewdney. 2016. Liberibacter superhighway between canopy and roots: Implications for huanglongbing management. International Citrus Congress. W9-447.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2016 Citation: E. G. Johnson, J. H. Graham, J.W. Grosser. 2016. Root infection and damage: Rootstock and canopy responses. International Citrus Congress. W9-446


Progress 10/01/14 to 09/30/15

Outputs
Target Audience:The main target audience of this is citrus growers. Results and outcomes of this research obtained so far were translated into 3 extension posters, 2 trade journal articles and 3 presentations at grower events. Additionally, one oral presentation was given at the American Phytopathological Society national meeting and 3 oral and 1 poster presentations at the International Research Congress on Huanglongbing to share information with other researchers working in this field and receive useful feedback to expedite and improve our work on managing HLB and root health issues. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided?One PhD student and one Master's student have received one on one training in multiple techniques and experimental design as they pursue their thesis and dissertation projects on pathogen movement into root systems and HLB-Phytophthora root rot interactions. An undergraduate student intern has gained experimental design and professional writing experience from their undergraduate research project. How have the results been disseminated to communities of interest?The results of this project have been presented at 7 grower meetings with management recommendations based on our current knowledge and with 4 posters at UF-IFAS extension displays. Additionally, results from this project have been incorporated into the appropriate chapters of the Florida Citrus Pest Management Guide. What do you plan to do during the next reporting period to accomplish the goals?We will complete existing field trials and greenhouse trials and analyze the results to design follow up experiments to expand our knowledge of Huanglongbing to provide better management recommendations and inform projects developing treatments for HLB.

Impacts
What was accomplished under these goals? Prior to this research project, the above ground symptoms of huanglongbing (HLB or citrus greening) were the primary focus of research. This project has identified that the root system is highly impacted by the disease as well and is likely one of the major contributors to crop loss caused by the disease. Current work is aimed at understanding how this root loss occurs, so that appropriate management techiniques can be developed and implemented, which will improve tree health and reduce unnecessary costs to citrus growers by targeting the most effective management strategies. 1. Strong field based evidence showed that root growth is stimulated by Candidatus Liberibacter asiaticus infection leading to the inference that root lifespan is greatly reduced and the increased root growth is likely to accelarate carbohydrate limitations within the HLB affected tree that is developing a phloem disfunction problem during this stage. While initial experiments with rhizotrons were only qualitatively successful, modifications were made to make quantitative data collection easy, reproducible, and reliable. 2. A PhD student continued work on understanding how HLB increases Phytophthora root rot with the hopes of providing better management tools. Collaborations with the citrus nematologist allowed a greater understanding of the larger influence of HLB on soil-borne citrus pests. It was also found that HLB reduces the efficacy of chemical management of Phytophthora root rot, complicating root health management on HLB-affected trees. 3. Results from objective 1 demonstrate that the easiest, observations by some citrus growers suggest that high soil pH and irrigation water bicarbonate content may negatively impact root health and speed up the decline of trees with HLB. Under certain conditions acidification of soil or irrigation water seems to improve root health on HLB-affected trees. Sampling of citrus breeders rootstock trials is giving early indications that 2 rootstocks differ in their response to HLB compared to most other rootstocks 4. Detection of pathogen DNA in root and leaf samples from the field sites in objective 1 continues to provide informaiton on seasonal variations in distribution and detection of the pathogen to improve sampling methods. Young flush on symptomatic branches with suspect symptoms have been the best location for pathogen detection in the canopy, but root samples remain the most reliable.

