Management of phloem-limited bacterial diseases is very challenging. These bacteria employ unusual and sometimes unique strategies by which to optimize their niche occupation and obtain their nourishment from the host plant. Their location within the living (sieve tubes) plant cells, rather than in the intercellular spaces, offers different challenges and opportunities for them to avoid the host plant’s defense system. Phloem is also difficult for any bactericides to reach to control the pathogen population. Among the phloem-limited bacterial diseases, citrus Huanglongbing (HLB, greening) is one of the most devastating diseases. The current management strategy of HLB is to chemically control psyllids and scout for and remove infected trees. However, the current management practices have not been able to control HLB and stop spreading of Candidatus Liberibacter asiaticus (Las). The goal of the proposed study is to develop HLB management strategies which boost plant defense to protect citrus from HLB by exploiting the interaction between Las and citrus and understanding how Las manipulates plant defense. As requested by CRDF and SAB, we have revised project and will focus on the following two objectives: to characterize how Las causes HLB disease symptoms and how Las manipulates plant defense response by investigating the roles of putative virulence factors; to test different compounds in controlling HLB and characterize their mechanisms in controlling HLB. Recently, we compared the gene expression of PR1, PR2 and PR5 in healthy trees and Las infected citrus plants. The expression of PR1, PR2 and PR5 was significantly reduced in HLB diseased grapefruit as compared to healthy grapefruit after inoculation with Xac AW. We also tested whether infection by Las can make citrus more susceptible to infection by Xanthomonas citri subsp. citri. We also sprayed four times with different chemicals in 17 different combinations on citrus to test their effect in controlling HLB in one grove. Multiple compounds showed control effect. To further test those compounds, we have selected two more groves to expand the field test. The disease index of the two groves have been investigated. We compared the SA levels in HLB infected and healthy grapefruit after the inoculation with Xac AW. We also compared the SA levels in HLB infected and healthy Valencia citrus. We are continuing to evaluate the effect of different compounds on management of HLB both in greenhouse and in citrus grove. We have applied different compounds at three separate field trials. Following up study is ongoing. Four compounds were shown to have positive effect on controlling HLB based on two year field test results. To confirm this result, two more field trails were initiated in Lake Wales to further test the effect of these chemicals on HLB, and the follow up investigations are ongoing, including monitoring the HLB symptoms, disease incidence and Las titer in leaves. We are also testing the mechanism of those compounds showing positive effect on HLB control. We are characterizing the two putative virulence genes sndA and stbA of Las, e.g. subcellular localization and host proteins interacting with them using yeast two hybrid system. Interestingly, some targets identified are transcription regulators, transporters, zinc ion binding proteins.
The goal of this study is to understand the role of biofilm formation and quorum sensing (QS) in X. citri ssp. citri infection of citrus fruit and to prevent its infection by interfering with biofilm formation and QS. Recently, we compared the attachment of the QS mutants on the citrus fruit surface. Compared with wild type stain Xac 306, the quorum sensing mutant ‘rpfF showed significantly reduced attachment to the fruit surface as revealed by CLSM (confocal laser scanning microscopy) observation with the GFP-labeled bacterial strains. We also evaluated the effect of nine compounds on Xac biofilm formation on abiotic surfaces using the crystal violet staining method. The data obtained showed that three compounds were active in inhibiting Xac biofilm formation in NB liquid medium at. Three compounds exhibited a significant reduction in biofilm formation both on polystyrene surface and in glass tubes compared to the untreated control, where the level of biofilm formation were reduced to 50% and 60% of control, respectively. Plant test in greenhouse showed that treatment with the three compounds prior to infection could reduce biofilm formation of Xac on leaf surface, reduce the formation of canker lesions on spray-inoculated grapefruit leaves with the wild-type strain. Effects of the three compounds on Xac on detached immature citrus fruit were also tested using spray inoculation. Preliminary results showed that these small molecules affected Xac 306 infection of unwounded and wounded citrus fruits at sub-inhibitory concentrations. We have completed testing the effect of those compounds in different combinations with copper based bactericides in controlling Xac infection of grapefruit plants in the greenhouse. The sensitivity of biofilm and planktonic cells of Xac 306 to copper (copper sulfate) were evaluated by measuring the MICs. Biofilms are less susceptible to copper than planktonic cells. Effect of the selected compounds on sensitivity of Xac planktonic cells and biofilm cells to copper sulfate was also investigated. In the NB medium, planktonic cells exhibited a MIC of 0.50 mM CuSO4 without biofilm inhibitor. In the presence biofilm inhibitors at sub-MIC concentrations , the MICs of CuSO4 against Xac 306 planktonic cells were decreased to 0.25 mM. In a in vitro biofilm system test, the combined use of copper sulfate and the compounds individual or both resulted in significantly increased killing compared to killing by copper sulfate alone. The results have been accepted for publication consideration by Phytopathology. One patent is filed based on the results. We also identified multiple new biofilm inhibitors. The effect of those biofilm inhibitors to control citrus canker is being investigated. We tested the survival of both biofilm deficient and QS mutants on fruit surface. Effects of biofilm formation inhibitors on Xac infection on detached immature citrus fruit were tested using spray inoculation. The inhibitors affected the infection of Xac on both unwounded and wounded citrus fruits. We further tested three potential biofilm inhibitors. We are setting up the field trial to test the effect of the identified biofilm inhibitors to control citrus canker.
