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. We are approaching this problem in three ways. First, we are attempting to find products that will control the greening bacterium in citrus trees. We have chosen initially to focus on antibacterial peptides because they represent one of the few choices available for this time frame. We also are testing some possible anti-psyllid genes. Second, we are developing virus vectors based on CTV to effectively express the antibacterial genes 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. With effective antibacterial or antipsyllid genes, this will allow protection of young trees for perhaps the first ten years with only pre-HLB control measures. Third, we are examining the possibility of using the CTV vector to express antibacterial peptides to treat trees in the field that are already infected with HLB. With effective anti-Las genes, the vector should be able to prevent further multiplication and spread of the bacterium in infected trees and allow them to recover. We have completed several large screenings of antibacterial peptides against Las in sweet orange trees. About 40 different antibacterial peptides have been tested in trees. We initially found three peptides that allow much better growth of trees that were grafted with HLB-infected buds. Some trees had no symptoms and no detectable Las, some trees had no symptoms and low levels of Las, and other trees had leaf symptoms but continued growth of the trees with normal levels of Las. Another result is that we found that leader peptides for the export of the peptide from the CTV-infected is not needed for HLB but is needed for citrus canker. Because we were concerned that graft inoculation of HLB into the trunks of small trees is a too severe challenge that might cause peptides that could work in the field to be missed, we developed a system that only allows inoculation by infected psyllids. We have established a containment plant growth room in which psyllids inoculate the plants expressing the peptides. Using this system, we have found two peptides that appear to effectively protect sweet orange trees from HLB. To speed up the search for effective anti-HLB genes, Falk (UC Davis) has developed a tobacco-tomato psyllid/liberibacter model to screen for effective genes against the similar bacterium. This system is working and screening is on-going. We also are improving the CTV-based vector to be able to produce 2-5 peptides at the same time. This will allow expression of genes against HLB and canker or multiple of genes against HLB. We have developed a vector that can be re-added to trees if the anti-Las gene is lost or a better gene becomes available. A major objective that we are pursuing is to make a vector that cannot be transmitted by aphids. Another major goal is to do a field test of the CTV vector with antibacterial peptides, which is an initial step in obtaining EPA and FDA approval for use in the field. We have received permission for USDA APHIS for the field test, but were delayed by EPA. We are now submitting a revised application to USDA APHIS to include EPA requirements and are expecting to establish the field test this spring. In addition, we are screening a series of transgenic sweet orange and grapefruit expressing antibacterial genes for Erik Mirkov of Texas A&M and Mike Irey of Southern Gardens.
A manuscript on the sequence of symptom development and the identification of callose and phloem protein 2 (PP2) as the amorphous and filamentous plugging materials was accepted: DS Achor, et al. 2010.Plant Pathology J. 9(2): in press. The upset of normal phloem translocation of carbohydrates to other plant parts and ultimately the starvation of the root system (Etxeberria et al, 2009, Physiol. Mol. Plant Pathol. 74:76-83) may be the main reason for tree decline from HLB infection. Field samples of several cultivars were collected and prepared to determine the amounts of amorphous versus filamentous plugs disrupting phloem sap flow. If one type of plugging is more prevalent, that pathway for plugging may be more important. Earlier work by a colleague indicated that three common rootstocks do not express HLB symptoms when infected with HLB and grown in a greenhouse. One of these, Swingle, was found infected in the field, but it developed typical leaf mottle symptoms and phloem plugging. Carrizo trees are being monitored until they become PCR positive. Poncirus trifoliate trees are being monitored in two field sites also as they may have HLB tolerance. Since insufficient bacteria are present to directly cause phloem plugging, work is underway to understand the mechanism by which the bacterial infection leads to this phloem plugging. To determine how the bacteria elicits the over expression of phloem plugging materials, one Agilent microarray was designed based on the genome sequence of Candidatus Liberibacter asiaticus. Bioinformatics analysis was performed to identify potential virulence factors. Six potential virulence factors were cloned into pGEMT-easy vector. The insertions were confirmed using PCR and three virulence factors