Objective 1: We conducted tests with nutrient analyses of leaf samples collected from HLB-infected and healthy trees to establish relationships which could be used for (early) diagnosis of HLB. A protocol was established to correct the distortion of leaf nutrient data caused by fluctuations in leaf dry weight (DW). Starch accumulation in symptomatic leaves can significantly alter the interpretation of nutrient status when diagnosed on a DW concentration basis. Conversion of leaf nutrient data to a leaf area basis is one accepted approach which we are using for correcting the influence of DW fluctuation. The Diagnosis and Recommendation Integrated System (DRIS) is another approach to leaf nutrient diagnosis which eliminates any erroneous background noise due to changes in DW. In DRIS, the DW values simply cancel out in a series of nutrient concentration ratios. Thus nutrient interpretation with DRIS considerably reduces the bias from undesirable nutrient concentration or dilution effects due to uncontrollable changes in leaf tissue DW. Furthermore, versions of the DRIS computations are available which can estimate an index of the leaf DW (DWI). In our studies with HLB-infected blotchy mottled leaves where starch accumulation caused DW to increase, the DWI calculated by DRIS was correlated with measured DW. Analytical labs do not routinely report DW for leaf samples, but by using DRIS, we are able to calculate DWI and obtain an estimate of undesirable DW changes which we need to be aware of. Furthermore, DWI may be valuable for diagnosing early onset of HLB in asymptomatic leaves even before PCR methods can detect the Candidatus Liberibacter spp. DNA. Objective 2: An HLB infected commercial citrus grove near Haines City, Florida has been located for field trials to determine the effects of remedial applications of plant nutrients on HLB. The experimental design and treatments are currently being developed in cooperation with the grower and will be reported on in future reports. Objective 3: Seeds of ‘Valencia’ and ‘Hamlin’ sweet orange, and ‘Duncan’ grapefruit have been germinated to produce trees for use in a controlled greenhouse hydroponics study of the effect of specific plant nutrients on HLB expression. The seedlings are currently being grown in sand culture with a complete nutrient solution applied every two days. The seedlings will soon be transferred to larger containers, but maintained in sand culture, to grow up to a sufficient size for budding, at which time a sufficient number of trees will be bud inoculated with HLB. After successful bud inoculation, infected and healthy trees will be transferred to hydroponics culture where specific nutrients (identified through objective 1) will be deleted or supplemented in the nutrient solution to determine their effects on HLB expression and plant growth.
Reducing the amount of vegetative growth produced annually by citrus trees in Florida would reduce the opportunities for Asian citrus psyllid reproduction and thereby the spread of Hunaglongbing. This could be done in citrus without detrimental effects on yield because citrus trees in Florida produce an excess of leaves above that required to support maximum fruit yield. Excess tree growth is routinely controlled through hedging, but little research has been done to examine the effects of hedging on vegetative growth. Recent research in Florida has shown that branch re-growth can be reduced when hedging is performed in fall under Florida conditions because of the onset of cool temperatures. Additionally, research in Florida indicates that late-summer hedging may be able to synchronize a final late-season flush and thus, reduce new flush leaves present during the winter to support over-wintering psyllids. In other fruit crops, such as apple, where excessive vegetative growth can be problematic plant growth regulators (PGRs) are routinely used. However, the use of PGRs in citrus has been limited to influencing fruit development and for maintaining post-harvest quality. PGRs not only control vegetative growth, but also offer the potential to reduce insect pest populations either by reducing pest-required vegetative growth or by altering host plant metabolites or nutrition. In addition, some PGRs have been shown to enhance pesticide efficacy. The PGR prohexadione calcium (Apogee, BASF Corp.) has also been shown to be effective at reducing vegetative growth of potted citrus seedlings under greenhouse conditions. Apogee was capable of reducing extension growth of shoots by more than 50%. Psyllid ovipostioning was also reduced by approximately 50% on Apogee treated seedlings compared to untreated control plants, but variation was large and obscured statistical differences. A 16-acre research block, managed according to standard commercial practices, has been designated at the CREC for further field evaluations of various PGRs and hedging treatments. In addition, plants have been obtained from commercial citrus nurseries to begin greenhouse screening trials to determine efficacy of a variety of different PGRs. The first data from these two trials will be reported in forthcoming reports.