Development of alternative or complementary approaches for effective management of citrus greening is highly desirable and will greatly help the citrus industry due to the difficulty to control the HLB disease. Considering the highly destructive nature of HLB disease and the lack of control measures, there is a huge potential to develop antimicrobial small molecules against the causal agent thus to suppress the population of Ca. L. asiaticus in planta and to reduce the innoculum for psyllid transmission. The most common targets for antimicrobial agents development include receptors, proteins and enzymes, DNA, RNA and ribosomal targets. Among them, proteins have become the major target due to their druggable characteristics. In this study, we presented our research on screening small molecule inhibitors against SecA. SecA is one essential component of the Sec machinery which provides a major pathway of protein translocation from the cytosol across or into the cytoplasmic membrane. The Sec pathway was also shown to be required for virulence of Ca. L. asiaticus in our study. SecA is the protein translocase ATPase subunit, which is involved in pre-protein translocation across and integration into the cellular membrane in bacteria. In our study, we expanded our previous study in identifying lead antimicrobial compounds with higher activities by targeting SecA using various computational techniques like homology modeling, virtual screening, molecular docking & minimization. Due to the uncultivable nature of Ca. L. asiaticus, we tested the potential inhibitory effect of the selected compounds against Agrobacterium tumefaciens, which is phylogenetically related to Ca. L. asiaticus. Twenty compounds were selected for biological activity study against SecA of Ca. L. asiaticus and A. tumefaciens. Five compounds were found to inhibit the ATPase activity of SecA of Ca. L. asiaticus in nano molar concentrations and showed antimicrobial activities against A. tumefaciens with MBC ranging from 128 ug/ml to 256 ug/ml. These compounds appear to be suitable as lead compounds for further development of antimicrobial compounds against Ca. L. asiaticus. Those findings were published in the article entitled: Identification of small molecule inhibitors against SecA of Candidatus Liberibacter asiaticus by structure based design. on Eur J Med Chem (http://dx.doi.org/10.1016/j.ejmech.2012.05.035). To test the application potential of those compounds on plants, the phytotoxicity studies were performed on the five compounds against citrus. At higher concentrations (0.25mg/ml), all five compounds showed phytotoxicity. When the concentrations were diluted to 0.025mg/ml, the toxicity went down to mild to low. Currently, we are testing the inhibitory effects of the five compounds again Ca. L. asiaticus in planta. We are evaluating different solvents and adjuvants for the five compounds. Their antimicrobial activities against Liberibacter crescens, which is closely related Ca. L. asiaticus and could be cultured, have been tested. Currently, we are optimizing these five antimicrobial compounds to identify compounds higher antimicrobial activity. Fourteen new small molecule inhibitors were identified. Further studies are being conducted on those 14 compounds. Currently, one manuscript is in preparation to report the latest findings. We are also in collaboration with IBM to further expand this study. Optimization of C16 was conducted. Extension of this project was approved by CRDF and we are continuing this project.