were expressed in tobacco; one of these caused plant death. These factors and others will be inserted into citrus with a phloem promoter to study their potential roles in virulence. Transgenic approaches to achieve over-expression of the citrus ‘-1,3-glucanase gene using different promoters, though protoplast/GFP co-transformation and Agrobacterium-mediated transformation are underway in order to minimize HLB associated callose-plugging. Initial transgenic grapefruit trees that over-express 1, 3-. glucanase were grown out on Macrophylla rootstocks and challenged with HLB by bud inoculation. Only 4 of 44 plants had successful bud take and none of these show symptoms 4 months after the inoculation attempt. All these plants were re-inoculated with infected buds in February. The citrus ‘-1,3-glucanases gene from Valencia embryogenic callus and young leaves (McCollum et al., 1999) was cloned. Citrus .-1,3 glucanase cDNA (GenBank accession number AJ000081) was synthesized from Valencia leaf and embryogenic callus. A 1011 bp citrus .-1,3 glucanase gene fragment was amplified. To obtain the suitable restriction site and additional cMyc tag (to facilitate subsequent western analysis) on the cDNA, another PCR reaction was performed using a new primer set with a different restriction site including the cMyc tag. The final PCR product (BG3) was purified, cloned into the pGEM T-Easy vector (Promega) and sequenced. The cloned BG3 fragment was ligated into a vector (pUCLON) between the 35S promoter and 35S terminator, and was transformed into host E coli DH5. cells. Plasmid DNA was isolated using 5prime kit and checked through restriction digestion and PCR analysis. From the ligated plasmid ( pUBG3), the HindIII fragment was excised and ligated into the HindIII site on another vector pCIT101 holding the GFP/NPTII fusion gene. The final vector , pCITBG3, was transformed into Agrobact. Co-transformation experiments can now be done. Plasmid vector pARS108 with the ER-targeted GFP gene was used to make a new construct for efficient protoplast transformation. Another vector, pGASS, was constructed to target BG3 expression in phloem tissues only. Two new sweet orange callus lines are available to carry out transformations. >Antisense or other knockout methods for phloem protein 2 and callose genes will be used in year 2.
A RAPID SCREENING PROGRESS FOR CHEMICAL CONTROL OF HUANGLONGBING Annual Research Report-2009 In the second year, the project was mainly focused on: 1) Screening and evaluating anti-microbial molecules for suppression of liberibacter using the optimized screening systems in periwinkle; 2) The effective compounds were tested and evaluated in HLB-affected citrus in the greenhouse 1. Screening of chemical compounds that eliminate or suppress the Las bacteria in periwinkle using the optimized regeneration system. Antibiotics (penicillin G and streptomycin, oxytetracyclin and Metronidazole), a biocide (DBNPA), a fungicide (zineb), two peptides (D4E1 and D2A21) and three SARs (SA, antiguard, and ortho-phenylphenol) were tested for their ability to eliminate or suppress Las bacteria and to promote the growth of severe HLB-affected cuttings using the optimized regeneration system. More than 75% of plants could be regenerated from the severe HLB-affected periwinkle cuttings treated with a combination of penicillin and streptomycin (PS). All regenerated plants from the HLB-affected cuttings treated with PS tested negative for Las, even by nested PCR. The Las bacteria were undetected in these regenerated plants with average Ct value of 39.33. When treated with oxytetracyclin, metronidazole or peptides (D4E1 or D2A21), the regenerated plants also tested negative by nested PCR or qPCR with Ct values over 32. However, the regeneration percentage was lower than 30%. The regenerated control plants treated with water tested positive for Las by PCR and qPCR with low Ct values <26.0, indicating that the bacteria titers were at least 100-fold higher than those in the antibiotic or peptide-treated, regenerated periwinkle. DBNPA can also suppress the Las bacteria. The severe HLB-affected cuttings had a regeneration rate of 33.1 %. The Ct values of the regenerated plants ranged from 27 to 30. The fungicide zineb and three SARs (SA, antiguard and ortho-phenylphenol) were not effective in controlling Las bacteria. Whether treated with zineb or not, the Las bacteria can keep reproducing. The Ct value was lower in the zineb-treated, regenerated plants than those treated with antibiotics or peptides, and similar to the water control. Zineb and ortho-phenylphenol are not very soluble. The water solubility is only 10 mg/L. When PS was foliar-sprayed at one-week intervals for three consecutive weeks on the Las-infected periwinkle plants at different rates (1x, 5x and 10x), no significant differences were found among treatment rates, but each treatment eliminated the Las-bacteria in the infected periwinkle as compared to the water control. DMSO or Silwet L-7 as emulsifiers has no different effect on Las-infected periwinkle plants. 