Disease control of citrus huanglongbing (HLB) by interplanting with guava. HLB is the most devastating disease of citrus worldwide and presently threatens the existence of the citrus industry in Florida. In Vietnam guava has been shown to be an effective deterrent to HLB. Guava trees, (Vietnamese white cultivar) were propagated and grown to appropriate size. Two nursery sites, a guava protected citrus nursery versus and unprotected nursery, have been established. Disease free, PCR-negative citrus trees (2 sweet orange and 1 grapefruit cultivars) were located in the protected and unprotected plots in June 2009. Trees will be assayed for HLB every 60 days. Psyllid populations will also be monitored. Guava plantings have also been established at three field locations in Florida. Two of these are commercial plantings with collaborators in Southern Gardens Citrus and Consolidated Citrus totaling over 120 acres. Citrus will be interplanted with the guava later this Summer and Fall of 2009 in the commercial plantings. A third planting was established at the USDA, ARS in Fort Pierce, Florida and will be interplanted with citrus in late July early August 2009. The sampling of the plots and monitoring of the guava-effect research will commence immediately following interplanting with citrus and will be conducted every 3 months. Psyllid vectors will also be monitored within plots to document any repulsion of the vector due to guava. Data collection is presently ongoing.
Methods and models for the control of HLB disease of citrus. Citrus huanglongbing (HLB) is the most serious disease of citrus worldwide and presently for the very existence of citrus industry of Florida. The approach is two-fold: First was the examination of the effect of various control strategies on HLB, in control plots, established in 2007. I this phase of the study, five treatments were examined: Minimal control, Insecticide vector control, Roguing, Roguing via PCR+, and Comprehensive. Results indicated that although treatments were significantly different, there was no benefit of any control treatment over another. Small differences were due to plot location, not treatment effects. The test will be repeated, however, the results point to a need for regional control strategy and that small plantings that canÕt control neighbors cannot control the disease. New plots have been established at the USDA, ARS , in Fort Pierce, Florida. Our estimates indicated that for each tree with visual symptoms, there were an average of 13 (range 2-52) that were infected but asymptomatic, i.e., infections that have occurred over the duration of the epidemic but that have not yet expressed symptoms. The second approach is to develop epidemiological models of HLB disease dynamics which improve the understanding of vector-driven disease transmission and analyze disease control policies aimed at disrupting vector population dynamics. In previous and current work we have developed a model for citrus canker. This is the basis upon which we have built a preliminary model for HLB. The HLB simulation model is stochastic based on biological, epidemiological, and meteorological parameters using Markov Chain Monte-Carlo simulation methods and SIR modeling protocols. The model is fit to the HLB data collected in various observed epidemics from Florida and SE Asia by thousands of simulations. Linked-differential equations are used that describe the temporal increase in HLB infected trees and explicitly characterize the population dynamics of the vector. The effectiveness of different disease control measures such as intercropping with guava, roguing and insecticide use will be analyzed via this model and a suite of mathematical tools to identify the most effective strategies. HLB data sets will be correlated with various disease mitigation strategies/events from our epidemiology trials in Florida. The stochastic models allow testing of multiple disease management strategies in thousands of simulated epidemics to determine which will have the optimum effect and in what combination these methods can best be deployed for maximal disease control. The HLB model will continue to be augmented and improved over the next 2 years.
The objectives of this work are to 1) investigate the susceptibility of various rutaceous plant species to Candidatus Liberibacter asiaticus (Las), Ca. L. americanus (Lam), and Ca L. africanus (Laf) and the transmission from these hosts to citrus and 2) using a limited number of these hosts determine if passage through alternative hosts affects the biology (pathogenicity) of the pathogen. This work also will determine if certain alternative hosts are better hosts for the suspected HLB bacterial pathogen and can serve as a reservoir for infection to citrus. During this quarter work was performed at all four investigator locations. At the University of Florida, CREC, alternative plant materials were obtained and PCR tested for Las infection. HLB infected citrus materials to be used for inoculum were propagated in the greenhouse and verified as PCR positive for Las . The materials were used to graft inoculate Severinia buxifolia, Zanthoxylum clava-herculis, Citrus