ASHS 2025 Conference

Crop protection measures, cultural practices, nutrient and irrigation management, rootstock selection, and other studies have been conducted to alleviate the devastating bacterial disease of huanglongbing (HLB, aka citrus greening) in recent years. However, there is no single cure for HLB caused by Candidatus Liberibacter asiaticus, which is spread by the Asian citrus psyllid, Diaphorina citri Kuwayama. The objectives of the study were to assess whether tree growth, soil and leaf tissue nutrient concentrations, fruit yield, and postharvest quality can be improved through improved nutrient management using selected rootstocks. The study was conducted on ‘Hamlin’ citrus trees [Citrus sinensis (L.) Osbeck] budded on Cleopatra mandarin (Citrus reshni) or Swingle rootstocks [Citrus paradisi Macf. Poncirus trifoliata (L.) Raf.] from 2019 – 2021. The study was conducted in a split plot design with two rootstocks as main plots, three nitrogen rates (168, 224, and 280 kg ha-1) as sub-plots, and secondary macronutrients (control, Ca or Mg-thiosulfates at 45 kg ha-1 each, and a combination of Ca and Mg at 22.5 kg ha-1 each) replicated four times. Results indicated that soil ammonium nitrogen (NH4-N) and nitrate nitrogen (NO3-N) were significantly accumulated in an oscillating (between spring and summer seasons) pattern for soil NH4-N and persistent accumulation of NO3-N on both rootstocks. Leaf Ca and Mg nutrient concentrations were significantly greater in Cleopatra than Swingle rootstocks. Trees on Swingle rootstocks react greater than those on Cleopatra rootstocks. Control trees had a lower LAI than those that received either Ca and/or Mg treatments, and the impact is significantly prevalent on Swingle than on Cleopatra rootstocks. Trees on Cleopatra had greater canopy volumes than Swingle rootstocks. However, the impact of secondary macronutrients manifested only after the third year of the study. Fruit yield significantly increased with a magnitude of 1.8× in the trees that received combined Ca and Mg treatments than a control tree. Greater fruit total soluble solids were detected in trees that received Mg or combined Ca and Mg treatments. Hence, nutrient management can enhance tree growth, fruit yield, and postharvest quality and can be used as an immediate solution until the HLB can be permanently eradicated.

Huanglongbing (HLB), a.k.a. citrus greening, is an endemic disease significantly impacting Florida and Texas citrus production. HLB-affected trees are compromised in the canopy and root health, often found with heavy loss of root mass that impairs water and nutrient uptake, leading to nutrient deficiencies. The prevalent presence of the suspected causal organism of HLB, Candidatus Liberibacter asiaticus (CLas) bacterium, in the citrus phloem leads to a cascade of active defense responses triggered by plant-pathogen interactions. One of the crucial outcomes of the interactions is the production of reactive oxygen species, leading to excessive oxidative stress that, over time, weakens tree health. Therefore, this research investigated the therapeutic strategies to mitigate nutrient deficiencies and HLB symptoms, attenuate reactive oxygen species load, and improve the production of premium grapefruits in trees affected by HLB by combining an integrated approach of micronutrient applications with antioxidants. The experiment design consisted of the foliar application of micronutrients, zinc (Zn), iron (Fe), and manganese (Mn) varied in two rates: 2 quarts/acre (control) and 3 quarts/acre that were conducted alone or in combination with gamma-aminobutyric acid (GABA), uric acid, and gibberellic acid (GA). The micronutrients, GABA, and uric acid applications were conducted at an interval of three months, while GA sprays were limited between blooming to the pre-harvest period in the HLB-affected mature grapefruit grove with a tree density of 104 trees/acre in the Lower Rio Grande Valley of Texas. The results demonstrated that merely increasing micronutrient application to 3 quarts/acre improved canopy health, with healthy (non-HLB-symptomatic) canopy quadrants reaching an average of 30% compared to 16.7% in the control treatment. Moreover, the standalone 3-quarts/acre significantly improved the yield of premium-size grapefruit to 25% compared to 6.2% in 2-quarts/acre only. The combination of GA (10 Fl.Oz/acre) and GABA (8 gm/tree) with control micronutrient treatment enhanced healthy canopy quadrants to 33% and 38%, respectively, compared to 19% in the micronutrient-only control. In 3 quarts/acre, GA and GABA showed synergistic effects in improving the canopy health by alleviating HLB severity. This applied research provides a practical framework for translating laboratory findings into field applications for managing HLB-affected trees. The integrated approach of combining enhanced micronutrient applications with antioxidants and phytohormones shows promise in mitigating HLB symptoms and improving fruit yield, particularly in the premium size category.

Nitrogen (N) is an essential nutrient required to support citrus yield and fruit quality. Its bioavailability to the tree must be continuous to support adequate biomass production, especially at key phenological stages such as vegetative flush and fruit setting. However, most commercial nutrition programs in southern Texas deliver synthetic fertilizer on average twice per year when not drip-irrigated. Although adoption of fertigation strategies continues to gain ground, the rate of transition is of low velocity, and a percentage of orchards is still flood-irrigated. In these systems, trees rely on the soil storage capacity to meet permanent nutrient demand. This field study investigated the effects of traditional N management practices on two orchards with different rootstock * scion combinations. We evaluated the effects of different N fertilization rates on fruit growth and postharvest traits such as brix, and size-class distribution. Our preliminary data indicates that despite trees growing in similar soil properties, they exhibited differences in fruit growth rate, and size-class distribution of fruits. Moreover, certain rootstocks * scion combinations exhibited superior fruit yield and quality. Continue refinement of nutrient management practices, especially N application is imperative to support tree long-term productivity under the constant pressure of pathogens in commercial settings.

