Progress report for the fourth quarter of the 2019/2020 project yearThe purpose of the project is to develop new guidelines for restoring root health and improving overall tree nutrition for Florida oranges and grapefruit. The objectives of the project are to:1. Determine optimal nutrient concentrations in roots and leaves for multiple grapefruit and orange varieties.2. Compare and contrast fertigation, soil, and foliar fertilization to identify best application method for uptake of nutrients into both underground and aboveground components.3. Investigate the relationship between root and leaf nutrient contents to tree health, yield, and fruit quality as well as bacteria titer.4. Generate updated and new guidelines for optimal nutrient contents for roots and leaves for HLB-affected trees. Progress to date:The project is being conducted at three sites: Citrus Research and Education Center (CREC), Southern Gardens Citrus near Clewiston, FL and Indian River Research and Education Center (IRREC). Data collection continued during this quarter on root scanning, canopy size determinations and soil sampling on the central Ridge and southwest Flatwoods along with fertilizer treatment applications. Trends on soil and root nutrient levels in Valencia orange, for example, continue to follow the pattern of treatments e.g. 4x>2x>1x, for soil applied treatments in summer and fall 2020. Similarly, for foliar applied treatments, the pattern in leaf nutrient concentration follows 1x<2x<4x. All sampling for the fourth quarter is complete. The aim of the study at the UF/IFAS IRREC in Fort Pierce, FL was to relate nutrient concentrations in grapefruit leaves and roots to indicators of tree health and root growth. The research was conducted on flatwoods soils in a randomized complete block design field study on `Ruby Red' grapefruit. Micronutrients (B, Fe, Mn and Zn) were applied using three different concentrations (1x, 2x, and 4x current UF/IFAS guidelines) in the form of either dry granular water-soluble fertilizer, controlled-release fertilizer, or liquid fertilizer. A total of 600 trees divided in 40 experimental units were employed. We collected leaf and root nutrient concentrations, canopy volume and tree height twice a year. Mini-rhizotrons were installed at the beginning of the experiment and root images were taken four times a year. Results showed increased micronutrient concentrations in the leaves among all treatments. There were no significant differences in tree height, canopy volume, root length, and root diameter. Graduate student Lukas Hallman presented his research at the American Society of Horticultural Science virtual Annual Meeting on August 10-13, 2020: Hallman, L.; Ferrarezi, R. S.; Kadyampakeni, D.; Wright, A. L.; Lange, J.; Johnson, E.; Rossi, L. 2020. Micronutrient uptake and root growth and development in HLB-affected grapefruit on Florida Flatwoods soils. HortScience 55(9): S3 (Abstr.). Graduate student Tanyaradzwa Chinyukwi presented her research at the American Society of Horticultural Science virtual Annual Meeting on August 10-13, 2020: Chinyukwi, T., S. Kwakye, and D. Kadyampakeni. 2020. Response of HLB-Affected trees to differential foliar, and Soil Macro- and Micronutrient Applications. HortScience 55(9) (Abstract) Graduate student Tanyaradzwa Chinyukwi presented portions of her research at the Florida State Horticultural Society virtual Annual Meeting on October 19, 2020. She has submitted a conference proceedings paper that will be in print soon. Chinyukwi, T., S. Kwakye, and D. Kadyampakeni. 2020. Performance of HLB-affected trees to soil macro- and micronutrient applications. Proceedings of the Florida State Horticultural Society 133:xx-xx. Plans for Next QuarterThe team will continue with fertilizer treatments and data collection including HLB rating assessments, canopy size and root growth measurements and reporting on the progress of the project.
1. Please state project objectives and what work was done this quarter to address them: Objective. To determine the influence of compost during the first three years of tree establishment on growth, productivity, and root and soil health of citrus trees on rootstocks with different vigor-inducing capacity. Leaf samples were collected for nutrient analysis. Leaf and soil nutrient data analysis was completed. Significant differences between compost plots and no-compost (control) plots were found for several variables. Leaf K concentrations were higher in trees on compost plots than control plots whereas the reverse was found for leaf Ca and B. None of the other nutrients were affected. A rootstock effect was measured only for leaf Mg concentrations which were higher in trees on US-812 compared with the other rootstocks. Soil K, Mg, Ca, and B concentrations were significantly higher in the compost plots than in control plots; the reverse was found for Mn and copper. Compost plots also had a higher organic matter content, a higher CEC, a higher pH, and a higher base saturation for Mg and Ca than control plots. Root cores were collected for root physiological (root respiration) and root structural analyses. Root length and structure was not significantly affected by the compost treatment, but some structural differences were found among rootstock cultivars. Root respiration was neither affected by the compost nor the rootstock cultivar. 2. Please state what work is anticipated for next quarter:We will conduct the one-year horticultural measurements.We will continue with the rhizospere sample processing.The 2nd annual compost application will be conducted. 3. Please state budget status (underspend or overspend, and why): Approximately 30% of funds have been spent, which is in accordance with the timeline.
