Matthew Mattia

The global citrus industry has been severely affected by huanglongbing (HLB), a citrus disease associated with the bacterium Candidatus Liberibacter asiaticus ( C Las), vectored by the Asian citrus psyllid. HLB symptoms are seen in the tree's canopy, fruit, and root systems, which include canopy dieback, increased fruit drop, lower fruit Brix, and reduced fine root growth. Eliminating HLB from trees is challenging, as no single method can completely control C Las. There has been an increase in research focused on finding ways to mitigate the effects of HLB. Oxytetracycline hydrochloride (OTC), an antibiotic used in citrus trunk injections, has been gaining attention in citrus growing regions like North America, South America, and South East Asia and Brazil. Research on OTC trunk injections has shown a reduction in HLB symptoms and bacterial titer; however, little is known about the interaction between OTC trunk injections, genotype, environmental stresses, and other biotic stress. Although there is a wealth of information on the HLB pathogen and its effects on trees, a succinct analysis has yet to be done to draw conclusions about whole tree effects across a range of geographies and demonstrate the broad effects of the use of OTC in an HLB environment.

Frequent and increasingly severe freezing events threaten citrus production in northern Florida, underscoring the need for strategies that enhance freezing resilience in citrus cultivars. Grafting scions onto tolerant rootstocks provides a physiologically integrative approach to improve stress tolerance. This study aims to elucidate how these interactions modulate physiological and metabolic responses under freezing stress, thereby identifying mechanisms that contribute to enhanced freeze resilience in citrus. Here, we grafted Citrus reticulata (cv. UF-950) onto eight rootstocks (Bitters, Blue-1, C-146, Sour Orange, UFR07TC, UFR09TC, UFR5, and US942) to evaluate scion–rootstock interactions under normal (20 °C) and freezing (−6 °C) conditions. Freezing stress caused a sharp increase in oxidative stress markers, lipid peroxidation, and membrane damage while reducing photosynthetic performance across most combinations. Antioxidant capacity, osmolyte accumulation, and carbon–nitrogen metabolic responses varied significantly among rootstocks, revealing strong genotype-dependent modulation of scion physiology. Among the tested combinations, UF-950 grafted onto UFR5 displayed the highest freezing tolerance, characterized by robust activation of antioxidant enzymes, elevated proline and glycine betaine accumulation, reduced oxidative damage, and sustained carbon–nitrogen metabolic fluxes under freezing stress. These results demonstrate that rootstock genotype governs the extent of scion defense activation and metabolic homeostasis under freezing conditions. Our findings identify UFR5 as a promising rootstock for enhancing freezing resilience in citrus and provide mechanistic insight into how scion–rootstock interaction orchestrates integrative stress tolerance pathways. Future work should focus on multi-omics dissection of rootstock-mediated signaling networks and long-term field validation to optimize rootstock selection for enhanced cold resilience under variable climatic conditions.

Since the citrus greening disease (aka Huanglongbing, HLB) was detected in 2005, orange (Citrus sinensis) juice production has decreased by more than 80% in Florida. Over the years, some mandarins (C. reticulata) and mandarin-like hybrids with Poncirus trifoliata in their pedigree have shown tolerance to the disease and have the potential to make up the deficit in the volume of oranges. However, current FDA rules allow for only 10% mandarin juice in orange juice. If citrus, other than C. sinensis, could be added to orange juice in amounts greater than 10%, it would give more flexibility to processors under the current production environment where oranges are declining due to citrus greening. The objective of this study was to evaluate the effect of blending mandarins and other citrus hybrids with orange juice on overall orange flavor. In the first year, a 9-member panel was trained to distinguish orange from mandarin flavor using reference standards, rate sweetness, sourness and bitterness, and to describe potential off flavors. Fruits were washed, sanitized, and manually juiced. Blends were made with 60:30 ‘Valencia’:‘Hamlin’ juice, and 10% mandarin juice including ‘Superna’, FF-5-51-2, and a Poncirus introgressed mandarin hybrid, FF-1-89-11. These blends were compared with the standard 60:40 ‘Valencia’:‘Hamlin’ juice. Panelists found no differences in orange flavor among samples. Mandarin flavor was slightly higher in blends containing FF-5-51-2, and sweetness and sourness were higher in blends containing FF-1-89-11, in comparison with the reference 60:40 ‘Valencia’:‘Hamlin’ juice. A similar comparison was performed between 60:40 ‘Valencia’:‘Hamlin’ juice and 60:20:20 ‘Valencia’:‘Hamlin’:mandarin juice, the mandarin being FF-5-51-2, ‘Superna’ or Sugar Belle®. The trained panelists could perceive the mandarin flavor in those blends, but the orange flavor remained unchanged. In the following year, blends were prepared with juice that had been commercially processed and pasteurized and presented to untrained panelists in a difference test (Tetrad). Most panelists could not detect differences between control base juice (60:40 ‘Valencia’:‘Hamlin’) and base juice to which 10%, 15% or 20% of FF-5-51-2 had been added. The test was also performed with commercial juice, and again, untrained panelists could not detect differences between blends and control. From these results, it is unlikely that untrained consumers would perceive mandarin or other hybrid added to orange juice at a level up to 20%.