Publications

  • Type: Conference Papers and Presentations Status: Published Year Published: 2015 Citation: J. Wu, E. G. Johnson, D. B. Bright, K. M. Gerberich, J. H. Graham. 2015. Citrus morphological root damage caused by Phytophthora spp. and Candidatus Liberibacter spp. Phytopathology 105(Suppl. 4)S4.150.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2015 Citation: E. G. Johnson, J. H. Graham, J. Grosser. 2015. Differential response of citrus rootstock breeding lines to Huanglongbing-induced root loss. Phytopathology 105(Suppl. 4):S4.66.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2015 Citation: J. Wu, E. Johnson, D. Bright, K. Gerberich, J. Graham. 2015. The interaction between Phytophthora spp. and Candidatus Liberibacter spp. damage to citrus fibrous roots. Proceedings of the American Society for Horticultural Science Annual Conference.
  • Type: Conference Papers and Presentations Status: Published Year Published: 2015 Citation: J. Graham, K. Gerberich, D. Bright, E. Johnson. 2015. Excess bicarbonate in soil and irrigation water increases fibrous root loss and decline of Huanglongbing-affected citrus trees in Florida. Proceedings of the IRCHLB IV:1.4
  • Type: Conference Papers and Presentations Status: Published Year Published: 2015 Citation: J. Graham, K. Gerberich, D. Bright, J. Wu, E. Johnson, J. Taylor. 2015. Relationship between HLB-induced fruit drop, fibrous root loss and the interaction with Phytophthora spp. Proceedings of the IRCHLB IV:3.3
  • Type: Conference Papers and Presentations Status: Published Year Published: 2015 Citation: E. G. Johnson, K. M. Gerberich, J. Wu, J. H. Graham. 2015. Citrus huanglongbing stimulates root growth while causing overall root loss. Proceedings of the IRCHLB IV:4.2
  • Type: Conference Papers and Presentations Status: Published Year Published: 2015 Citation: J. Wu, E. Johnson, D. Bright, K. Gerberich, J. Graham. 2015. The interaction between Phytophthora nicotianae and â¿¿Candidatus Liberibacter asiaticusâ¿¿ damage to citrus fibrous roots. Proceedings of the IRCHLB IV:4.20
  • Type: Conference Papers and Presentations Status: Published Year Published: 2015 Citation: E. G. Johnson, J. Wu, D. B. Bright, J. H. Graham. 2015. Huanglongbing reduces the effectiveness of Phytophthora fungicide control. Proceedings of the IRCHLB IV:8.5
  • Type: Other Status: Published Year Published: 2015 Citation: E. Johnson, J. Graham. Aug. 2015. Root health in the age of HLB. Citrus Industry. 96:8
  • Type: Other Status: Published Year Published: 2015 Citation: E. Johnson, J. Grosser, J. Graham. Oct. 2015. Rootstocks and HLB: Whats happening below ground? Citrus Industry 96:10


Progress 06/11/14 to 09/30/14

Outputs
Target Audience:The main target audience of thisis citrus growers. Results and outcomes of this research obtained so far were translated into an extension poster and a grower talk at one of the major citrus grower events (Citrus Expo in Ft. Myers, FL)in the state of Florida. Additionally one oral and 2 posters were presented at the national meeting of the American Phytopathological Society to share information with other researchers working in this field and receive useful feedback to expedite and improve our work on managing HLB and root health issues. Changes/Problems: Nothing Reported What opportunities for training and professional development has the project provided?One PhD student and one masters student have received one on one training in multiple techniques and experimental design as they pursue their thesis and dissertation projects on pathogen movement into root systems and HLB-Phytophthora root rot interactions. How have the results been disseminated to communities of interest?The results of this project have been presented at grower meetings both as extension presentations with management recommendations based on our current knowledge and with posters at the UF-IFAS extension displays. Knowledge has also been distributed with one on one interactions with growers over the phone and in their production fields. What do you plan to do during the next reporting period to accomplish the goals? Nothing Reported

Impacts
What was accomplished under these goals? Prior to this research project, the above ground symptoms of huanglongbing (HLB or citrus greening) were the primary focus of research. This project has identified that the root system is highly impacted by the disease as well and is likely one of the major contributors to crop loss caused by the disease. Current work is aimed at understanding how this root loss occurs, so that appropriate management techiniques can be developed and implemented, which will improve tree health and reduce unnecessary costs to citrus growers by targeting the most effective management strategies. 1. Field sites were identified and sampling initiated to compare root loss between presumed healthy, early disease, and late disease trees. Prototype rhizotrons were built and tested for analyzing root growth and providing samples for phytohormone analysis. 2. A PhD student has begun work on understanding how HLB increases Phytophthora root rot with the hopes of providing better management tools 3. Although the primary work on this objective will be initiated after we begin to get results from objective 1 and 2, observations by some citrus growers suggest that high soil pH and irrigation water bicarbonate content may negatively impact root health and speed up the decline of trees with HLB. Studies have begun to determine the effect of soil and irrigation water pH on citrus root health in relation to HLB. Citrus breeders rootstock trials are also being sampled to determine if some rootstocks reduce HLB-associated root loss. 4. Detection of pathogen DNA in root and leaf samples from the field sites in objective 1 is providing informaiton on seasonal variations in distribution and detection of the pathogen to improve sampling methods for early detection. Work on detecting pathogen DNA in juice from citrus fruit is also improving DNA extraction methods for consistent and efficient methods with the possibility of automation.

Publications