Objective 1. To illustrate whether application of bactericides via trunk injection could efficiently manage citrus HLB and how bactericides via trunk injection affects Las and HLB diseased trees. 1.1. Determination of the in planta minimum bactericidal concentrations (MBCs) of bactericides against LasThis has been completed for both streptomycin and oxytetracycline against Las. A manuscript entitled: “Residue dynamics of streptomycin in citrus delivered by foliar spray and trunk injection and effect on Candidatus Liberibacter asiaticus titer” was submitted to Phytopathology for publication.1.2. Effect of bactericides via trunk injection on citrus HLB disease progression, tree health, yield and fruit quality in different aged trees with a different disease severityThe field experiments were performed at four different groves on different aged trees with a different disease severity. They are one located in Avon Park, FL, 3-year old Valencia trees; one in Bartow, FL, 2-year old W. Murrcot trees; and one in Auburndale, FL, 7-year old Hamlin trees (planted in 02/2012). The last one is in CREC-, Lake Alfred, FL, 20-year old Hamlin trees. The HLB disease severity and tree size (canopy volume and trunk diameter) in the four groves were estimated immediately prior to treatment application. For the field tests, the experiment design is a randomized complete block design (RCBD) for 9 treatments, including 6 injection treatments (3 different doses for OTC or STR), 2 spray treatments (OTC or STR spraying), and one No treatment as a negative control. Each injection treatment consisted of 9 or 15 trees divided into 3 blocks of 3 or 5 trees each. Each spray treatment consisted of 30 trees divided into 3 blocks of 10 trees each. For all the four field trials, the injection treatment applications were completed by the end of April 2019. The 1st application of spray treatments were completed during spring flushing in February or March 2019, the 2nd applications were conducted in late June to early July 2019, and the 3rd applications were conducted in early to middle October 2019. Leaf samples have been collected from the treated trees at the following time points: 0 (pre- injection), 7, 14, 28 days, 2, 4, 6, 8, 10 and 12 months after treatment (MPT). The estimation of Las titers in these leaf samples are ongoing with qPCR assays. The first estimation of HLB disease severity and growth performance (height, trunk diameter, and canopy volume) of immature trees after treatment were performed in May 2019 (three months after the injection) and continued in a 3-months interval. Fruit yield and quality data were collected for the Bartow trial (W. Murrcot), Auburndale trial (Hamlin), and CREC trial (Hamlin) in January 2020. Fruit yield was estimated for the Avon Park trial (Valencia) in April 2020. Leaf samples were collected for Las population and antibiotic residue assays for those trials. Objective 2. To examine the dynamics and residues of bactericide injected into citrus and systemic movement within the vascular system of trees and characterize the degradation metabolites of bactericides in citrus. Leaf and root samples have been collected from OTC or STR treated trees in the Avon Park grove at the following time points:0 (pre- injection), 2, 4, 7, 14, 28 days, 2, 4, 6, 8, 10, and 12 months after injection. The samples have been processed for OTC or STR extraction, and the concentrations of OTC and STR in these samples were determined by HPLC assays. Fruit samples were collected for the Bartow trial (W. Murrcot), Auburndale trial (Hamlin), and CREC trial (Hamlin) during harvest in January 2020, and for the Avon Park trial (Valencia) in April 2020. The samples were processed for OTC or STR extraction, and the concentrations of OTC and STR in these samples were determined by HPLC assays. We have collected data for 60 and 360 days post treatment. Objective 3. To determine whether trunk injection of bactericides could decrease Las acquisition by Asian citrus psyllids (ACP)Twenty 1.5-year old citrus (Valencia sweet orange) plants were graft-inoculated by Las carrying buds in February 2020. These plants are being tested for Las infection and 4 plants were confirmed with Las infection (Ct values are between 34.0 and 35.0) at 4 months after grafting. They will be subjected to OTC or STR treatment by trunk injection and ACP acquisition access for 7 to 14 days. This experiment is ongoing. Objective 4. To monitor resistance development in Las against bactericides and evaluate potential side effects of trunk injection of bactericides Monitoring resistance development in Las against bactericides. Leaf samples for this test have been collected from 5 trees injected with OTC and 5 trees injected with STR at the highest doses in each of the three groves at 6 and 9 months after the injection, respectively. PCR-sequencing analysis on Las 16SrRNA gene showed there was no mutation compared with the reported sequence. We are further confirming the results. Evaluation of potential side effects of trunk injection of bactericides have been completed.