2. Effect of screened chemical compounds on HLB-affected citrus in the greenhouse. Nutrition and HLB development in citrus. The nutrients K, Zn, N, and Ca were tested for their ability to suppress HLB symptoms in citrus. Potassium and Zn, but not N or Ca could delay HLB symptoms. All-treated citrus remained infected by Las bacterium after 9 months. K and Zn reduced titers of Las bacteria at 5 months after treatment. Effect of SAR-inducer on HLB development. Seven months after treatment with SAR-inducer (SA and antiguard), HLB-affected citrus tested positive for Las by PCR with an average Ct value of lower than 28, indicating SAR was not effective in suppressing or inhibiting the Las bacterium. This had also been the case with a perwinkle host. Effect of antibiotics on HLB-affected citrus by root soaking or foliar-spray. HLB-affected citrus was soaked or foliar-spraye
This report covers the period of July 1, 2009 through September 30, 2009. This project was funded July 1, 2009. This project was organized and many activities were completed prior to July 1, 2009. There were four organizational meetings between FDACS, CHRP, USDA, University of Florida, the Indian River Citrus League and consulting personnel. There were 42 organizational meetings of the field personnel. The Indian River Citrus League held three meeting with their board to explain the areawide suppression program and to invite growers to participate. The project was organized into Martin, St. Lucie and Indian River Counties. Growers were contacted for access to groves, blocks were surveyed for trap placement and trees and blocks were mapped and GPSed. All reports were designed and supplies were ordered. The tracking and analysis of data was organized. Personnel were trained in procedures for trapping and identification of Asian citrus psyllid. This program cooperated in providing additional materials for aerial application for Asian citrus psyllid and Caribbean fruit fly control. Materials were provided by the pesticide industry and efficacy experiments were designed by the University of Florida. Three large, replicated efficacy experiments were conducted. One additional experiment was designed and organized, but not implemented due to weather. Materials were applied by air with psyllid monitoring before and after applications and comparisons with psyllid numbers in control plots. The efficacy of materials for Caribbean fruit fly control was assessed in the same experiments. Sterile Caribbean fruit flies were released before applications and fly numbers were monitored before and after application. Materials not previously registered are now moving toward label inclusion of aerial application for Asian citrus psyllid and for Caribbean fruit fly. For the period July 1-Sept 30, 2009: Traps placed by county were: Indian River, 205; St. Lucie, 225; Martin, 72. Trees surveyed were: Indian River, 2,665; St. Lucie, 2925; Martin, 975. Traps set and retrieved were 6,565. Asian citrus psyllids caught were: Indian River, 5,583; St. Lucie, 8,414; Martin, 4,672. This project was successfully organized and initiated during this period.
Objective 1: Sprays of copper (Cu) formulations, containing copper hydroxide (Kocide, Champ, Kentan, Badge) or copper sulfate (Cuprofix), were moderately to highly effective for control of canker on fruit of susceptible Ruby red grapefruit and Hamlin orange in the lack of high disease pressure due to early and late season wind-blown rain storms in 2009. A chelated Cu (Magna-Bon, copper pentahydrate) at a 50% lower rate of Cu per application than standard Cu formulations performed as well for reducing fruit disease incidence for grapefruit or canker-induced fruit drop for Hamlin. Early season infection and fruit drop of grapefruit and Hamlin was minimal because April was relatively dry and Cu treatments were initiated before significant rainfall occurred in May. Cumulative fruit drop due to early season infection of untreated Hamlin amounted to about 5%. Five sprays of Cu formulations reduced the incidence fruit drop due to canker by about 40-50% to 2.5% cumulative fruit loss. Objective 2: In Marsh grapefruit, canker control increased with number of Cu sprays from 3 to 11 (April to October), canker infection and copper burn occurred after rains commenced in July. In August, fruit were growing most rapidly which would produce a thinning of fruit cuticle and an increase in the rate that new stomates open due to more rapid expansion of the fruit surface. Season-long copper spray also gave the best control of late season scab and melanose on fruit. In Hamlin, sprays beyond mid-July provided additional canker control of fruit drop confirming that late season lesions do not cause fruit drop like early season lesions. Objective 3: In two grapefruit trials, Firewall (streptomycin[Sm]) applied alone or in combination with a reduced rate of Kocide 3000 in July and early August gave equivalent control on grapefruit to Kocide alone. The adjuvant, Polymer Delivery System (PDS) did not increase the residual activity of Cu on grapefruit or control efficacy of Cu. The residual activity of Cu on fruit was not affected by Kocide rate but