Soil organic matter (SOM) is a key determinant of soil health, directly influencing structure, moisture retention, nutrient availability, and microbial activity. Among soil amendments, biochar has gained attention for its potential to enhance soil fertility and support plant productivity. However, its influence on citrus root system architecture and nutrient uptake across different rootstocks remains underexplored. This study evaluated the effects of four biochar concentrations (0%, 10%, 20%, and 30% v/v) mixed with native field soil on four citrus rootstocks: ‘Sour Orange’, ‘Swingle’, ‘US-802’, and ‘US-942’. Plants were grown under controlled greenhouse conditions for 16 weeks. Root system architecture traits: total root length, surface area, root volume, and number of forks, were quantified using WinRHIZO™ analysis. Soil parameters, including pH, cation exchange capacity (CEC), moisture content, and SOM, were assessed post-harvest, alongside plant tissue nutrient concentrations (P, K, Ca, Mg, Mn). The 20% biochar treatment significantly enhanced root morphological traits across all rootstocks, with increases of up to 35% in total root length and 40% in surface area compared to the control. Plant tissue analysis revealed elevated macro- and micronutrient concentrations in the 20% treatment, particularly for P, K, Ca, and Mg. Soil analysis showed progressive improvements in pH, SOM, CEC, and moisture content with increasing biochar concentration, with the highest values recorded at 30%. While higher biochar rates improved soil physicochemical properties, the 20% concentration provided the most balanced benefits, optimizing both root development and nutrient uptake. These findings suggest biochar, particularly at moderate rates, may be an effective amendment for improving soil fertility and citrus root system performance in nutrient-limited soils.

Workshop Summary for Conference Program
Huanglongbing (HLB), commonly known as citrus greening disease, continues to challenge citrus growers and researchers worldwide. Its devastating impact on tree health, fruit quality, and overall orchard longevity makes it essential to explore adaptive horticultural strategies that can sustain production in affected regions. This workshop provides an in-depth exploration of the horticultural approaches being used to mitigate the effects of HLB, with a focus on real-world applications and current research. The content is designed to equip participants with actionable insights into how orchard management can be adjusted to maintain tree vigor and maximize productivity despite HLB pressure. Topics will include:

Physiological impacts of HLB on citrus growth and development: Understanding how the disease disrupts water, nutrient, and carbohydrate transport, and its implications for tree management.
Nutritional and irrigation strategies: Optimizing inputs to support trees with compromised root systems and reduce stress.

Soil amendments and root health: Exploring the role of organic matter, compost, biochar, soil conditioners, and microbial inoculants in enhancing soil health and nutrient availability to support stressed trees.
Canopy and crop load management: Best practices for pruning, thinning, and canopy maintenance to enhance light interception, reduce disease severity, and promote new growth.
Rootstock and scion selection: Insights into tolerant or better-performing combinations under HLB pressure, and how rootstock choice can influence disease expression.
Replant and tree replacement strategies: Managing orchard renewal while dealing with persistent disease pressure.
Cultural practices integration: How horticultural management intersects with pest control (e.g., Asian citrus psyllid suppression), disease diagnostics, and emerging technologies.

Workshop Format:
This workshop will include:
-Expert presentations from researchers, university extension specialists, and industry professionals sharing the latest findings and innovations.
-Grower case studies detailing the successful implementation of horticultural practices in HLB-endemic orchards.
-Interactive Q&A sessions and moderated discussions to encourage knowledge exchange and problem-solving tailored to regional conditions.
Learning Objectives:

By the end of the workshop, participants will be able to:

• Identify the key horticultural impacts of HLB on citrus trees.
• Apply practical orchard management techniques including soil amendments and nutrient strategies to mitigate disease effects.
• Make informed decisions about rootstock, scion, canopy, and irrigation strategies.
• Integrate horticultural practices with broader HLB management approaches for sustainable production.

Speakers:

• Chater, John M. jchater@ufl.edu “Cultivar selection, OTC trunk injection, and nutrition are important horticultural strategies under observation for HLB mitigation in Florida”
• Wang,Nian nianwang@ufl.edu  “Genetic determinants underneath the Candidatus Liberibacter-triggered immune disease development of citrus Huanglongbing and its control”
• Killiny,Nabil nabilkilliny@ufl.edu  “Achieving the traditional control strategies through biotechnological approaches for sustainability: Attract-and-Kill and Repellency to control Asian citrus Psyllid”
• Albrecht,Ute ualbrecht@ufl.edu “Integrating trunk injection of oxytetracycline with other practices to sustain tree health and productivity under HLB-endemic conditions”
• Chandrika Ramadugu chandram@ucr.edu “Citrus Huanglongbing - use of disease tolerant and resistant varieties for sustainable cultivation.”
• Vincent,Christopher I civince@ufl.edu “Mitigating the physiological impact of huanglongbing by shading.”
• Dutt, Manjul manjul@ufl.edu “Rooted in Resistance: Breeding the Next Generation of HLB-Resilient Citrus rootstocks”
• Ashraf El-kereamy (ashrafe@ucr.edu) “Cultural practices to control HLB in California”