Objective 1: Investigate the efficacy of bactericides treatments for preventing new infections for young citrus trees protection. Hypothesis: Bactericidal treatment will protect young trees from CLas colonization.Initial leaf samples were collected before treatments to evaluate CLas titers in the uninfected trees. Antibiotic treatments were applied from May 2019 through September of 2020. From early June through September, CLas titer was monitored in leaf tissue in response to antibiotic treatments using quantitative real-time PCR analysis. In this report, the results of the CLas-infection rate in citrus leaves and ACP from May 2019 through June 2020 are described. Currently, citrus leaves tissue samples from July through September 2020are being processed to analyze the CLas-infection rate. Results: CLas titers were highest during the past quarter compared to previous quarters across all treatments. The average tree CT value prior to the beginning og the study was 37.5-39.5. Trees are considered CLas-infected (positives) when CT values are below 35. Trees receiving monthly or quarterly applications of Firewall/Fireline tested CLas-positive beginning January 2020 and December 2019, respectively. Trees covered by Tree Defender bags or treated with insecticide only tested CLas-positive beginning December 2019 and February 2020, respectively.During May-June 2020, CLas values reached the higest level across all treatments since the study was initiated. CLas titers were lowest in the insecticide only treatment during this period, followed by monthly applications of Firewall/Fireline rotation. Trees receiving quarterly applications of Firewall/Fireline had the second highest titer of CLas, compared to other treatments. The highest titers of CLas during May-June 2020 were observed in trees covered with Tree Defender bags. Objective 2. Determine the effect of bactericides application frequency on Las infection of citrus. Hypothesis: Bactericidal treatment will reduce CLas infection in mature trees. Antibiotic treatments were applied from May 2019 through September of 2020. From early May through June, CLas titer was monitored in leaf tissue and Asian citrus psyllid adults in response to antibiotic treatments. Currently, citrus leaf samples from July through September are being processed to analyze the CLas-infection rate. Results: The average CT value of citrus trees at the onset of the study was 28. From May 2019-June 2020, trees receiving monthly applications of Firewall/Fireline had the lowest CT values, as compared with trees receiving quarterly antibiotic applications or insecticide only, with the exception of January 2020 when mean CLas titers across all treatments were not significantly different.
Our project is examining phloem gene expression changes in response to CLas infection in HLB-susceptible sweet orange and HLB-resistant Poncirus and Carrizo (a sweet orange – Poncirus cross). We are using a recently developed methodology for woody crops that allows gene expression profiling of phloem tissues. The method leverages a translating ribosome affinity purification strategy (called TRAP) to isolate and characterize translating mRNAs from phloem specific tissues. Our approach is unlike other gene expression profiling methods in that it only samples gene transcripts that are actively being transcribed into proteins and is thus a better representation of active cellular processes than total cellular mRNA. Sweet orange, and HLB-resistant Poncirus and Carrizo (sweet orange x Poncirus) will be transformed to express the tagged ribosomal proteins under the control of characterized phloem-specific promoters; tagged ribosomal proteins under control of the nearly ubiquitous CaMV 35S promoter will be used as a control. Transgenic plants will be exposed to CLas+ or CLas- ACP and leaves sampled 1, 2, 4, 8, and 12 weeks later. Ribosome-associated mRNA will be sequenced and analyzed to identify differentially regulated genes at each time point and between each citrus cultivar. Comparisons of susceptible and resistant phloem cell responses to CLas will identify those genes that are differentially regulated during these host responses. Identified genes will represent unique phloem specific targets for CRISPR knockout or overexpression, permitting the generation of HLB-resistant variants of major citrus cultivars.During the 3rd quarter of the second year of our grant, the Stover lab continues producing transgenic plants and the Rogers lab continues to propagate the transgenic lines. However, currently ARS employees have been ordered to maximize telework due to the COVID-19 pandemic. This has continued to slow down progress on the grant milestones. Still, this quarter 72 new transgenic plants were evaluated for transgene expression levels in the Stover lab. A number of high expressor were identified. There are now 20 Hamlin transgenic lines, 2 Poncirus lines, and 7 Carrizo lines ready for shipment to the Rogers lab once both labs are ready to reopen. The Rogers lab now has a CLas infected colony of Asian citrus psyllid and is ready to start exposing transgenic lines. We are very much hoping to be allowed to move to the next phase of reopening soon, which will allow for citrus shipments and CLas infections of transgenic lines.