decreased with time after application due to increase in fruit surface area over 21 days. This result supports the recommendation for use of 21 day interval Cu sprays for adequate canker control and explains the reduced efficacy of 28-day interval sprays. Objective 4: The Cu resistance gene was identified as CopL on a plasmid from a resistant Xanthomonas citri subsp. citri (Xcc) strain from Argentina that was exposed long-term to Cu for canker control. The identical resistance gene sequence was found in Xanthomonas spp. causing bacterial spot in tomato and pepper. Primers constructed based on the gene sequence were used to screen the remaining Cu resistant strains of Xcc from Argentina and Cu 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. In addition, a non-pathogenic strain of Xanthomonas isolated from a citrus grove was found to be Cu resistant and may represent a pre-existing source of risk in citrus groves for horizontal transfer of CopL into Xcc. Cu and Sm resistance were monitored in Xcc and epiphytic bacterial populations on grapefruit trees sprayed with Cu or Sm every 21 days for two growing seasons (22 sprays total). Each season Cu and Sm sprays increased the ratio of epiphytic bacterial population with tolerance to these chemicals. Overall, the Sm resistant bacterial populations were proportionally lower than Cu tolerant bacterial population. No resistance to either Cu or Sm was verified in Xcc or epiphytic populations after two years of season long sprays. Objective 5: In 2009, canker management talks were given at county extension and other meetings. Updated 2010 canker management recommendations have been published in the Florida Citrus Pest Management Guide and Citrus Industry Magazine. Oral presentations have been made to the Florida Citrus Production Managers and at Florida Citrus Show.
Objective 1: Potential for soil application of the neonicotinoids, imidacloprid (Admire) and thiamethoxam (Platinum), and acibenzolar-s-methyl (Actigard), to provide long-lived SAR control of canker was evaluated in a trial of 4-yr old grapefruit trees in Ft. Pierce, FL. Despite above average rainfall in May, the disease on the spring flush that emerged in March when it was dry was free of canker. In contrast, spring-summer flushes evaluated in September had 62% incidence of canker diseased leaves. Several of the SAR treatments significantly controlled disease but not as well as with Kocide 3000 and Firewall (streptomycin) sprays at a 21 day interval. SAR treatments that failed to significantly reduce foliar disease were the two applications of Actigard, Platinum split rate and isonicotinic acid treatments. Incidence of canker on the spring- summer-fall flushes evaluated at the end of the season in November had a slightly higher canker incidence than on the earlier set of flushes. By this time all the SAR treatments significantly reduced disease compared to the non-treated check. The most effective treatment was 4 applications of Actigard at 2 oz and least effective treatment was 2 applications of Actigard at 1.0 oz. The best treatment matched the control on the flushes attained by the 11 sprays of Kocide 3000 and/or Firewall. The 4-yr old trees produced enough fruit to evaluate canker incidence. Effectiveness of treatment on incidence of canker fruit varied from moderate for the 4 applications of Actigard at 2 oz, Platinum and Admire to ineffective for isonicotinic acid and Actigard at lesser rates and applications. Kocide 3000 and Firewall were significantly more effective than SAR treatments for reducing fruit disease. Harvestable fruit was reduced by canker’induced premature fruit drop. The non-treated check had the lowest fruit harvested per plot while the 4 applications of Actigard at 2 oz had the highest number of fruit. The number of fruit harvested was significantly negatively correlated with the incidence of disease on the spring summer-fall flush disease, but was not correlated with fruit disease incidence. Control of canker on the leaves apparently reduced the inoculum available for early season infection of fruit. Based on the trial results, Syngenta the manufacturers of Actigard are supportive of seeking a label for soil application of Actigard for SAR control of canker on non-bearing citrus. Objective 2 Integration of soil applied SAR inducers with foliar applications of copper sprays for control of canker on grapefruit was evaluated in the 4 yr-old grapefruit trial above, the best control was Admire applied once at the beginning of the season followed by 11 Kocide 3000 sprays. A trial in 4 yr-old fruiting Hamlin trees was set up in Arcadia, FL to compare trunk and soil applications of Admire at 3x the label rate to account for effect of the larger tree volume. SAR control of canker on foliage and fruit was equal to that of six 21 day sprays of Kocide 3000 starting in March. This suggests that SAR and copper could be used in an integrated program for augmenting canker control for young fruiting trees. However, it is unlikely that either neonicotinoid will be approved for use on bearing trees due to potential risks of increasing