jambhiri and Murraya paniculata. The Murraya (Bergera) koenigii materials were not large or woody enough to graft and were therefore inoculated using infected psyllids. Successful inoculations were done with S. buxifolia, and C. jambhiria. Grafts were not successful with Z. clava-hericulis and are being repeated and there is no data to date on psyllid transmission to B. koenigii. Citrus jambhiri produced symptoms different than those seen in other citrus species. Psyllid transmissions from S. buxifolia to sweet orange and results with the receptor plants is pending. Z. clava-hericulis is not listed as a host of the Asian citrus psyllid, however we have found that the psyllid does feed on this host plant. A graduate student has joined Dr. Brlansky’s research program and will focus his research in this area. At the Texas A&M Citrus Center, Weslaco a collection of eight rutaceous species has been established which include two Amyris species (torch wood), Zanthozylum fagara (lime pricklyash), Helietta parvifolia (baretta), Esenbeckia berlandia, Casimiroa tetrameria and two Choisya species (Mexican orange) and will be tested as hosts for the psyllid. Since HLB is not in Texas some of the plant materials will be shipped to the USDA, ARS, Beltsville, MD for inoculation tests with HLB infected citrus materials. At the USDA, ARS, FDWSRU, Ft. Detrick, MD Murrraya paniculata, M. exotica and M. koenigii were established in quarantine. Two isolates of Las ( from Taiwan and Florida) were obtained from J. S. Hartung at the Exotic Citrus Diseases Collection, USDA, Beltsville, MD and were used in experiments to infect the the plants species listed above. In psyllid transmission tests Asian citrus psyllids transmitted Las to M. paniculata and M. exotica, but not M. koneigii. Disease symptoms rarely developed in Murraya plants, and positive infections were determined by PCR. Back-inoculations from M. paniculata to sweet orange was successful however there was some variability was in infection rates, titer, and persistence of the Las bacterium in M. paniculata. A publication is in preparation. Work will continue acquiring other rutaceous plants and testing them for susceptibility to Las, Lam and Laf.
Transmission of Huanglongbing causal bacteria by psyllid feeding is the natural route of transmission. The causal agent has recently been cultured but pathogenicity tests have not been completed to prove the causal nature of the cultured bacteria. Although the release of our funding was delayed for several weeks, we have initiated a staffing plan and begun experiments. For psyllids to acquire the cultured bacteria we had proposed to use membrane-feeding and/or micro-injection to charge the psyllids with the causal bacteria. We are developing protocols for membrane feeding using dilute (5%, 10%, and 20%) sucrose preparations within the membrane. We found psyllids survived for several days on all dilutions. We are beginning experiments to determine what the psyllids can inject into the membrane during feeding and how much they can obtain from feeding through membranes. We are proposing using 5th instar nymphs and adult psyllids for these studies. We have purchased a micro-injection system and are beginning to test the parameters for successful injection of bacterial culture into the psyllids hemolymph.
Under objective 1 (define rates and formulations of copper sprays for more effective control) one trials with Red grapefruit has been set up in Ft. Pierce, FL and one trial with Hamlin orange in Hardee County, each with 20 treatments of formulations, rates and combinations with other chemical treatments of interest. Under objective 2 (establish the period of fruit susceptibility for grapefruit and oranges), companion trials have been set up looking at whether longer duration of a copper spray program is necessary for control of crop loss. Under objective 3 ( evaluate the use of streptomycin [Firewall] and a polymer delivery system [PDS] to lower the rate and frequency of copper application) three trials (two non-bearing and one bearing) in grapefruit have been initiated to evaluate streptomycin for canker control, and two trials in bearing grapefruit have been initiated to evaluate PDS and the interaction with copper and timing of copper burn. Under objective 4 (To define risk for development of bacterial resistance to copper [Cu] and streptomycin [Sm] in FL citrus groves) one non bearing trial in grapefruit sprayed every 21 days with Kocide 3000 and/or Firewall (Sm) has been monitored since March 2008. No Cu or Sm resistant strains of Xcc have been isolated, however Cu and Sm sprays increased the ratio of epiphytic bacteria with resistance to these chemicals. Overall, the Sm resistant bacterial population is proportionally lower than Cu resistant bacterial population. Under Objective 5 (rapid transfer improved canker management technology to the Florida citrus industry), the 2009-10 canker management recommendations have been published in the Florida Citrus Pest Management Guide and Citrus Industry Magazine. Oral presentations have been delivered to the Florida Citrus Production Managers and at county extension meetings.