The goal of the proposed study is to characterize the effect of application of beneficial bacteria (MICROBE Program) on management of HLB. Currently, we are setting up the experiments to test different Microbe Products in management of HLB. We have developed a culture collection of approximately 400 bacteria initially isolated from the root and rhizosphere of citrus. These bacterial isolates have been screened for various beneficial traits . We are also evaluating the antagonistic activity of these bacterial strains against some well-known plant pathogenic fungi. Especially we have screened a bacterial isolate designated as 43A which possess multiple plant growth promoting activity and is also able to antagonize different fungi. We are also testing the plant growth promoting activity of 24 isolates using seed germination pouch in greenhouse. We have also selected several Bacillus spp. possessing multiple beneficial traits to develop bacterial consortium which can be further developed as carrier based bioformulation. Assay for compatibility between isolates using antagonistic survival tests showed that all the selected beneficial bacteria are compatible with each other. Plant growth promoting activity of six selected isolates was evaluated using the model plant Arabidopsis grown in vitro. The results suggested that three isolates could promote plant growth. The plant growth promoting activity of these six isolates was tested using citrus (grapefruit) seedlings in greenhouse. Greenhouse assays suggested that a consortium of three Bacillus and relative isolates (AY16, PT6 and PT26A) may delay the development of both HLB symptoms and pathogen population on citrus leaves after root inoculation. The potential of the consortium to recover the tree decline from HLB infection is being evaluated in greenhouse. The growth conditions of the three strains were optimized using a small fermenter. Three antifoam agents, A204, PPG200 or M-Oil did not affect the growth of the three bacterial strains. The initial neutral to alkaline pH values (7.0 ~ 8.0) favor growth of the three bacteria in LB, while acidic pH (5.0 ~ 6.0) suppress bacterial growth. The optimal cultural temperature was determined to be around 30C with average bacterial population of 109-1010 cfu/ml after 20-hour incubation, although the bacteria may grow slowly under room temperature (~ 23C). The shelf life of three different formulations of the bacterial culture is being evaluated under room temperature. In a six-month time course, the bacterial populations in LB broth, OPB broth and tape water are comparatively stable with initial and final both at ~ 108cfu/ml. Two field trails for estimation for bacterial activity improving the health of HLB diseased trees were initiated and the first round data were collected following bacterial inoculation, including HLB disease incidence and Las titer in leaves and root. A gyrB-based qPCR method for detection of ATY16, PT6 and PT26A is being developed to investigate the fate of these bacteria in the environment after application.
The goal of the research is to control citrus HLB using small molecules which target essential proteins of Candidatus Liberibacter asiaticus (Las). In our previous study, structure-based virtual screening has been used successfully to identify five lead antimicrobial compounds against Las by targeting SecA. SecA is one essential component of the Sec machinery. Those compounds showed promising antimicrobial activity. However, further work is needed to apply the compounds. We will evaluate the important characteristics of our antimicrobial compounds including solvents and adjuvants, phytotoxicity, antimicrobial activities against multiple Rhizobia, antimicrobial activity against Las, application approaches, and control of HLB. Those information are critical to for the practical application of those antimicrobial compounds in controlling HLB. We also propose to further optimize the five lead compounds. In addition, we propose to develop antimicrobial compounds against lipid A of Las. The lipid A substructure of the lipopolysaccharides (LPS) of Sinorhizobium meliloti, which is closely related to Las, suppresses the plant defense response. Las contains the complete genetic pathway for synthesis of lipid A. We hypothesized that Las uses lipid A to suppress plant defense. Thus, targeting lipid A could activate plant defense response. Lipid A is also an ideal target and has been targeted for screening antimicrobial compounds for multiple pathogenic bacteria. We plan to accomplish our goal by conducting the following objectives: Objective 1: Evaluate and quantify important characteristics of the antimicrobial compounds identified in our previous study. We are optimizing the compounds in collaboration with IBM. Currently, we are evaluating the best range of composition ratio among each component (%weight) of AIs, solvents and surfactants. The following characteristics are being evaluated: 1) emulsion stability and ease of emulsion; 2) stability of diluted concentrate; 3) freeze-thaw stability; and 4) phytotoxicity to citrus species. Objective 2: Control HLB with antimicrobial compounds targeting lipid A of Las. Pharmacophore based virtual screening and QSAR studies are being conducted to identify antimicrobial compounds targeting lipid A of Las.