This project is a continuation of previously funded CRDF grants to TWO BLADES focused on utilizing multiple This project is a continuation of previously funded CRDF grants to TWO BLADES focused on utilizing multiple strategies to produce canker-resistant citrus plants. The project has focused on transforming Duncan grapefruit with genes that express EFR or a gene construct designated ProBs314EBE:avrGf2 that is activated by citrus canker bacteria virulence factors. This project is a continuation of previously funded CRDF grants to TWO BLADES focused on utilizing multiple strategies to produce canker-resistant citrus plants. The project has focused on transforming Duncan grapefruit with genes that express EFR or a gene construct designated ProBs314EBE:avrGf2 that is activated by citrus canker bacteria virulence factors. We also are in the process of testing citrus that has been transformed to modify the bs5 gene to enhance resistance to the citrus canker bacterium. Objective 1. To determine if Bs3-generated transgenic grapefruit plants are resistant to diverse strains of the citrus canker bacterium or to alternate target susceptibility genes in greenhouse experiments and to the citrus canker bacterium in field experiments in Fort Pierce. In late March, 2019, in the field at Fort Pierce in collaboration Dr. Ed Stover thetransgenic material was planted. Citrus canker has developed on plants in the field and the trees were rated for disease in December and there was considerable disease on all susceptible Duncan trees, but no evidence on the transgenic, JJ5. Statistical analysis revealed that there was a significant difference in disease both at the second and third ratings. Because of Covid-19 I was unable to rate the plants until late on July 28, 2020 but have not had the time to analyze the data. We have two additional transgenics from plants received from Dr. Vladimir Orbovic. Both transgenic trees contain the avrGf2 gene (based on PCR for detection of avrGf2). These transgenics are reaching a stage where they can be assayed more thoroughly for resistance. We have placed our constuct in a different vector that is acceptable for future transgenic purposes. The previous constructs contain an additional selectable marker that allowed for identifying putative transgenics with a higher success rate that contained the targeted construct. Given that there was concern about the additional marker, the new construct contains only NPT as a selectable marker. The construct is with Vladimir Orbovic, who has additional transformants. We have screened these via inoculations and so far the ones we have received are susceptibe. More will be available in the future. Objective 2. To determine if EFR-generated transgenic grapefruit plants are resistant to the citrus canker bacterium in field experiments in Fort Pierce. We have grafted our two most promising EFR transgenic plants (based on ROS activity) onto two rootstocks (812 and Sour Orange) and planted them in the field at Fort Pierce in collaboration Dr. Ed Stover. They were planted in the field in late March and were recently rated in late July. I am in the process of analyzing the data. We have identified additional transgenics from plants received from Dr. Vladimir Orbovic that have been grafted onto rootstocks. The are in the process of testing this quarter for ROS activity and for EFR gene expression . Objective 3. To determine if bs5-generated transgenic Carrizo plants are resistant to X. citri and to generate transgenic grapefruit carrying the pepper bs5. We have recently received budwood from UC Berkeley. The budwood was from two transgenic events and a third was from a tree that was run through the transformation process but that was negative for the gene, serves as budwood that had undergone the transformation process but that was negative for the transgene. This will serve as a negative control. We have grafted the buds and several have developed into branches. We are currently growing these. Unfortunately the control grafts have not developed any branches as of now. Once they are of an appropriate size we will send DNA to Berkeley. Of course this will only occur once we are allowed to conduct research. We have grown the two CRISPR created bs5 Carrizo trees by pinprick inoculation and spray inoculation. For both inoculation methods we saw no clear-cut difference in susceptibility compare to Carrizo rootstock leaves. We have also compared bacterial growth of one of the CRISPR events with Carrizo rootstock leaves and saw no differences in populations. We plan to test the other CRISPR event when the leaves grow outl.