Under Objective 1 (define rates and formulations of copper sprays for more effective control) one trial with Red grapefruit has been set up in Ft. Pierce, FL and one trial with Hamlin orange in Hardee County, each with 20 treatments of formulations, rates and combinations with other chemical treatments of interest. Trials have low to moderate disease epidemics this season as a result of a dry spring, moderately wet summer and an absence of tropical storm events. Under Objective 2 (establish the period of fruit susceptibility for grapefruit and oranges), companion trials have been set up looking at whether longer duration of a copper spray program is necessary for control of crop loss. Fruit drop has been quantified in the Hamlin orange plots. Under Objective 3 ( evaluate the use of streptomycin [Firewall] and a polymer delivery system [PDS] to lower the rate and frequency of copper application) three trials (two non-bearing and one bearing) in grapefruit have been initiated to evaluate streptomycin for canker control, and two trials in bearing grapefruit have been initiated to evaluate PDS and the interaction with copper and timing of copper burn. Under Objective 4 (To define risk for development of bacterial resistance to copper [Cu] and streptomycin [Sm] in FL citrus groves) the copper resistance gene has been identified from a resistant Xcc strain from Argentina as CopL, the same resistance in X. vesicatoria the cause of bacterial spot in tomato. Primers constructed based on the gene sequence have been used to screen the remainder of copper resistant strains of Xcc from Argentina and the copper resistant strains of X. alfalfae. pv citrumelonis from Florida citrus nurseries with citrus bacterial spot. All strains screened thus far contain the CopL resistance gene. Under Objective 5 (rapid transfer improved canker management technology to the Florida citrus industry), the 2009-10 canker management recommendations have been published in the Florida Citrus Pest Management Guide and Citrus Industry Magazine. Oral presentations have been delivered to the Florida Citrus Production Managers and at county extension the copper meetings.
The Objectives are to: 1) Test the effect of soil applied imidacloprid as an inducer of SAR on HLB disease incidence in newly planted citrus trees subjected to psyllid mediated infection; and 2) test the effect of imidacloprid on HLB disease progress in newly planted citrus trees that are graft-inoculated with budwood from HLB infected field trees. The following treatments were set up in the USDA-ARS, Picos Farm in Ft. Pierce FL: 1) untreated check, 2) foliar insecticide to control psyllids, 3) soil applied imidacloprid/thiamethoxam to induce SAR, 4) soil applied imidacloprid/thiamethoxam plus the foliar insecticide to induce SAR and control psyllids, 5) HLB bud graft-inoculated untreated check, 6) HLB bud graft-inoculated with soil applied imidacloprid/thiamethoxam to induce SAR. The experimental design is a completely randomized block design with 50 trees per treatment (5 blocks of 10 trees per block). Tree and insecticide treatments commenced in May and HLB graft inoculations will occur in July. A HLB bud-grafted experiment of similar design will be set up in Brazil in spring time (September-October) for southern hemisphere.
Under Objective 1 (evaluate the potential for soil application of the neonicotinoids, imidacloprid and thiamethoxam, and acibenzolar-s-methyl, to provide long-lived SAR control of canker in young trees) two trials in 1 and 3 yr old grapefruit in Ft. Pierce, FL have been set up with 14 treatments in each location. A trial with complementary treatments will be set up in Brazil in the spring (August-September) in the southern hemisphere. Objective 2 (integration of SAR with foliar applications of reduced rates and frequency of copper sprays for control of canker on the most susceptible cultivar in Florida, grapefruit) and objective 3 (evaluation of the complementation of the use of neonicotinoids thiamethoxam and imidacloprid to increase and/or extend insect and disease control) are being addressed within the treatments of the two trials set up in Florida.
Under Objective 1 (Survey and confirmation of HLB in seedlings from HLB-affected trees) mild to severely affected fruit from the an HLB-affected Pineapple orange and Murcott tangor fruit were collected from groves in Collier County, FL and freshly extracted seed planted in the greenhouse for HLB detection in emergent seedlings. Under Objective 2, (Thermotherapy of HLB-affected seed) subsets of these same seed sources were treated at 125, 130 and 135F to test the effect of heat treatment on detection of HLB in seedlings. Pineapple orange and Murcott seedlings will be tested by PCR and any PCR products detected will be cloned and sequenced. Under Objective 3 (Follow-up on HLB-positive trees from DPI annual survey), DPI survey of seed source trees in Florida nurseries has, thus far, not detected seed trees PCR positive for HLB.
