Explore the vast range of titles available.

The systemic delivery of oxytetracycline (OTC) by trunk injection has emerged as a viable strategy to manage huanglongbing (HLB, also known as citrus greening), a bacterial disease devastating citrus production around the world. This study examines the efficacy of delivering OTC systemically into the trunk of young, HLB-affected citrus trees using a drill-based or a drill-free system to improve tree health and productivity. Two field trials were conducted in two commercial production sites in Florida. Trees were four years old at the start of the study and composed of ‘Valencia’ or ‘OLL-8’ sweet orange (Citrus sinensis) scion grafted on X-639 (C. reticulata × Poncirus trifoliata) rootstock. Injections were performed in spring or late summer/early fall in 2022 and 2023. Using the drill-based system, 0.79 g of OTC was administered into each tree, whereas 0.15 g or 0.3 g was administered using the drill-free system. Delivering a higher dose of OTC by drill-based injection increased fruit yield and improved juice quality more than delivering lower doses by drill-free injection, though responses varied between cultivars. Injections in late summer/early fall increased the juice total soluble solids content considerably more than injections in spring. However, fall injections resulted in OTC fruit residues exceeding the maximum allowed level. Trunk injury was more extensive when OTC was applied with the drill-free system than when it was applied with the drill-based system.

The global citrus industry faces unprecedented challenges due to Huanglongbing (HLB), which is associated with the bacterial pathogen Candidatus Liberibacter asiaticus (CLas). This study evaluates the field performance of 11 rootstocks, grafted with ‘Valencia’ orange (Citrus sinensis), under Florida’s HLB-endemic production conditions, while also examining the impact of systemic applications of oxytetracycline (OTC) via trunk injection. Mature trees received annual OTC injections and were assessed over two production seasons. In year 1, OTC-treated trees exhibited significant improvements regardless of the rootstock, including a 36% increase in yield, an 11% increase in juice TSS, and reduced leaf bacterial titers. During year 2, the positive effects of OTC were sustained, or even enhanced. CLas titers were reduced in both leaves and roots; yield increased by 70%; and fruit weight, juice color, and TSS also improved significantly. Moreover, OTC-injected trees exhibited a larger percentage of finer roots compared to non-injected trees. US-1688 and US-1672, both hybrids of C. maxima ‘Hirado’ and C. reticulata ‘Cleopatra’, emerged as the most productive rootstocks. These results demonstrate the importance of rootstock selection for sustainable citrus cultivation while highlighting the benefits of integrating the systemic delivery of OTC to manage HLB and maximize the resilience of citrus.

Florida citrus production has declined by over 90% since the bacterial disease huanglongbing (HLB) was found in the state. In the absence of an effective cure, growers are adopting more frequent fertilization and irrigation practices to improve tree health and prolong the life span of their orchards. However, Florida’s soils under citrus production are sandy, with little organic matter, a low water holding capacity, and a low cation exchange capacity (CEC), rendering them prone to nutrient leaching. Organic amendments can be used to improve soil health and the environment for citrus roots, but may promote a higher incidence of weeds competing with trees for water and nutrients. A large field trial was established in a commercial citrus orchard in southwest Florida to evaluate the effects of organic amendments and weed management on young tree growth. The organic amendment treatments were as follows: (1) plant-based compost, (2) humic acid, and (3) a non-amended control. The weed management (herbicide) treatments were (1) glyphosate, (2) glufosinate, (3) flumioxazin, and (4) a maintenance herbicide control. Trees were planted in August 2019, and treatments began in 2021. Tree growth and physiological variables and soil physicochemical properties were evaluated during the two-year study. Compost-amended plots had a higher volumetric water content throughout the experiment, and soil nutrient content, organic matter, CEC, and pH were higher after two years of application. Humic acid amendments were less effective in altering these soil properties. Compost’s effects on tree and fibrous root physiology were moderate, and tree growth, fruit yield and fruit quality were not affected by either organic amendment. In contrast, the use of post-emergent herbicides (glyphosate and glufosinate) improved tree growth and nutrient uptake. The results suggest that in Florida, the use of organic amendments needs to be integrated with weed management to prevent resource competition. In the short term, these practices did not improve the productivity of the trees in the current Florida production environment.