This project is a continuation of previously funded CRDF grants to TWO BLADES focused on utilizing multiple This project is a continuation of previously funded CRDF grants to TWO BLADES focused on utilizing multiple strategies to produce canker-resistant citrus plants. The project has focused on transforming Duncan grapefruit with genes that express EFR or a gene construct designated ProBs314EBE:avrGf2 that is activated by citrus canker bacteria virulence factors. This project is a continuation of previously funded CRDF grants to TWO BLADES focused on utilizing multiple strategies to produce canker-resistant citrus plants. The project has focused on transforming Duncan grapefruit with genes that express EFR or a gene construct designated ProBs314EBE:avrGf2 that is activated by citrus canker bacteria virulence factors. We also are in the process of testing citrus that has been transformed to modify the bs5 gene to enhance resistance to the citrus canker bacterium. Objective 1. To determine if Bs3-generated transgenic grapefruit plants are resistant to diverse strains of the citrus canker bacterium or to alternate target susceptibility genes in greenhouse experiments and to the citrus canker bacterium in field experiments in Fort Pierce. In late March, 2019, in the field at Fort Pierce in collaboration Dr. Ed Stover thetransgenic material was planted. Citrus canker has developed on plants in the field and the trees were rated for disease in December and there was considerable disease on all susceptible Duncan trees, but no evidence on the transgenic, JJ5. Statistical analysis revealed that there was a significant difference in disease both at the second and third ratings. Because of Covid-19 I was unable to rate the plants until late on July 28, 2020 but have not had the time to analyze the data. We have two additional transgenics from plants received from Dr. Vladimir Orbovic. Both transgenic trees contain the avrGf2 gene (based on PCR for detection of avrGf2). These transgenics are reaching a stage where they can be assayed more thoroughly for resistance. We have placed our constuct in a different vector that is acceptable for future transgenic purposes. The previous constructs contain an additional selectable marker that allowed for identifying putative transgenics with a higher success rate that contained the targeted construct. Given that there was concern about the additional marker, the new construct contains only NPT as a selectable marker. The construct is with Vladimir Orbovic, who has additional transformants. We have screened these via inoculations and so far the ones we have received are susceptibe. More will be available in the future. Objective 2. To determine if EFR-generated transgenic grapefruit plants are resistant to the citrus canker bacterium in field experiments in Fort Pierce. We have grafted our two most promising EFR transgenic plants (based on ROS activity) onto two rootstocks (812 and Sour Orange) and planted them in the field at Fort Pierce in collaboration Dr. Ed Stover. They were planted in the field in late March and were recently rated in late July. I am in the process of analyzing the data. We have identified additional transgenics from plants received from Dr. Vladimir Orbovic that have been grafted onto rootstocks. The are in the process of testing this quarter for ROS activity and for EFR gene expression . Objective 3. To determine if bs5-generated transgenic Carrizo plants are resistant to X. citri and to generate transgenic grapefruit carrying the pepper bs5. We have recently received budwood from UC Berkeley. The budwood was from two transgenic events and a third was from a tree that was run through the transformation process but that was negative for the gene, serves as budwood that had undergone the transformation process but that was negative for the transgene. This will serve as a negative control. We have grafted the buds and several have developed into branches. We are currently growing these. Unfortunately the control grafts have not developed any branches as of now. Once they are of an appropriate size we will send DNA to Berkeley. Of course this will only occur once we are allowed to conduct research. We have grown the two CRISPR created bs5 Carrizo trees and inoculated by pinprick inoculation and spray inoculation. For both inoculation methods we saw no clear-cut difference in susceptibility compare to Carrizo rootstock leaves. We have also compared bacterial growth of one of the CRISPR events with Carrizo rootstock leaves and saw no differences in populations. We plan to test the other CRISPR event when the leaves grow out.
At the start of this quarter, we halfway through the funding period of the project. Our field results in Polk County are showing some early signs of the efficacy of foliar phosphate fertilization. We have been spraying citrus with four levels of potassium phoshate every two months since April 2019 at Polk County and since August 2019 in Collier County.
The copy number of the CLas terC gene per ng of DNA had declined significantly with the 2 mM phosphate spray after one-year of treatment (p-value = 0.029). We were surprised to see such a result so early in this study. We expect to see yield improvements next spring.
Data is still being analysed from the Imokalee trial. We hope to have those done by the end of this month. This will give a sense of the progress from one year of spraying in Collier County.