In Summer 2021, a comprehensive evaluation of germplasm for the US Department of Agriculture (USDA), Agricultural Research Service (ARS) citrus breeding program was conducted to assess all citrus scions that may have commercial potential for use in not-from-concentrate orange juice (NFCOJ) to support Florida’s citrus industry that has been ravaged by Huanglongbing (HLB), causing production losses of 90%. The ‘Donaldson’ sweet orange tree ( Citrus sinensis ) stood out as being exceptionally healthy compared with industry standards that were planted close by and were in decline or had died. The tree tested positive for the presence of Candidatus Liberibacter asiaticus ( C Las), the presumed causal agent of HLB, indicating that the tree may have tolerance to HLB, as that tree was planted more than 30 years ago. The fruit was displayed at a field day in Dec 2021; the healthy-looking tree attracted growers’ attention and generated a records search for information. This is the only mature tree growing at the USDA, ARS Whitmore Foundation Farm. The results show that ‘Donaldson’ is an early season sweet orange variety that matures from December to January in HLB endemic conditions and has juice as a potential substitute for the early season orange variety ‘Hamlin’, contributing to NFCOJ. HLB tolerance is difficult to assess from a single tree and the HLB tolerance of ‘Donaldson’ will be confirmed in multiple replicated plantings.

In this letter, we present a new dataset to advance the state of the art in detecting citrus fruit and accurately estimate yield on trees affected by the Huanglongbing (HLB) disease in orchard environments via imaging. Despite the fact that significant progress has been made in solving the fruit detection problem, the lack of publicly available datasets has complicated direct comparison of results. For instance, citrus detection has long been of interest to the agricultural research community, yet there is an absence of work, particularly involving public datasets of citrus affected by HLB. To address this issue, we enhance state-of-the-art object detection methods for use in typical orchard settings. Concretely, we provide high-resolution images of citrus trees located in an area known to be highly affected by HLB, along with high-quality bounding box annotations of citrus fruit. Fruit on both the trees and the ground are labeled to allow for identification of fruit location, which contributes to advancements in yield estimation and potential measure of HLB impact via fruit drop. The dataset consists of over 32,000 bounding box annotations for fruit instances contained in 579 high-resolution images. In summary, our contributions are the following: (i) we introduce a novel dataset along with baseline performance benchmarks on multiple contemporary object detection algorithms, (ii) we show the ability to accurately capture fruit location on tree or on ground, and finally (ii) we present a correlation of our results with yield estimations.