rates of soil application leaching into groundwater and residues in the flowers. Because the best SAR treatment for fruit disease control in 4-yr -old tree trial was the 4 applications of the 2 oz rate of Actigard, trials of this SAR treatment for young bearing trees are planned. Objective 3 is to evaluate of the complementation of the use of Actigard and neonicotinoids Platinum and Admire to increase and/or extend canker control in 1-yr-old grapefruit trees. Canker was first observed in the trial area in September 2009 after a very high rainfall period in August (17 inches). The pattern of disease spread was across the trial area from southwest to northeast. Incidence of trees with canker was 56% in the non-treated check trees, whereas in most of the SAR treatment combinations the incidence was less than 10%. This trial will continue in 2010.
Objective 1: Survey and confirmation of HLB in seedlings from HLB-affected trees. 500 seedlings grown from seed extracted out of mild to severely HLB-affected fruit from Pineapple orange and Murcott tangor groves in Hendry Co. were assayed for HLB detection. All seedlings were negative for HLB detection by PCR in repeated assays in July and Nov. 2009. Objective 2: Thermotherapy of HLB-affected seed for the same seed sources under Objective 1 were treated at 125F, 130F and 135F to test for the effect of heat treatment on HLB detection in seedlings. Thermal treatments were lethal to Murcott, but not Pineapple seed. The emergent seedlings at 125F (142), 130F (98) and 135F (97) were PCR tested. All seedlings were negative for HLB in repeated PCR assay in July and Nov. 2009. Objective 3: In October 2009, seed source trees in two Florida nurseries were found positive for HLB by FDACS-DPI (Nursery 1) or Southern Gardens Diagnostic Lab (Nursery 2). Discovery of infected seed source trees in two Florida citrus nurseries identifies a potential (but unconfirmed) risk of outdoor seed source trees acting as a source of inoculum for introduction into nursery propagations. In Nursery 1, seed was collected from symptomatic branches of two sources of Swingle citrumelo, four sources of ‘Kuharske’ Carrizo citrange and one source of Cleopatra mandarin. In Nursery 2, seed was collected from one source of Sekwasha mandarin. From 63 to 205 seedlings from each source provide enough leaves for testing in February 2010.
Spatial and Temporal Incidence of Ca. Liberibacter in Citrus and Psyllids Detected Using Real Time PCR Objective 1. Assess seasonal patterns of pathogen incidence in citrus trees and psyllids in regions of high HLB incidence. A 12 acre block of ‘Valencia’ orange trees was selected at a commercial grove in 2008. A sample of psyllid adults collected in the block in November 2008 and analyzed at USDA-ARS Riverside labs contained 21% HLB positive psyllids. The block was divided into 16 plots, which receive, two levels of micronutrients+SAR, insecticide treatments, or left as control. Plant and psyllid samples are being collected every four months to test for Candidatus Liberibacter asiaticus using PCR. Insecticide applications significantly suppressed psyllids compared to control. No significant differences in the field distribution of HLB in plants were observed using quantitative geostatistical analysis between November 2008 (40% HLB infection) and April 2009 (33% infection). Results from the fall 2009 and spring 2010 are being currently evaluated. A tree determined to be PCR positive and one PCR negative tree in each plot was trimmed to induce new flush. On 3 June, 10 psyllid adults from HLB negative colony on orange jasmine (Murraya paniculata) were released on a new shoot that had been caged immediately after trimming. An additional cage was placed on a previously uncaged and psyllid-infested shoot on the same tree. All cages along with branches and psyllids were collected 2 weeks later for PCR analysis. Nineteen percent of shoots caged with psyllids from the HLB negative colony tested positive for HLB on trees, regardless of whether trees had previously tested HLB positive or negative. In contrast, shoots that were naturally infested with psyllids when caged were 63% and 56% positive on previously HLB positive and negative trees, respectively. It would appear that infestation with infected psyllids from the field was over 2.5 times more likely to result in a positive plant sample, whereas the previous history of testing had little bearing. However, results were different when the same experiment was repeated in July-August using same plants. Shoots caged with psyllids from HLB negative colony were 64% and 42% HLB positive on previously HLB positive and negative plants, respectively. Whereas, naturally infested caged shoots were 7% and 18% HLB positive on previously HLB positive and negative plants, respectively. Adults emerging from infested shoots caged on HLB symptomatic trees at SWFREC during Feb-March 2009 and analyzed at US Sugar and SWFREC were found to be 5% HLB positive by both labs. Psyllids collected at large at the same time and location were 25% HLB positive, followed by 11% in April. Adults that emerged