Nutrient assessment of plants, a key aspect of agricultural crop management and varietal development programs, traditionally is time demanding and labor-intensive. This study proposes a novel methodology to determine leaf nutrient concentrations of citrus trees by using unmanned aerial vehicle (UAV) multispectral imagery and artificial intelligence (AI). The study was conducted in four different citrus field trials, located in Highlands County and in Polk County, Florida, USA. In each location, trials contained either ‘Hamlin’ or ‘Valencia’ sweet orange scion grafted on more than 30 different rootstocks. Leaves were collected and analyzed in the laboratory to determine macro- and micronutrient concentration using traditional chemical methods. Spectral data from tree canopies were obtained in five different bands (red, green, blue, red edge and near-infrared wavelengths) using a UAV equipped with a multispectral camera. The estimation model was developed using a gradient boosting regression tree and evaluated using several metrics including mean absolute percentage error (MAPE), root mean square error, MAPE-coefficient of variance (CV) ratio and difference plot. This novel model determined macronutrients (nitrogen, phosphorus, potassium, magnesium, calcium and sulfur) with high precision (less than 9% and 17% average error for the ‘Hamlin’ and ‘Valencia’ trials, respectively) and micro-nutrients with moderate precision (less than 16% and 30% average error for ‘Hamlin’ and ‘Valencia’ trials, respectively). Overall, this UAV- and AI-based methodology was efficient to determine nutrient concentrations and generate nutrient maps in commercial citrus orchards and could be applied to other crop species.

There is increased interest by the agricultural industry in microbial amendments that leverage natural beneficial interactions between plants and soil microbes to improve crop production. However, translating fundamental knowledge from laboratory experiments into efficient field application often has mixed results, and there is less clarity about the interaction between added microbes and the native microbial community, where microorganisms belonging to the same phylogenic clades often reside. In this study, four commercially available microbial amendments were examined in two greenhouse experiments using field soil to assess their impact on tomato plant growth and the native soil microbial communities. The amendments contained different formulations of plant growth-promoting bacteria (Lactobacilli, Rhizobia, etc.), yeasts, and mycorrhizal fungi. The application of the tested amendments in greenhouse conditions resulted in no significant impact on plant growth. A deeper statistical analysis detected variations in the microbial communities that accounted only for 0.25% of the total species, particularly in native taxa not related to the inoculated species and represented less than 1% of the total variance. This suggests that under commercial field conditions, additional confounding variables may play a role in the efficacy of soil microbial amendments. This study confirms the necessity of more in-depth validation requirements for the formulations of soil microbial amendments before delivery to the agricultural market in order to leverage their benefits for the producers, the consumers, and the environment.

Citrus crops have a long history of cultivation as grafted trees on selected rootstock cultivars, but all current rootstocks have significant limitations and traditional methods of rootstock breeding take at least 2–3 decades to develop and field test new rootstocks. Citrus production in the United States, and other parts of the world, is impaired by a wide range of biotic and abiotic problems, with especially severe damage caused by the disease huanglongbing (HLB) associated with Candidatus Liberibacter asiaticus. All major commercial citrus scion cultivars are damaged by HLB, but tree tolerance is significantly improved by some rootstocks. To overcome these challenges, the USDA citrus breeding program has implemented a multi-pronged strategy for rootstock breeding that expands the diversity of germplasm utilized in rootstock breeding, significantly increases the number of new hybrids evaluated concurrently, and greatly reduces the time from cross to potential cultivar release. We describe the key components and methodologies of this new strategy, termed “SuperSour,” along with reference to the historical favorite rootstock sour orange ( Citrus aurantium ), and previous methods employed in citrus rootstock breeding. Rootstock propagation by cuttings and tissue culture is one key to the new strategy, and by avoiding the need for nucellar seeds, eliminates the 6- to 15-year delay in testing while waiting for new hybrids to fruit. In addition, avoiding selection of parents and progeny based on nucellar polyembryony vastly expands the potential genepool for use in rootstock improvement. Fifteen new field trials with more than 350 new hybrid rootstocks have been established under the SuperSour strategy in the last 8 years. Detailed multi-year performance data from the trials will be used to identify superior rootstocks for commercial release, and to map important traits and develop molecular markers for the next generation of rootstock development. Results from two of these multi-year replicated field trials with sweet orange scion are presented to illustrate performance of 97 new hybrid rootstocks relative to four commercial rootstocks. Through the first 7 years in the field with endemic HLB, many of the new SuperSour hybrid rootstocks exhibit greatly superior fruit yield, yield efficiency, canopy health, and fruit quality, as compared with the standard rootstocks included in the trials.

Key messagePlant biostimulants did not improve citrus health and productivity in an HLB-endemic environment after two years, but fulvic acids increased concentrations of some nutrients and influenced the rhizosphere microbial community.Fibrous root loss is one of the main factors associated with citrus tree decline due to Huanglongbing (HLB) disease in Florida, and there is increased interest to use soil amendments such as plant biostimulants to improve root health and crop production. The effect of individual and combined applications of seaweed extracts, humic substances, and microbial inoculants on root and tree health, tree nutritional status, and productivity was investigated in an HLB-endemic environment. Experiments were conducted in two differently managed commercial citrus orchards. The effect of the treatments on the composition and structure of the citrus rhizosphere bacterial and fungal community was examined and linked to variations in root macro- and micronutrient concentrations. Overall, treatments had no effect on disease index ratings, total and specific root lengths, root physiological responses, and fruit quality and yield after two years of application. Only the individual applications of fulvic acids resulted in statistically significant increases in root and leaf macro- and micronutrient concentrations compared to the control treatment, an effect that differed between trials. These increases in root macro- and micronutrient concentrations were linked to variations in the relative abundance of specific bacterial and fungal taxa in the citrus rhizosphere. Our results suggest that longer-term and higher application rates of fulvic acids may be necessary to translate rhizosphere effects into measurable effects on tree health and productivity in HLB-endemic conditions. We show that factors such as different management practices and soil and environmental conditions may determine efficacy of biostimulants on root and tree health.