Mid Florida Citrus Foundation (MFCF) a 501c5 not for profit organization which has supported (past 25 years) and currently supports citrus research efforts of scientists from the University of Florida, USDA and private industry. The MFCF supports citrus research through the employment of a full time grove manager whom works closely with researchers to ensure that their projects are handled properly and that the grove is an excellent condition. The management of this grove requires extra financial commitment as grove care costs tend to be higher than commercial groves due to the nature of many of the research projects. Current projects being conducted at the MFCF are Asian citrus psyllid (ACP) pesticide evaluation control trials, low volume applicator trials, windbreak evaluation, HLB nutritional programs, new and existing herbicide trials, variety and rootstock evaluation trials. During the recently completed quarter (July 1 to September 30, 2013), the following highlights occurred at the Mid Florida Citrus Foundation ‘ A.H. Krezdorn Research Grove: ‘ Plant Improvement Team o Scion selections being evaluated for HLB tolerance established ‘ Dr. Singh initiated a trial evaluating herbicide tolerance of selected USDA rootstocks to various residual herbicides continues ‘ Evaluation of NpHuric to decrease pH and bicarbonates demonstration continues ‘ UAS of America evaluation on supplemental materials applied to the soil and/or foliage to increase tolerance to the affects of HLB and citrus canker continues ‘ Dr. Futch evaluations: o Continued evaluations of trifoliate rootstocks for HLB tolerance o Established an herbicide trial for Dow Agroscience which resulted in significant damage in some treatments ‘ Applications of the ‘Boyd Program’, Keyplex and Ben Hill Griffin programs continued in the ‘commercial scale’ nutritional trial. ‘ Conducting late summer/early fall fertilizer and pest management programs for the groves o Herbicide program on schedule o Heavy psyllid pressure requiring extra applications of insecticide o Evaluation of ‘chemical mowing’ as a cost savings vs. mowing in a portion of the grove proving successful ‘ Applications of seven nutritional treatments continue in MFCF replicated nutritional programs evaluation and plant growth data taken in July ‘ Commercial Trials: o Eurofins evaluations on disease and insect management continue o Evaluations of Agri Quest Citrus Root Health Improvement Project continue o Keyplex nutritional trial evaluations continue o Bayer demonstration of Optiva programs for citrus canker management continue o Ag Consulting established a rust mite trial o DuPont demonstration for row middles management with Matrix Herbicide established ‘ Drs. Stelinski and Rogers have continued evaluations of Asian citrus psyllid and citrus leafminer management in their areas.
The primary goal of this project is to develop cryotherapy as a practical, reliable method of eliminating graft transmissible pathogens from Citrus and citrus relatives without inducing juvenility. Permits have been obtained to ship pathogen infected citrus material from Riverside, CA and from the Exotic Disease Quarantine Laboratory, Beltsville, MD to Ft. Collins, CO. Once the material has been treated in Ft. Collins, CO, permits have been obtained to enable shipping of leaves from the recovered plants to Riverside, CA for pathogen testing using laboratory methods. Personnel at Riverside, CA and Beltsville, MD have been trained on the cryotherapy protocol, and cryotherapy experiments are now being conducted also at the Riverside and Beltsville locations. Emphasis at the beginning of this project was on elimination of citrus viroids and Citrus tatterleaf virus, as these pathogens are the most difficult to eliminate using the traditional methods of thermotherapy and shoot tip grafting. We have data showing the effective elimination of these pathogens from citrus, and a manuscript is being prepared to report the results. Current research is concentrated on other graft transmissible pathogens of citrus, with the domestic pathogens being the focus in California and the exotic pathogens being the focus in Maryland.