from infested shoots caged on HLB symptomatic trees were 23% HLB positive according to the USDA-ARS Riverside lab. Adults that emerged in the cages on HLB symptomatic trees from same location during July-August 2008 were 27% positive according to the Riverside lab. Therefore, psyllid origin and time of collection are major sources of variation on PCR results that we will need much more data to sort out. This report contains analyzed data through fall 2009. We currently have additional fall collections of psyllids that are being assayed and data compiled and analzyed. Additional psyllid samples from these experiments up to present 2010 are being processed. As our methods for addressing the first objective are refined as described above, the second objective is being addressed by the identification of additional sites to survey for psyllids which differ with respect to tree age, variety, and rootstock, and block size as described in grant proposal. A survey to administer to growers for tracking HLB and grove management tactics in the selected sites is in preparation and has been administered to a few, but data is still being collected. Despite the delay in receiving funds, the project is progressing in adherence to timeline and objectives.
The objectives of our research project are: Objective 1. Characterize the microbial community of healthy and Liberibacter-infected citrus phloem tissue by serial-section electron microscopy; Objective 2. Localize antigens and DNA sequences specific to Liberibacter in citrus phloem tissues by immunofluorescence microscopy, immunoelectron microscopy, and electron microscopic in situ hybridization. For objective 1, we completed three-dimensional (3D) serial-section electron microscopy analysis of 7 sites in the citrus phloem where Liberibacter cells are detected (One of the sites reconstructed into a 3D model is shown at http://news.ifas.ufl.edu/2009/12/09/uf-researchers-find-lone-culprit-behind-greening). In all the sites, the bacterial cells displayed uniform morphological features including diameter, length, cytoplasmic staining, and cell wall staining, suggesting that Liberibacter is the sole bacterial species in the citrus phloem or it vastly outnumbers other bacteria, if there are any. While we were examining the Liberibacter-infected (Las+) citrus phloem cells by 3D reconstruction, we discovered that the plasmodesmata in the Las+ phloem cells were swollen and heavily labeled by anti callose-specific immunogold particles. In parenchyma cells of Huanglongbing (HLB) symptomatic leaves, chloroplasts are disrupted by massive accumulation of starch granules and the cells eventually die. By fluorescence microscopy with aniline blue staining, we demonstrated that the callose accumulation at the phloem plasmodesmata precedes starch build up in the parenchyma cells of Las+ leaves. Callose deposition in the plasmodesmata reduces their transport efficiency and photosynthetate loading into the phloem is mediated by phloem plasmodesmata. So we tested phloem-loading in the Las-, callose-rich Las+ leaves by injecting carboxyfluorescein dye into their intercellular space. Phloem loading was inhibited in asymptomatic Las+ and symptomatic Las+ leaves while phloem loading was not inhibited in the Las- leaves. This phloem occlusion in the Las+ citrus leaves is likely to interfere with export of photosynthetate from the leaf parenchyma cells and provides an explanation for their excessive starch accumulation. Our results also suggest that the death of non-vascular cells in Las+ leaves is due to an inappropriate plant response rather than damages directly done by the phloem-limited bacterial cells. These results were presented in an international meeting (Plasmodesmata 2010) and a manuscript is being prepared for publication in the Plant Physiology. For objective 2, we have prepared citrus phloem samples ready for immunogold labeling and in situ hybridization. We were able to detect polysaccharides by immunogold labeling but several antibodies against bacterial proteins (from Dr. Duan) have not provided consistent immunogold labeling results. We are testing more antibodies from Dr. Duan. We tried two oligonucleotides for localizing Liberibacter by in situ hybridization but their specificity has not been satisfactory. We are improving our protocols and designing new oligonucleotides. During the first year, we focused on structural characterization of Liberibacter cells in the citrus phloem and elucidating plant responses against Liberibacter infection. In the second year, we will carry out in situ hybridization and immuno-microscopy research using molecular markers of Liberibacter strains as planned in the original proposal. We will also elaborate callose detection/phloem loading research to characterize citrus varieties with different HLB susceptibility. The second year research will complement our structural research of HLB, help understanding the HLB disease development process, and improve current microscopy tools that will be applied not only to HLB but also to the ‘Zebra Chip’ disease, a Liberibacter-associated disease of potato.