Meanwhile, a greenhouse trial continues to show great promise for prevention of HLB by spraying phosphate on uninfected trees. Nabil Killinys group graft infected young saplings with in February 2019. These plants were given a potassium phosphate foliar spray, a calcium phosphate soil drench (to mimic FL soils), or no added phosphate other than what is in the typical fertilizer mix. Eighteen months later, the K-phosphate treated plants have no symptoms. The other treatments show severe symptoms. We jut sampled this plants and hope to have CLas titer done very soon.
We plan to submit a paper soon based on the results from the grafting experiment.
I am pleased to report that this project is working as planned so far. Earlier we showed that foliar potassium phosphate does reduce citrate levels and the levels of other organic acids in phloem. As organic acids, particularly citrate, are the preferred carbon source for Liberibacter crescens, we expect this treatment to starve the pathogen.
We are seeing that foliar K-phosphate prevents HLB symptoms in young saplings. We are seeing the beginnings of improvements in older trees with high titers. CLas is decling in the Polk County plots. To early to see symptom relief in the older field trees. We expect to see a yield improvement in the spring.
Our team (Triplett, Vincent, Killiny, and Wang) are working very well together and meet to discuss the project regularly.
Spring 2020 nematicide treatments (same treatments as in spring 2019) occurred during mid-April to late May. The timing was later than optimal due to the covid-19 lockdown. As done for the fall treatments, all liquid nematicides except oxamyl (which was sprayed by hand during the final third of the irrigation cycle) were injected for two hours, beginning 30 minutes after irrigation began and ending a half hour before the irrigation run ended. Nematicide effects on nematode populations were measured in July, as previously described. The average number of sting nematodes in aldicarb plots was 20% higher (NS) than that in untreated plots, whereas oxamyl plots had 68% fewer sting nematodes (P<0.05) than in untreated plots. Nematode numbers in the plots treated with the other nematicides ranged from 51%-82% of that in untreated plots, none of which were significant differences. The average cumulative numbers of sting nematodes measured 60 days following each of the three seasonal applications thus far during the project (areas under the curve) were 63%, 62%, 34%, 29%, and 6% lower than those in untreated plots for the Syngenta compound, oxamyl, Nimitz, Salibro, and Velum Prime, respectively. Sting nematodes in aldicarb treated plots were 18% more numerous than in untreated plots.Average fibrous root mass density for all nematicide treatments ranged between 10%-77% greater than for non-treated plots. Roots were significantly more abundant on trees treated with oxamyl, and both combinations of Salibro and the Syngenta material than those of untreated trees. The growth of the tree trunks during 1.5 years between February 2019 and July 2020 was 29% greater for trees treated with oxamyl than that of untreated trees (Dunnett, P<0.05). Growth rates of trees treated with other nematicides were between 1-13% greater than the untreated trees, but the differences were not significant. There was a strong inverse linear relationship (r= -0.35, n=56, P=0.01) between trunk growth during 1.5 years and the average sting nematode population density (log-transformed) during that time. In a second trial comparing untreated trees to trees treated with aldicarb in April 2019 and 2020, there were no differences in the trunk cross-sectional areas of either treatment at 15 months after the first treatment (untreated=1988 mm2 vs aldicarb=1973 mm2). Although young tree growth in this grove is very uneven, the cause(s) may be other than sting nematodes which were below detectable levels in most plots.In the perennial peanut trial, an incursion of weeds was noted in several peanut plots. Sting nematodes in those plots were significantly fewer in July 2020 (13.3 nematodes per 250 cm3 soil) than in the row middles with native vegetation (28.3 nematodes) but signifcantly more numerous than in pure stands of peanut (1.6 nematodes). Oxamyl reduced sting nematodes in the citrus rows by 55% compared to untreated plots (17 vs 37 nematodes per 250 cm3 soil; P=0.10). The tree girth was 15% greater in the oxamyl treatment, and 10% greater in the peanut treatment than in untreated plots, but the differences were not significant. There continued to be a strong inverse relationship between the size of the tree trunks and the (log) numbers of nematods in July 2020 (r= -0.63, P=0.01).