The primary goal of this project is to develop cryotherapy as a practical, reliable method of eliminating graft transmissible pathogens from Citrus and citrus relatives without inducing juvenility. Permits have been obtained to ship pathogen infected citrus material from Riverside, CA and from the Exotic Disease Quarantine Laboratory, Beltsville, MD to Ft. Collins, CO. Once the material has been treated in Ft. Collins, CO, permits have been obtained to enable shipping of leaves from the recovered plants to Riverside, CA for pathogen testing using laboratory methods. Personnel at Riverside, CA and Beltsville, MD have been trained on the cryotherapy protocol, and cryotherapy experiments are now being conducted also at the Riverside and Beltsville locations. Emphasis at the beginning of this project was on elimination of citrus viroids and Citrus tatterleaf virus, as these pathogens are the most difficult to eliminate using the traditional methods of thermotherapy and shoot tip grafting. We have data showing the effective elimination of these pathogens from citrus, and a manuscript is being prepared to report the results. Current research is concentrated on other graft transmissible pathogens of citrus, with the domestic pathogens being the focus in California and the exotic pathogens being the focus in Maryland.
We have completed the analysis greenhouse treated trees and concluded that each of the therapeutic spray applications could be validated by corresponding changes in gene expression of pathways that would be impacted by the sprays. We analyzed the transcription of 52 individual genes using Real Time PCR (qRT-PCR), 30 gave us valuable information on how each of these treatments was working. Our focus is on using these spray applications on field trees. On June 25, 2013 we treated a young orchard of Valencia on Swingle rootstocks in the Indian River Region of Central Florida. We treated 3 trees per treatment for a total of 24 trees for the 8 treatments. Six of these were therapeutic treatments included: 2, L- Arginine, 2, gibberellin in combination with 6-benzyl adenine (BA), and 2, atrazine in combination with sucrose. The remaining two were control treatments that accounted for the surfactant Silwet, K-phite and LDKP3XTRA that were added to all treatments. We have sampled leaves at day 0, 3 and 6 collecting 72 samples of leaves that were flash frozen in liquid nitrogen and used to extract DNA for CaLas titer determinations and RNA for the real time PCR analysis for gene expression and pathway analysis. The analysis of the DNA revealed that greater than a third of the plants were infected. Average CLas titer in field treated trees prior to spraying was ca. 1×107 cells per g fresh weight leaf tissue. The analysis of the 72 RNA samples is underway with 9 of the 30 genes measured earlier. We are conducting phenotypic analysis of all 24 trees. Trees were rated for the presence of HLB symptoms, stem diameters and canopy height and diameter were developed. These measurements will continue throughout the duration of the experiment and represent a phenotypic analysis of the trees during this experiment. We are currently doing another sampling of DNA and RNA from the 24 trees, a 90 day time point from the first spray in June 2013. We plan to examine the expression of the 9 genes to compare the expression levels 3 months post spray treatment. We are also planning a second spray of the same 24 trees but will also spray an additional 48 trees in the same location, 6 each for each of the treatments. We will do a CaLas titer estimation of all 72 trees but isolate RNA from 48 trees 6 tree replicates for each of the 8 sprays. We plan the 2nd spray for early October 2013 and a third spray in Feb 2014. The sprays coincide with the flush observed in this location to have a maximum impact of the spray treatments.