Freezing temperatures are a severe issue in North Florida, primarily due to occasional cold snaps and frost events in winter and early spring that cause damage to citrus groves, resulting in reduced fruit yield. The apoplasm is the primary cell component that interacts with environmental stress and is essential for plant tolerance to freezing temperatures. The present study was conducted to gain insight into how the application of silicon (Si) and potassium (K) are involved in the leaf apoplasm contributes to freezing stress tolerance, and regulates carbohydrate metabolism. We used Satsuma mandarin (Citrus unshiu Marc.), the most successful commercially grown citrus cultivar in North Florida, and treated trees with two concentrations of Si and K (50 and 100 ppm) both individually and combined as foliar spray to determine their effect as they relate to improving cold hardiness. Freezing stress (-6 °C) caused a severe reduction in photosynthesis, and modulations in leaf carbohydrate metabolism resulted in inhibited plant growth. The exogenous application of Si and K both improved the photosynthesis rate, soluble sugars, and activities of enzymes of carbohydrate metabolism such as fructokinase, phosphofructokinase, hexokinase, sucrose and phosphate synthase, and acid and neutral invertase. Applying Si (100 ppm) and combined treatment (Si + K-50) showed the best response by inducing the maximum tolerance to freeze stress. Our data demonstrated the ameliorative effect of Si and K under freezing stress in citrus is associated with modification in carbohydrate metabolism in the leaf apoplasm. This study provides direction for future research to investigate the effect of Si and K on the transcriptome and metabolome in citrus plants and their tissues under freezing stress. Graphical Abstract

Sweet orange ( Citrus sinensis ) exhibits limited genetic diversity and high susceptibility to Huanglongbing (HLB). Breeding HLB-tolerant orange-like hybrids is in dire need. However, our understanding of the key compounds responsible for orange flavor and their genetic regulation remains elusive. Evaluating 179 juice samples, including oranges, mandarins, Poncirus trifoliata , and hybrids, distinct volatile compositions were found. A random forest model predicted untrained samples with 78% accuracy and identified 26 compounds crucial for orange flavor. Notably, seven esters differentiated orange from mandarin flavor. Cluster analysis showed six esters with shared genetic control. Differential gene expression analysis identified C. sinensis alcohol acyltransferase 1 ( CsAAT1 ) responsible for ester production in orange. Its activity was validated through overexpression assays. Phylogeny revealed the functional allele was inherited from pummelo. A SNP-based DNA marker in the coding region accurately predicted phenotypes. This study enhances our understanding of orange flavor compounds and their biosynthetic pathways and expands breeding options for orange-like cultivars.

Two of the major postharvest diseases impacting grapefruit shelf life and marketability in the state of Florida (USA) are stem-end rot (SER) caused by Lasiodiplodia theobromae and green mold (GM) caused by Penicillium digitatum. Here, we investigated the in vitro and in vivo efficacy of vapors of thymol, a natural compound found in the essential oil of various plants and the primary constituent of thyme (Thymus vulgaris) oil, as a potential solution for the management of GM and SER. Thymol vapors at concentrations lower than 10 mg L⁻¹ significantly inhibited the mycelial growth of both pathogens, causing severe ultrastructural damage to P. digitatum conidia. In in vivo trials, the incidence and lesion area of GM and SER on inoculated grapefruit were significantly reduced after a 5 d exposure to 50 mg L⁻¹ thymol vapors. In addition, the in vitro and in vivo sporulation of P. digitatum was suppressed by thymol. When applied in its vapor phase, thymol had no negative effect on the fruit, neither introducing perceivable off-flavor nor causing additional weight loss. Our findings support the pursuit of further studies on the use of thymol, recognized as safe for human health and the environment, as a promising strategy for grapefruit postharvest disease management.

Mandarins have many unique flavonoids with documented health benefits and that help to prevent chronic human diseases. Flavonoids are difficult to measure and cannot be phenotyped without the use of specialized equipment; consequently, citrus breeders have not used flavonoid contents as selection criteria to develop cultivars with increased benefits for human health or increased tolerance to diseases. In this study, peel, pulp, and seed samples collected from many mandarin accessions and their hybrids were analyzed for the presence of selected flavonoids with documented human health benefits. A genome-wide association study (GWAS) was used to identify SNPs associated with biosynthesis of flavonoids in these mandarin accessions, and there were 420 significant SNPs were found to be associated with 28 compounds in peel, pulp, or seed samples. Four candidate genes involved in flavonoid biosynthesis were identified by enrichment analysis. SNPs that were found to be associated with compounds in pulp samples have the potential to be used as markers to select mandarins with improved phytonutrient content to benefit human health. Mandarin cultivars bred with increased flavonoid content may provide value to growers and consumers.