The goal of the proposed research is to understand how Candidatus Liberibacter asiaticus causes Huanglongbing (HLB) disease and how tolerant plant resists Ca. L. asiaticus infection. 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. Major achievements: 1. One refereed journal article was published. Kim et al. 2009 Response of sweet orange (Citrus sinensis) to ‘Candidatus Liberibacter asiaticus’ infection: microscopy and microarray analyses. Phytopathology. 99:50-7. Investigation of the host response was examined with citrus microarray. The microarray analysis indicated that HLB infection significantly affected expression of 624 genes related to plant pathogenesis/stress, anthocyanin biosynthesis, cell wall metabolism, cell division, detoxification, lipid metabolism, metabolite transport, metal transport, nucleotide metabolism,phenylpropanoid / flavonoid / terpenoid metabolism, phytohormones, protein kinase, protein metabolism, protein-protein interaction, signal transduction, sugar metabolism, transcription/translation factors and unknown/hypothetical genes. 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 ‘Ca. Liberibacter asiaticus’ 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. The cDNA sequence of PP2 has been requested by four different research groups to use as the target to suppress the HLB symptom development. 2. Leaf samples from different varieties including grapefruit, Murcott, and Hamlin were collected from Florida citrus groves. Both healthy and infected trees were sampled. It is expected gene expression of those trees in the citrus grove will reveal more information when compare with the gene expression profile in greenhouse. Those samples will be used for gene expression analysis using microarray or SSH approaches. 3. Gene expression of Valencia leaf samples (healthy vs infected) in citrus grove are being conducted. Currently, two biological replicates were included in the preliminary test. Further microarray analysis is needed. 4. To further expand our current understanding of Ca. L. asiaticus-host interaction, we are currently comparing two susceptible and two resistant/tolerant cultivars grown in greenhouse. The plants have been inoculated in greenhouse. Finding key genes involved in HLB symptom development will reveal potential management strategy and lead to innovative research to control HLB. Research plan 1.1 Host response in greenhouse condition. Both Carrizo citrange and Poncirus trifoliata exhibited tolerance in the initial test. However, the inoculation has been inconsistent. The tolerance of Poncirus trifoliata to HLB is inconclusive. Thus host response of those varieties to Ca. L. asiaticus infection could not be done. Instead, we have adjusted our plan based on the most recent data on pathogenicity assays of different varieties to Ca. L. asiaticus. Both Persian lime and Eureka lemon are tolerant to HLB. 1.2 Host response in field condition. Due to the differences in greenhouse and field conditions, the disease development in greenhouse and field is quite different. We would to focus on sweet orange. we will use the following samples: leaf, stem, and root.
Work proceeded on several fronts. First, updated budgets that reflect the increase in grove maintenance costs were completed. These budgets indicate that a full program to minimize the effects of greening can increase grove care-taking costs by $450 per acre. Under typical Florida yields, this increase adds more than one dollar per box to the breakeven price. Second, plans are being formulated to hold an “economic summit” to held in April at the CREC. The participants will include economists from both Florida and Sao Paulo. During the last half-day of the meeting, representatives from the citrus industry are invited to listen and respond to the discussion. Third, a model of disease spread through a particular grove that was first developed in Sao Paulo by FUNDECITRUS is being adapted to Florida.