At the request of the CRB, as of 10-1-11, Grafton-Cardwell and Morse merged their core entomology research efforts under a single project, 5500-501 (Morse’s portion of the project is 5500-501b). We have always coordinated our research efforts but this arrangement formalizes the situation. This report summarizes the Morse lab’s recent research under this coordinated project (all arthropod research except ACP which is a separate project, i.e. 5500-189). Our major effort this last year has focused on helping the industry to deal with Fuller rose beetle in relation to citrus exports to Korea. For the last several years, Korea has put increasing pressure on the CA industry to reduce egg mass levels on export fruit and there is a concern that in 2013-14, loads found to be infested with viable egg masses may be denied entry into that country. Morse presented a summary of recent research on Fuller rose beetle at the Korea Export meeting held in Tulare the morning of 9-4-12 and two talks, one on bean thrips and a second on mite contaminants of export citrus, during the 9-4-12 afternoon Australia / New Zealand Export meeting. An update on the FRB situation was given to the CCM Marketing committee 2-20-13, to growers and PCAs in Exeter 2-21-13, and to the CCQC Board in Pomona 3-26-13. A meeting with packing house personnel, CCM, and CCQC was held 4-12-13 in Exeter, a FRB field day at the Lindcove REC on 4-22-13, and an FRB presentation at a CCQC meeting in Visalia 6-17-13. We contributed to a 24(c) application requesting that the present 2ee label allowing use of 0.5 lb ai/a/yr bifenthrin as a trunk spray be expanded to allow a total of 1.0 lb ai/acre. To support the 24(c) request, three field studies were conducted to determine fruit residues after either 4 applications of 0.25 lb ai/a spaced out every 6 weeks (4 week PHI) or 2 applications of 0.5 lb ai/a spaced 12 weeks apart (9 week PHI). Studies were run in a commercial grove north of Bakersfield (Morse lab), at Lindcove (Grafton-Cardwell), and in Riverside (Morse). A method of analyzing bifenthrin residues on trunks was developed in collaboration with Dr. Jay Gan (UCR Dept. of Environmental Sciences) and is being used to analyze levels of residues on trunks at various dates post application with different application rates. Data from a FRB adult bioassay run at Lindcove concurrent with trunk residue analysis allows us to determine when trunk sprays are no longer effective and an additional study was recently completed at Lindcove evaluating the impact of sprinkler irrigation impacting the trunk. We have also set up a large FRB experiment at a site in the Pauma Valley that has very high levels of FRB. Citrus thrips resistance to Delegate has been confirmed in the San Joaquin Valley. Two products nearing registration on citrus that will be useful in control of citrus thrips (as well as other pests) are Bexar and Closer. Fall fruit scarring evaluations were taken at a spring 2012 mandarin field trial comparing the efficacy of abamectin, Delegate, Bexar, and Closer but citrus thrips levels were relatively low at location in 2012 (despite extreme levels in 2011), making it difficult to draw strong conclusions. We are examining contaminants of export citrus using 10 randomly selected cartons per load from a variety of citrus packing houses. To date, data have been collected from 11 navel 1 Valencia, 1 pummelo, 5 grapefruit, and 4 mandarin loads (22 total).
A field trial was established during the period on Header Canal Road (St. Lucie county) where a planting of Corymbia torelliana (Myrtaceae), a relative of Eucalyptus, was established in fields formerly occupied by citrus and infested by Diaprepes root weevil. Males and females, mating pairs and egg masses of Diaprepes were observed throughout the plantation. In addition to documenting the susceptibility of this fast-growing tree to Diaprepes, the infestation presents an excellent opportunity to study the distribution and movement of Diaprepes in the field in response to the deployment of potential attractant blends. The simple and open architecture of the tree (compared with citrus trees) is favorable for comprehensive sampling of Diaprepes. We initiated a survey of 20 acres of C. trorelliana to describe the distribution of males and females and determine the degree of aggregation prior to deployment of male pheromone and kairomone blends. Post-deployment surveys will determine the extent to which odor sources influenced distribution. If movement in response to deployment of volatile blends can be demonstrated in the field, design of appropriate traps will be pursued. Collections of volatile compounds from the headspace over field-collected and lab-reared female adult Diaprepes were initiated during the period. Extracts will be concentrated and analyzed by GC-MS and GC-coupled electroantennography to determine the presence of a female-produced pheromone. Synthesis of the male-produced pheromone methyl (E)-3-(2-hydroxyethyl)-4-methyl-2-pentenoate was completed in-house. This will be used in olfactometer trials to identify an optimal blend of pheromone and kairomones, and in the field as described above.