Sugarbelle appears to be one of the most HLB tolerant scion varieties and UFR-4 is a rootstock that does not suffer from HLB-induced root loss. Valencia and Swingle are susceptible to HLB damage. This project has two main goals based on different graft combinations of these two scion and two rootstock varieties. First is to determine if there is a benefit to only half of a grafted tree being tolerant or if both scion and rootstock need to be tolerant to ameliorate some of the physiological effects of HLB. Second is to identify the mechanism of UFR-4 resistance to HLB-induced root loss and to identify the systemic signal of HLB-induced root loss with the hope of finding CRISPR targets to move this resistance into other rootstock varieties. These experiment will use a previously developed split root rhizotron system to maintain CLas infected and uninfected root systems on the same tree Rootstock seedlings (Swingle) and cuttings (UFR-4) have been selected from our greenhouse for grafting of SugarBelle and Valencia. The first round of grafting had an unexpectedly low success rate, so a new source of budwood was found and trees were regrafted. In both grafting events combinations containing Sugarbelle or UFR-4 had much lower success rates than the standard Valencia on Swingle. This has led to a delay in the initial inoculation until Spring to allow trees to caliper up and avoid inoculations during the winter, which have more unpredictable CLas movement. Meanwhile we are starting propagation of trees for the second experiment, to stay on track overall. This may require the production of more than expected aluminum rhizotrons.
HLB is known to make citrus roots more susceptible to Phytophthora root rot. It also reduces the efficacy of chemical management of Phytophthora root rot, creating a difficult management scenario. Current Phytophthora management recommendations are based on pre-HLB work done in the 1980s. These three conditions raise the question of whether yield improvement from Phytophthora management is enough to pay for the management costs themselves. The goal of this project is to develop new soil propagule density managment thresholds and recommendations for chemical management of Phytophthora root rot based on ecomonic analysis of yield responses in different soil conditions. Finding and getting to sites during the research ramp-up has proven more difficult than predicted. We are still looking for suitable grower sites with Phytophthora pressure. Meanwhile we are planning an additional greenhouse Phytophthora and HLB experiment to determine if the newly labeled chemistries have the same limitation on HLB-affected plants as fosetyl-Al and mefanoxam have shown.
The objectives of this study are to identify optimal pH range for root function and minimize root turnover on HLB-affected rootstocks and how uneven pH levels in the root zone (e.g. irrigated vs. row middle portions of root system) affect the overall health of the tree. This is being done in a split root system in the greenhouse where pH of different parts of the root system can be controlled an maintained. The following progress reported is based on a July 31st report date. The breakdown of the first experiment has been completed and preparation for setup of the 2nd experiment is underway. Initial findings from the first experiment will be presented virtually as a poster at the American Phytopathological Society annual meeting the 2nd week of August. Final dry mass, gene expression, and CLas population data is still being collected on samples from the first breakdown. Initial observations from breakdown suggest that roots suffer minimal damage at some soil pH environments pH 7.5 for US942 and pH 5.5 for Swingle. Swingle roots were most damaged by HLB at pH 7.5 and US942 roots were most damaged at pH 5.5. More information will be provided when the quantitative data has been analyzed from the first experiment.
Progress report for the second quarter of the 2019/2020 project yearThe purpose of the project is to develop new guidelines for restoring root health and improving overall tree nutrition for Florida oranges and grapefruit. The objectives of the project are to:1. Determine optimal nutrient concentrations in roots and leaves for multiple grapefruit and orange varieties.2. Compare and contrast fertigation, soil, and foliar fertilization to identify best application method for uptake of nutrients into both underground and aboveground components.3. Investigate the relationship between root and leaf nutrient contents to tree health, yield, and fruit quality as well as bacteria titer.4. Generate updated and new guidelines for optimal nutrient contents for roots and leaves for HLB-affected trees. Progress to date:The project is being conducted at three sites: Citrus Research and Education Center (CREC), Southern Gardens Citrus near Clewiston, FL and Indian River Research and Education Center (IRREC). Data collection continued during this quarter on root scanning, canopy size determinations and soil sampling on the central Ridge and southwest Flatwoods. While it is early to determine some trends, it appears soil and root nutrient levels, for example, follow the pattern of treatments e.g. 4x>2x>1x, for soil applied treatments. Similarly, for foliar applied treatments, the pattern in leaf nutrient concentration follows 1x<2x<4x. We are following through with correlations in roots, leaves and soil with canopy size, trunk size, yield, bacteria titers and so forth. Root growth tends to be more pronounced with soil applied nutrients compared with leaf applied nutrients and this will be confirmed and validated further in the following year of measurements. At the southeast Flatwoods, all grove caretaking activities were performed on time and the treatments have been applied as expected along with data collection on tree size, soil/leaf/root nutrient levels, and HLB ratings. All sampling for the third quarter is complete and data analysis is underway and will be reported in the next quarter. Graduate students Tanyaradzwa Chinyukwi and Lucas Hallman presented portions of their research at the American Society of Horticultural Science virtual Annual Meeting between August 10 to 13, 2020. Plans for Next QuarterThe team will continue with fertilizer treatments and data collection including HLB rating assessments, canopy size and root growth measurements and reporting on the progress of the project. Graduate students working on the project will present at the Florida State Horticultural Society. One graduate student working on the project will be submitting her MS thesis to the UF graduate school this fall and will prepare articles for refereed journal publications.
