Location: Citrus and Other Subtropical Products Research
2023 Annual Report
Objectives
Objective 1: Determine genetic and horticultural inputs that provide superior citrus production and quality under endemic HLB disease pressure. [NP 305, Component 1,Problem Statement lB]
• Sub objective 1a. Evaluate susceptibility of standard and novel citrus scion and rootstock combinations in grower-cooperator trials.
• Sub objective 1b. Determine the effects of prophylactic treatments on prevention of HLB.
• Sub objective 1c. Determine effects of pre-plant soil treatments on citrus tree health and productivity.
Objective 2: Investigate plant physiology and symptoms during disease development to design rational approaches for treatment. [NP 305, Component 1, Problem Statement lB
Sub objective 2a: Determine the effects of CLas infection on photosynthesis and carbon partitioning in citrus.
Approach
This project includes field plot, greenhouse and laboratory components to evaluate factors related to citrus susceptibility to Huanglongbing disease.
Field plots will be conducted with grower cooperators to evaluate performance of scion/rootstock combinations for susceptibility to Huanglongbing disease under commercial management systems. Some such trial were established in the previous project and data collection will continue in the current project. New plantings will also be established in the current project. Citrus is a long term crop that does not begin to produce fruit until three to four years after planting. During the first three years of the experiments, tree mortality is really the only metric for tree performance. When trees come into bearing yield and fruit quality data will be collected.
Greenhouse trials will be conducted to determine the efficacy of prophylactic treatments to prevent Haunglongbing disease. In the previous project realistic model system including citrus, Asian citrus psyllids and Candidatus Liberibacter asiaticus. This model has a small foot print and is rapid, and robust. Numerous therapeutic treatments to mitigate Huanglongbing symptoms, but there have been none of these treatments have proven efficacious. In this project trees will be treated prior to transmission of Candidatus Liberibacter asiaticus by Asian citrus psyllids. The hypothesis is that preventing the establishment of Candidatus Liberibacter asiaticus will be effective whereas therapeutic treatments have unsuccessful. Data will be collected to determine the effects of treatments on incidence of Candidatus Liberibacter infection incidence and titer, growth and development of Huanglongbing symptoms. If promising treatments are identified in the greenhouse, small scale field trails will be established to determine efficacy of these treatments in the field.
Grower cooperator trials will also be established to determine if anaerobic soil disinfestation, a treatment that alters the soil microbiome and has proven beneficial for some annual crops and may improve the growth of newly planted citrus trees. Data collection will be as described for the scion/rootstock evaluations.
Greenhouse trials will also be conducted to determine how citrus leaf ontogeny relates to the development of Huanglongbing symptoms. It is apparent that it is only the very youngest of citrus leaves is where transmission of Candidatus Liberibacter asiaticus. Anatomical changes that develop very early following Candidatus Liberibacter asiaticus will be investigated using light and electron microscopic evaluation. In addition, because Huanglongbing is characterized by massive accumulation of starch, the effects of Candidatus Liberibacter on photosynthetic activity will be determined by quantifying starch accumulation. The effect of infection on the rate of carbon dioxide fixation (photosynthesis) will be determined using commercially available instrumentation. Carbon partitioning (dry weight) following infection will also be investigated by determining root and shoot dry weights post infection.
Progress Report
We made progress this year that allowed us to fully meet all of our objectives.
Subobjective 1.a Evaluate susceptibility of standard and novel citrus scion and rootstock combinations in grower cooperator trials. Of the original 20 field trials planted in five counties around Florida consist of pummelo, grapefruit, orange, and mandarin types, 14 continue to be evaluated. Each trial consists of 12-15 citrus hybrids on three rootstocks. Annual data sets were collected on the susceptibility to huanglongbing (HLB) of various hybrid/rootstock combinations. In established experiments, mandarin hybrids are the most tolerant to HLB. Some orange and grapefruit varieties with low amounts of Poncirus in their heredity also show tolerance. Fruit quality, crop load and post-harvest production data will be collected on those varieties. Additional trials that were planted last year with the most promising materials continue in the early stages of development. All 14 trials were evaluated in an aerial drone assessment and collected imagery data on tree health, size, and color was compared to ground truthing data collection.
Subobjective 1.b Determine the effects of prophylactic treatments on prevention of HLB. Three antibiotic compounds, SAMP peptide, flavonoid, putrescine, and ferulic acid as prophylactic treatments of HLB were evaluated using the rapid-assessment citron system. After treatment with the compounds, plants are exposed to CLas-positive ACP, and monitored for the development of HLB symptoms. Experiments were repeated comparing citrus plants treated with spray applications of the antibiotics oxytetracycline, streptomycin, and penicillin. Infusion experiments were completed using multiple concentrations of penicillin and oxytetracycline. The SAMP peptide was tested using three different delivery methods: spray, drench, and infusion and flavonoid, putrescine, and ferulic acid were applied in spray form. Infused penicillin resulted in the most consistent reduction in disease.
Subobjective 1c. Determine effects of pre-plant soil treatments on citrus tree health and productivity.
Two field trials were completed in which anaerobic soil disinfestation (ASD) was compared to fertilizer alone for citrus tree establishment. One scion hybrid was planted onto three rootstocks and each combination was replicated in ASD-treated and fertilizer-treated soil. Initial measurements of trunk diameter, canopy size, and tree survival were completed. Early yield was higher from ASD-established trees, but there was no difference in the expression of disease caused by CLas.
Sub objective 2a: Determine the effects of CLas infection on photosynthesis and carbon partitioning in citrus.
First year experiments were completed to determine the best method for using the LI-6800 Portable Photosynthesis System on citrus. Survey measurements and diurnal curves were completed for citron and Valencia, providing the tool needed to conduct the fixation experiments.
Sub objective 2b: Characterize the relationship between citrus leaf ontogeny and CLas infection. Starch analysis was completed for samples preserved during the pandemic. Results confirm previous experiments in which starch accumulation is higher in CLas positive leaves. Data from these trials are being compiled for analysis.
Accomplishments
1. Drone data and ground rating comparisons were conducted for 14 field trials containing multiple scion and rootstock replicated plantings in commercial groves. Drone data and ground rating comparisons were conducted by researchers at Fort Pierce, Florida, for 14 field trials containing multiple scion and rootstock replicated plantings in commercial groves. Under high huanglongbing (HLB) pressure, correlation between imagery and ground-collection HLB rating data was high (0.70 and above) and several mandarin scions from these trials were identified as having acceptable tolerance to HLB. One scion, 5-51-2, was identified as having the most consistent HLB tolerance and acceptable physical and quality characteristics for release.