CREC-ACPS project at Auburndale A Citrus Research and Education Center Field Day was held on December 13, 2012, at the Auburndale ACPS experiment and jointly at the St. Helena Road rootstock experiment of Jude Grosser and Fred Gmitter. About 200 growers attended the morning’s presentations and demonstrations in the citrus experiments. See http://128.227.177.113/ACPS/Documents/Fielddayhandout2012mergedV1.pdf for a full discussion of the day’s proceedings. Highlights of the ACPS field research for year 4: The tour consisted of four stops, shown in the tour plan. Selected trees had been harvested early and fruit yield and vital statistics were displayed on posters at each harvested tree in each location. ‘ Stop 1 is to view and discuss the grower standard treatment ‘ Stop 2 is a walk-through the high density (363 trees/acre) ‘Hamlin’ trees with moderate to strong size control caused by high fruit set and C-35 rootstock. ‘ Stop 3 is to discuss the fertigation strategy used in year 4, and to view the prototype custom hedger on a 4 foot-wide tractor, to be used for hedging and topping in high density groves. ‘ Stop 4 is a walk-through to view the comparison between the microsprinkler-OH, Swingle rootstock and the drip-OH high density, C-35 rootstock performance. The field day was a great success despite the inclement weather. The high, early fruit yield on HLB-asymptomatic trees was self-evident. Also striking was the very high yield achieved per acre on relatively small trees (maximum height 7 feet). The weak performance of HLB-symptomatic trees, especially the high rate of pre-harvest fruit fall, was an issue that generated much discussion among growers, researchers and consultants. The actual plot harvests which were picked a week later will be analyzed and presented in the next quarterly report (March 2013). CREC-ACPS project at block 22, CREC The new 4-acre ‘Valencia’ orange ACPS experiment which was planted in August, has established well during the mild fall weather, and the growth is already above average due to the intensive daily fertigation schedule. Regular plot measurements of tree growth, soil properties and foliar nutrition will begin in the spring of 2013.
IRREC-ACPS project (Barrett Gruber, co-PI) The second objective of this project is to establish an ACPS solid-set trial at the UF-Indian River Research and Education Center (IRREC, Fort Pierce). The goal of this objective is to evaluate the performance of ACPS for citrus fresh-market citrus varieties and regional disease problems such as canker. After receiving funds in July 2012, RTK-DGPS mapping was used to stake the end-of-tree-rows on 8 acres of research property at IRREC and was done to obtain an estimate of total tree numbers. This mapping was used to plan for two different planting densities: 8′ . 25′ (218 trees / acre, ‘conventional modern planting’); and, 9′.1/4(3’+19’+3’+25′) (387 trees / acre, ‘higher density ACPS planting’). After estimates were obtained, a total of 3,098 new grapefruit trees (1,305 Ray Ruby/US897, 1,305 Ray Ruby/Kuharske, and 488 Ray Ruby/Swingle) were ordered on contract from a certified, commercial nursery. The nursery growing these trees provided a revised estimate of delivery. As of June, 2013, the revised delivery estimate for these trees is late July and August of 2013. The irrigation systems for the ACPS grapefruit trial at IRREC have been designed and we are in the process of ordering the necessary poly-tubing, emitters, valves, pumps, tanks, and controllers. A Ph.D. graduate student was recruited and hired through the UF-Horticultural Sciences Department to assist in executing this objective. Currently, this student is in Gainesville completing her required coursework. The student would be able to start contributing work towards this objective in August, 2013, and the summer of 2014. A part-time OPS technician will also be hired in fall, 2013, to begin contributing work towards executing this objective. An OPS technician has not yet been hired because we are awaiting delivery of trees from the nursery (see above). CREC-ACPS project at Auburndale A comprehensive survey of HLB incidence in this 14-acre ‘Hamlin’ experiment was completed in May 2013. The total percentage of HLB-symptomatic trees now stands at 44%, and has more than doubled during the past year. Clearly HLB disease is the most serious threat to the long-term survival and production of trees in this experiment. However this spring we documented that the most seriously declining trees were also impacted by Diaprepes root weevil larvae, sting nematodes, and Phytophthora – all having negative effects on root health. The appearance symptoms on HLB-affected trees was most severe during the dry spring season, despite efforts to optimize both nutrition and water relations with daily computerized fertigation. When the summer rains began in June, there was an immediate improvement in the appearance of the trees, seen as a reduction in HLB symptom severity. Fruit set on all 7 ACPS treatments in the experiment is very good on asymptomatic trees and about average on symptomatic trees. We anticipate picking fruit yields of 800-900 boxes/acre from asymptomatic trees of the best treatment in year 5 – a measure of the potential for ACPS to produce early, high yields. The highest average yield in year 4 was 622 boxes/acre.