The main focus thus far has been to improve the current media to increase growth of the group of bacteria known as Candidatus Liberibacter. Our main focus for improving the growth has focused on understanding the phloem chemistry of citrus. Several methods of extracting the phloem, including micro-dissection of the phloem elements have been performed. Samples are being analyzed at the UC Davis MetaCore facility using GC/MS. A metabolic analysis of the available Ca. L. asiaticus genome was also performed to indicate which metabolic pathways are present and may be important for the growth of the bacteria. The analysis indicated that the bacteria had specialized transport mechanisms for the micronutrient zinc and the amino acid proline. A previous study of the xylem content (includes the phloem) of citrus indicated a preponderance of proline and the symptoms of HLB have been compared to zinc deficiency. Current formulations of the published media are focusing on optimizing the pH of the media, and the additions of zinc and proline. Media improvements will be on-going during the course of the project. Our lab has successfully shown that the bacteria known as Ca. L. asiaticus, Ca. L. americanus and Ca. L. africanus are a causal agent of HLB by completing Koch’s postulates. To replicate the experiment under field conditions and study symptoms in the flower and fruiting body, a study has started at Ft. Pierce, FL. Healthy plants are in place and a Florida-derived culture of Ca. L. asiaticus has been started at Ft. Pierce. Inoculations should occur in the next quarter. The second focus has been sequencing the genomes of the three Liberibacter species from pure culture. In cooperation with the National Center for Genome Resources, we will use Illumina sequencing to sequence the strains used in the published pathogenicity trials (China1 Ca. L. asiaticus and Brazil1 Ca. L. americanus) and a representative strain from Ca. L. africanus (Africa2). An agreement is now being processed. Currently we have grown enough bacteria to be used in the Illumina process for all three of these strains and are focused on ensuring quality and purity before sequencing. Barring any sequencing and alignment issues, all three genomes should be publicly available by the end of the year.
This is a 3-year project with 3 main focal points: 1. To build a foundation of understanding of the host- Candidatus Liberibacter asiaticus (LAS) interactions that involves testing multiple genetic variants of citrus, examination of environmental and seasonal effects on symptoms and pathogen replication and movement, and understanding effects of genetic variation in the pathogen system, so that better methods of controlling the vector, detecting and monitoring the disease, and growing trees will allow better production of citrus; and, 2. To develop an understanding of how Las interacts with citrus genotypes to cause disease in sensitive varieties and to not cause disease in tolerant varieties in the hope that this information will lead to the development of approaches to produce citrus economically in the present situation with citrus greening in Florida; and, 3. To provide knowledge and resources to support and foster research in other laboratories. A substantial number of projects to be supported are based on our research and reagents (propagated healthy and HLB-infected plants, HLB inocula, nucleic acid extracts, etc.) supplied by our laboratory. We view this as one of our most important goals. Year 1 milestones: 1) Propagate and prepare trees for year 2. We have received seed from a range of different citrus relatives from the National Citrus Repository in Riverside, CA, and these plants are growing to be tested for response to HLB. 2) Begin screening of commercial varieties and additional citrus relatives to Las. We have an experiment underway to test a range of different sweet orange and grapefruit varieties for sensitivity to HLB. The experiment also has some citrus relatives. HLB symptoms have just started to develop. 3) Obtain seeds fo Poncitrus trifoliata and poncirus hybrids from a series of independent sources, propagate plants, and start experiments to evaluate phenotypic differences in their response to HLB. We have several independent selections of Poncirus trifoliata growing. We also have a series of experiments underway to analyze Poncirus rootstocks and interstocks on the development of disease. 4) Evaluate C. latipes response: repeat an attempt to initiate infection of C. latipes trees using different sources of the trees, on different rootstocks, and using different sources of HLB inoculum. We have a series of C. latipes trees infected with a series of different Las isolates, but nothing has induced the original symptoms that were associated with non-detection of Las. However, we have an interesting observation that the original trees are greatly inhibited when used as budwood. We are investigating this phenomenon. 5) Begin microscopy studies and develop an approach to analyze sugar metabolism in sensitive vs tolerant hosts in an attempt to understand what determines a differential host response to HLB. We have found that phloem necrosis in stems parallel the degree of disease severity. Citrus that is tolerant to HLB has much less phloem necrosis than sweet orange and grapefruit.