Trunk injection is a targeted delivery of pesticides, insecticides, nutrients, or other plant protection materials into the stem or trunk of woody plants as an alternative to spraying or soil drenching. Trunk injection has historically been used for disease and pest management of high-value forest tree species or ornamental plants when aerial applications are problematic due to spatial problems and health-related concerns. An interest in using the injection technique for protection of agricultural crops in commercial production systems has emerged more recently, where foliar applications and soil drenches have proven ineffective or pose environmental hazards. This review provides an overview of the basic principles of trunk injection and the plant physiological implications, its current use in commercial agriculture and other plant systems, and associated risks.

This study investigated the effects of supplemental light-emitting diode (LED) light spectra and intensity on the winter growth of grafted citrus nursery plants. The primary objective was to determine how different combinations of red, blue, white, and far-red LED light, applied at varying intensities, influence vegetative budbreak and scion shoot growth in young citrus trees during short winter days. The experiment used two citrus rootstocks, Carrizo (a hybrid of sweet orange and trifoliate orange) and Rich 16−6 (trifoliate orange), each budded with Washington Navel orange. Plants were grown in a temperature-controlled greenhouse and received natural sunlight, with supplemental LED lighting used to extend the photoperiod to 16 hours. The study employed a 4-factor response surface design, testing multiple combinations of light spectra and intensity. Eight plant growth and physiological responses were measured, including percentage budbreak, scion shoot length and diameter, rootstock diameter, internode length, leaf area, scion dry weight, and leaf chlorophyll index. Results showed that both the quality and intensity of supplemental LED light significantly affected all measured responses. Blue and white light, particularly at higher intensities, increased vegetative budbreak in both rootstocks, while red and far-red light reduced budbreak. The greatest scion shoot growth was observed when red, blue, and white light were combined at maximum intensity. Far-red light alone did not enhance growth, but when combined with red or white light, it further increased shoot elongation and leaf area. Chlorophyll content was highest under blue and white light and lowest under far-red light. These findings demonstrate that optimizing supplemental LED light spectra and intensity can improve winter nursery production of grafted citrus by enhancing budbreak and shoot growth. The results provide a basis for refining light management strategies in commercial citrus nurseries to increase propagation efficiency during periods of limited natural light.

Background While the rootstock genotype (belowground part of a plant) can impact rhizosphere microbial communities, few studies have examined the relationships between rootstock genotype-based recruitment of active rhizosphere bacterial communities and the availability of root nutrients for plant uptake. Rootstocks are developed to provide resistance to disease or tolerance of abiotic stresses, and compost application is a common practice to also control biotic and abiotic stresses in crops. In this field study, we examined: (i) the effect of four citrus rootstocks and/or compost application on the abundance, diversity, composition, and predicted functionality of active rhizosphere bacterial communities, and (ii) the relationships between active rhizosphere bacterial communities and root nutrient concentrations, with identification of bacterial taxa significantly correlated with changes in root nutrients in the rhizosphere. Results The rootstock genotype determined differences in the diversity of active rhizosphere bacterial communities and also impacted how compost altered the abundance, diversity, composition, and predicted functions of these active communities. Variations in the active bacterial rhizobiome were strongly linked to root nutrient cycling, and these interactions were root-nutrient- and rootstock-specific. Direct positive relationships between enriched taxa in treated soils and specific root nutrients were detected, and potentially important taxa for root nutrient uptake were identified. Significant differences in specific predicted functions were related to soil nutrient cycling (carbon, nitrogen, and tryptophan metabolisms) in the active bacterial rhizobiome among rootstocks, particularly in soils treated with compost. Conclusions This study illustrates that interactions between citrus rootstocks and compost can influence active rhizosphere bacterial communities, which impact root nutrient concentrations. In particular, the response of the rhizobiome bacterial abundance, diversity, and community composition to compost was determined by the rootstock. Specific bacterial taxa therefore appear to be driving changes in root nutrient concentrations in the active rhizobiome of different citrus rootstocks. Several potential functions of active bacterial rhizobiomes recruited by different citrus rootstocks did not appear to be redundant but rather rootstock-specific. Together, these findings have important agronomic implications as they indicate the potential for agricultural production systems to maximize benefits from rhizobiomes through the choice of selected rootstocks and the application of compost. 6QRnEBPSbTfGkWz6eGytjb Video Abstract