We want to obtain an in vitro culture of Candidatus Liberibacter by co-culturing the bacteria with insect cells to study Candidatus Liberibacter physiology, metabolism, virulence and its interactions with the insect vector. We tested various stable insect cell lines to in vitro culture Candidatus Liberibacter asiaticus (LAS), the Asian form of HLB disease, also found in Florida. We tested different inoculums (from various citrus and periwinkle plants) on different insect cell lines. We checked for the presence of LAS in inoculated cell cultures by direct PCR. Current results: – After Mamestra, Spodoptera, Drosophila cell lines, we tested an Aedes albopictus insect cell line. We didn’t detect LAS presence after inoculation. Actually, citrus inoculums have a deleterious effect on Aedes cells and we are now focusing on periwinkles inoculums. – We detected LAS in two lines of drosophila cell cultures by direct PCR. One line lost the detection after 6 transfers. Signal of detection was confirmed to be LAS. For one line, we are currently reaching the 15th culture transfer with a detection by PCR to be confirmed by sequencing. The detection remains very weak and we need to improve the conditions to get a higher titer. – The detection of the bacteria by direct PCR peaks at day 7 after transfer and then declines. The bacteria are multiplying and seem to follow the drosophila cells growth. We began to set up multiplex qPCR assays. Primer sets suitable for qPCR and specific of LAS and drosophila detection were selected and tested with success. – In order to reach higher bacterial concentrations, we tested complements of the culture medium: various sugars, vitamins or trace elements. We analyzed metabolic pathways potentially encoded by the released Liberibacter genome sequences to improve growth conditions and to define limiting factors and/or growth inhibitors. Of the complements tested some had a positive effect on the detection of LAS but negative effects on insect cells survival. With the decline of the insect cell cultures the bacteria was no longer detected. Currently, sodium pyruvate and sterile citrus juice had a positive effect on LAS detection but a lethal effect on insect cells. Proline and fructose had a positive effect on the bacteria detection and are now added to our co-cultures. – We analyzed insect cell culture medium sugar and amino-acids variations over culture time to identify potential LAS growth limiting factors. Rapid depletion of glucose and of some amino-acids by drosophila cells could be limiting factors and are candidates for our medium improvement. Current work: – We are now setting up multiplex qPCR assays to monitor the ratio of LAS and drosophila DNA over culture time. – We are combining complements to the insect cell culture media and looking for new ones to improve LAS concentration – We are analyzing insect cell culture medium minerals and trace elements variations over culture time to identify potential LAS growth limiting or improving factors. – We are testing new insect cell lines to get more consistent presence of the bacteria and higher concentrations. We are reaching our milestones for the first year of this project and we will look into the axenization of our primo-cultures.
The citrus extension agents conducted the “Low Volume Application Technology for Citrus Pests” program in six locations around central and south Florida in October. These programs discussed: low volume labeled materials; low volume application; application considerations; and worker safety. Program attendance exceeded 300 participants. Full program presentations can be viewed by going to the Citrus Agents web site at http://citrusagents.ifas.ufl.edu/events/PDF/LowVolumeApplication/LVA_program.htm . As part of greening control, selected agents are actively involved in assisting with the development of area wide spray events. These targeted sprays are to suppress psyllids over a wider geographic area. In addition to the above program, agents are actively involved in conducting grower programs (6), field visits (47), educating Master Gardeners (2) and the general public in greening identification and control issues.
With the program being recently funded, we are in the process of receiving order items to conduct the study. All materials were ordered in December of 2009 and are expected to be received within the next 30 days. Once delivered, studies will be designed to conduct proposed studies.
Six months after shearing or clipping citrus trees off above the soil, the study has shown that Remedy is effectively controlling stump sprouts at all treatment times of 0, 24, 48 or 72 hours after shearing the tree off above the soil. However, in a few cases lateral roots did exhibit limited sprout formation even with the stump being treatment with Remedy. Complete and thorough coverage of the cut surface is essential for adequate control. Sprouts were produced from the untreated stump as well as on limited lateral roots. The size of stump also impacts sprout formation with larger trunks producing more sprouts and quicker after shearing as compared to smaller stumps. Rootstock also has an impact on sprout formation. Rough lemon rootstock tends to produce more sprouts from the trunk and lateral roots than other rootstocks in this study. However, with limited data the difference between rootstock is not able to be determined. Sprouts from stumps will be collected and tested for HLB (greening) during the next quarter.
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