This is a continuing project to find an interim control measure to allow the citrus industry to survive until resistant or tolerant trees are available. With the loss of trees due to canker eradication and development and the continuing losses to greening, there is a major concern whether sufficient fruit will be produced to keep processing plants open. We are approaching this problem in two ways. First, we are attempting to find products that will control the greening bacterium in citrus trees. We have chosen to focus on antibacterial peptides because they represent one of the few choices available for this time frame. Secondly, we are developing virus vectors based on CTV to express the antibacterial peptides in trees in the field as an interim measure until transgenic trees are available. We think that this approach could be used beginning 2-3 years from now and until probably 15 years from now when resistant trees should be available. The milestones for this year’s research are: 1) Continue to screen peptides for activity against Las in citrus trees using the CTV vector. We now have built 45 different peptide constructs that are being tested against HLB or citrus canker. We have 5-7 peptides that we think have activity against these bacteria. We also are developing a more natural screening assay using psyllids to infect trees with HLB. 2) Begin high-throughput screening in tobacco against solanaceous liberbacter. (Falk, UC Davis) The subcontract to UC Davis has not been completed yet. However, Dr. Falk has established colonies of the solanaceous psyllid. 3) Improve CTV-based vector to produce 2-5 peptides and to overcome cross-protection. We have built several new versions of CTV vector and have shown that two foreign genes can be expressed efficiently at the same time. We are making progress in understanding the process of cross protection to allow development of a CTV vector that will allow subsequent application of antibacterial gene products. 4) Examine survival of peptides in fruit and juice. We have developed the plants for these assays, but this project is just beginning. 5) Prepare trees for year 2. As we develop new anti-bacterial or anti-psyllid constructs into the CTV vector, they immediately begin their pathway to citrus trees to be tested for resistance or tolerance to HLB or citrus canker.
The goal of the proposed research is to understand how Candidatus Liberibacter asiaticus causes Huanglongbing (HLB) disease on citrus. Citrus HLB is the most devastating disease on citrus. There are very few options for management of the disease due to the lack of understanding of the pathogen and citrus interaction. Understanding the citrus and citrus HLB pathogen interaction is needed in order to provide knowledge to develop sustainable and economically viable control measures. We proposed to expand our current research to different citrus varieties which are either susceptible or tolerant to HLB and a more facile model system tobacco (Nicotiana tabacum Xanthi). The specific objectives of this proposal are: transcriptional and microscopic analyses of citrus varieties which are either susceptible or tolerant to Ca. L. asiaticus infection at different infection stages in greenhouse and citrus grove; and transcriptional and microscopic analyses of host response to Ca. L. asiaticus infection with a model system tobacco. Microarray analysis and/or suppressive subtractive hybridization libraries approaches will be used to study the host response to the HLB pathogen infection followed by confirmation with Northern blot or quantitative reverse transcriptional PCR. Anatomical study will be performed with light microscopy using different staining methods. The following objectives related to this project were pursued: (A) An initial microarray analysis of host response of sweet orange to Las infection in greenhouse and in field; and (B) A preliminary study of the effects of HLB on phloem at the microscopic level and phloem transport of sweet orange. The results of these experiments are detailed below. Investigation of the host response was examined with citrus microarray hybridization based on 33,879 expressed sequence tag sequences from several citrus species and hybrids. The microarray analysis indicated that HLB infection significantly affected expression of 624 genes whose encoded proteins were categorized according to function. The expression of a total of 624 genes was significantly changed: 307 genes were up-regulated and 317 genes down-regulated in inoculated trees. The host genes that showed changes in expression pattern induced by Las infection were related to plant pathogenesis/stress (10.4% of the total), anthocyanin biosynthesis (0.5%), cell wall metabolism (6.6%), cell division (0.96%), detoxification (6.2%), lipid metabolism (2.4%), metabolite transport (5.8%), metal transport (2.6%), nucleotide metabolism (1.6%), phenylpropanoid/flavonoid/terpenoid metabolism (5.6%), phytohormones (3.0%), protein kinase (5.0%), protein metabolism (3.8%), protein-protein interaction (1.1%), signal transduction (1.8%), sugar metabolism (4.3%), transcription/translation factors (7.2%) and unknown/hypothetical genes (31.1%). The anatomical analyses indicated that HLB bacterium infection caused phloem disruption, sucrose accumulation, and plugged sieve pores. The up-regulation of three key starch biosynthetic genes including ADP-glucose pyrophosphorylase, starch synthase, granule-bound starch synthase and starch debranching enzyme likely contributed to accumulation of starch in HLB affected leaves. The HLB-associated phloem blockage resulted from the plugged sieve pores rather than the HLB bacterial aggregates since ÔCa. Liberibacter asiaticusÕ does not form aggregate in citrus. The upregulation of pp2 gene is related to callose deposition to plug the sieve pores in HLB-affected plants. To further expand our current understanding of Las-host interaction, we are currently comparing two susceptible and two resistant hybrid/cultivars. Currently, we are collecting samples for more than 4 months. The gene expression and anatomical study are under way.
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