Objective 1 – Determine the effect of the ratio and concentration of Fe2+ + organic acid on hydroxyl radical production and stability. Objective complete. Objective 2 – Determine the phytotoxic levels of Fe2+ + organic acid solutions on citrus. Objective complete. Objective 3 – Determine the effect of Fe2+ + organic acid solutions on HLB titer using a rapid greenhouse, HLB-infected citron, rooted shoot bud assay. Continuing work to develop a rapid greenhouse screening system. Current systems are not yet ready for screening methods to cure or manage HLB. Experimentally robust screening systems are probably the single most important research tool that are needed by the HLB research community. Objective 4 – Requires screening system – see objective 3 discussion.Objective 5 – Requires screening system – see objective 3 discussion. Objectives 6 and 8 (Note: there is no 7) – These are the field tests for the various ferrous iron (Fe2+) and citric acid treatments on HLB status and horticultural measures for both mature (HLB symptomatic) and nonbearing (non-symptomatic) trees. The seven treatments have been applied according to the schedule specified in the project protocol to both bearing and nonbearing age class trees. The nonbearing trees as a class continue to exhibit vigorous growth and are maturing with a good canopy density. There were no significant differences in growth between treatments based on trunk calipers measured on 06/05/2020. Interestingly, there were significant differences in the incidence of HLB symptomatic leaves between treatments (Table 1). Three treatments (Aqua F11-C, Tracite Iron and chelated iron) exhibited fewer incidences of HLB leaf patterns than the untreated control (Table 1).———————————————————————————-Table 1. Number of trees with no HLB symptoms. Each treatment is composed of 12 trees. Treatment # of Trees w/ no HLB Symptoms F11-C (30 ppm) 7 F11-C (60 ppm) 6 Tracite 5% Fe 7 FRC 249 4 Chelated Fe 10 Ferrous sulfate + citric acid 6 Fe Solution 2 Control 5 ———————————————————————————- Aerial images were taken of the young trees and images processed by machine learning to extract various size and shape measures, including canopy area (2D) and canopy density. These data were analyzed and no significant differences were detected between the treatments and the untreated control trees. The bearing grapefruit trees as a class were abandoned on 03/01/2020 due to severe decline caused by irreversible root damage, primarily due to the Phytophthora/Diaprepes complex exacerbated by HLB. Aerial images have been taken of the mature (bearing age) plot immediately prior to tree removal. Images are ready for processing, quantification, and analysis. Also, tree condition assessments, crop yield, and fruit drop counts were made and documented in early January 2020 and revealed no significant differences between treatments. Throughout this quarter as well as the project timeline, conventional pesticide spray applications were applied to all the treatments in the trial and were based on scouting and were in accordance with IFAS guidelines. Similarly, irrigation events were made based on tree and field conditions as determined by soil feel and appearance, tensiometer readings, water table observation well measurements and visual assessment of tree canopy. Fertilizer applications were made via fertigation and were `spoon fed’ with frequent small applications bi-weekly. No dry fertilizer was used.
1. Please state project objectives and what work was done this quarter to address them: Objective. To determine the influence of compost during the first three years of tree establishment on growth, productivity, and root and soil health of citrus trees on rootstocks with different vigor-inducing capacity. Horticultural measurements, scheduled for the last quarter but not completed due to the Covid-19 related halt of all research activities, were completed. These include tree height, canopy size, and trunk diameters.Fibrous root samples were collected for assessment of root traits. Rhizosphere samples were collected for microbial community analysis. Root trait analysis and rhizosphere DNA extractions are in progress. Soil sample were collected for nutrient and physical attributes analysis. 2. Please state what work is anticipated for next quarter: We will complete the leaf sample collection for nutrient analysis and continue with processing of samples in the laboratory and statistical data analyses. 3. Please state budget status (underspend or overspend, and why): Approximately 28% of funds have been spent, which includes funds for the initial tree establishment. Spending is in accordance with the timeline.
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