CREC-ACPS project at Auburndale The ‘Hamlin’ orange harvest from this ACPS experiment at 4 years demonstrated the continued synergistic interaction between hydroponics fertigation, high density planting, and superior rootstock performance (C35 better than Swingle). The average yield of 622 boxes/ac for the best treatment combination of drip OHS fertigation, high density planting of 363 trees/acre with C35 rootstock was produced with only 186 lb N/ac and about 7 acre-inches of irrigation water. Conventionally grown large citrus trees typically use more nitrogen and water to produce this amount of fruit, after more years. Every year saved in the citrus production cycle represents a year of production costs and resources (water, fertilizer, pesticides etc) saved. Moreover, every year saved due to accelerated production helps to offset losses caused by encroaching diseases, and, potentially early return on the investment. The cumulative 4-year fruit yield from the best ACPS treatment was more than double the cumulative yield from the conventionally grown citrus treatment (218 trees/acre, Swingle rootstock, dry granular fertilizer, standard microsprinkler irrigation with scheduling by grower). The fruit/juice quality of the 4th year harvest was only significantly different for Brix, Acid, and Ratio (see table below) and overall quality was good. We emphasise that all the fruit yield and quality data presented represent the asymptomatic trees in the experiment; fruit yield of HLB-symptomatic trees was reduced by up to 90%, mainly due to pre-harvest fruit drop. Treatment Brix Acid (%) Ratio Solids (lb/box) Fruit weight (g) Juice (%) STD-Sw (218′) 10.41 a 0.643 a 16.2 b 5.33 164.3 56.9 MS-OH-Sw (218) 9.95 b 0.623 a 16.0 b 5.15 173.6 57.5 Drip-OH-Sw (218) 9.90 b 0.567 b 17.5 a 4.60 170.3 57.8 Drip-OH-Sw (303) 10.00 b 0.588 b 17.0 ab 5.19 161.5 57.7 Drip-OH-C35 (303) 10.16 ab 0.588 b 17.29 a 5.26 167.1 57.6 Drip-OH-C35 (363) 10.51 a 0.593 b 17.8 a 5.22 159.7 55.1 Significance (F-pr) 0.011 ** <0.001 *** 0.007 ** 0.519 NS 0.116 NS 0.096 NS LSD (0.05) 0.380 0.0263 0.970 1.22 11.12 2.43 ' Trees per acre CREC-ACPS project at block 22, CREC The mild winter season of 2012/13 allowed most of these young 'Valencia' trees to produce leaf flush throughout the normally dormant months of December to February. The first soil samples collected in the spring showed early signs of rhizosphere acidification in the ammonium nitrate - based hydroponics fertigation treatment (calcium-free) versus the non-acidifying calcium nitrate - based fertigation treatment (calcium-containing). The soil samples were collected at a radius of 4 inches from the drip emitters, to a depth of 6 inches. The average soil pH differences of about 0.5 units were significant and quite dramatic, given the short duration of the trial at that time. Ammonium nitrate is an acidifying nitrogen source, primarily from the oxidation of ammonium to nitrate ions in the soil (nitrification). Calcium nitrate on the other hand does not oxidise and remains neautral or slightly basic in reaction with the rhizosphere.
Experiments to determine the efficacy of different nano-particle systems to deliver nutrients to citrus leaves were started by optimizing conditions for experimental treatments. In general, the experimental design consists of delivering of Mg to citrus leaves by means of three different nano-particle systems. Given their chemical and physical properties, we have selected PAMAM-dendrimers (~ 5 nm), kaolin particles (~1-5 um) and liposomes (~70 nm). To maximize efficiency, conditions of Mg deficiency were created by irrigating trees with a nutrient solution without Mg. Trees developed Mg deficiency symptoms within a few months and are now ready for experimental use (Phase 1). In phase 2, kinetic studies on the binding or carrying capacity of the different nano-particles to Mg were conducted. Concentration dependent kinetic experiments indicated that each PAMAM-dendrimer nano-particle is capable of binding a specific amount of Mg ions. Binding of Mg was demonstrated to be reversible depending on the pH of the environment. Concurrently, kaolin particles were treated similarly and showed a stronger Mg binding capacity than PAMAM-dendrimers. Chemical analysis showed that, although kaolin particles already contain Mg, they have sufficient exchangeable binding sites to bind additional cations. Liposomes, by their hydrophobic nature, will trap solutions of any concentration as they form spherical vesicles from lipid sheets. Greenhouse treatments have commenced with PAMAM dendrimers and the corresponding control of aqueous Mg solution at similar concentration. Treatments with other nano-particles will follow.
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