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In Florida, almost all citrus trees are affected with Huanglongbing (HLB), caused by Candidatus Liberibacter asiaticus (CLas). We characterized various parameters of HLB-affected sweet orange trees in response to yield-improving nutritional treatment, including canopy volume, canopy density and CLas Ct values, and found that the treatment improved yield and maintained canopy density for over three years, whereas untreated HLB-affected trees declined in canopy density. The nutritional treatment did not affect CLas titer or the tree canopy volume suggesting that canopy density is a better indicator of fruit yield. To further validate the importance of canopy density, we evaluated three independent orchards (different in tree age or variety) to identify the specific traits that are correlated with fruit yields. We found that canopy density and fruit detachment force (FDF), were positively correlated with fruit yields in independent trials. Canopy density accurately distinguished between mild and severe trees in three field trials. High and low producing HLB trees had the same Ct values. Ct values did not always agree with CLas number in the phloem, as visualized by transmission electron microscopy. Our work identifies canopy density as an efficient trait to predict yields of HLB-affected trees and suggests canopy health is more relevant for yields than the CLas population. [ABSTRACT FROM AUTHOR]

• Fruit size and peel thickness are reduced in HLB symptomatic mandarin fruit. • Degreening effect on peel color development is cultivar dependent. • Degreening marginally influences sensory and physicochemical quality during storage. • Degreening enhances fruit peel color in 'LB8-9' and 'Tango' HLB-affected mandarins. Sustainability of Florida citrus industry has become the biggest concern for growers due to widespread of Huanglongbing (HLB) in almost 100% of Florida orchards with no cure in sight. Some mandarin varieties, 'LB8-9' (Sugar Belle) and 'Tango', are tolerant against HLB as compared to sweet oranges and grapefruit cultivars. Mandarins are a fresh market produce, so uniform and attractive peel color must align with consumer expectations. Florida's subtropical weather inhibits peel chlorophyll breakdown and the presence of HLB further exacerbates peel color development. To improve peel color and quality (thus marketability of fruit) of Florida grown mandarins, a study was conducted to evaluate how HLB symptomatic (S) and asymptomatic (AS) Sugar Belle and 'Tango' mandarin fruit respond to degreening treatment (5 µL L−1 ethylene at 25 °C and 85-90% relative humidity, for 72 or 96 h, respectively). Degreened and control fruit were stored at 24±1 °C and 80-85% relative humidity for 14 d with weekly evaluations. With degreening, fruit (AS and S) of Sugar Belle and 'Tango' attained lower peel hues and higher chroma (more orange and pure color) compared to untreated control, in a cultivar dependent manner. In both cultivars, S fruit retained higher peel hues (less orange) than AS fruit at harvest and during storage. S fruit also had reduced fruit size, weight, and peel thickness compared to AS fruit. At harvest, S fruit of both cultivars had higher compression forces, peel puncture resistance forces, sugars (glucose and sucrose), and citric acid than AS fruit. During storage, S fruit retained higher citric acid than AS fruit in both cultivars. With degreening, AS and S fruit from both cultivars had lower citric acid than non-treated control during storage. Upon sensory quality evaluation of Sugar Belle, lower sourness scores were recorded for degreened Sugar Belle fruit compared to untreated control irrespective of HLB fruit symptoms. Overall, degreening treatment enhanced peel color development without accelerating storage decay incidence or deteriorating fruit quality. This suggests that degreening treatment can be a potent industry tool for achieving marketable fruit peel color from HLB-affected mandarin trees grown in subtropical climates without altering the flavor of the fruit. [ABSTRACT FROM AUTHOR]

Huanglongbing (HLB), a citrus disease, has caused >70% decline in Florida citrus production over the past decade. Fruit from HLB-affected trees are small, lopsided, uneven colored, with bitter and acidic taste, making them undesirable for the fresh market. 'Tango' mandarin is fairly tolerant to HLB, but achieving acceptable peel color under Florida conditions is challenging. Therefore, preharvest foliar-applied mineral nutrients (K + Ca, K + B, and Ca + B) applied to HLB-affected 'Tango' trees followed by postharvest degreening were evaluated for their effects on fruit quality and storage life. In addition, degreening followed by thiabendazole fungicide and storage conditions (4 ± 1 °C or 24 ± 1 °C) were evaluated. The K + B treatment resulted in fewer HLB-symptomatic fruit, as well as greater fruit weight and diameter, and better peel color development. Degreening fruit allowed marketable color to be achieved. Compression forces for both healthy and HLB-affected fruit, and peel puncture resistance forces for healthy fruit were higher in cold storage than ambient storage. Thiabendazole-treated fruit had less storage decay incidence than non-treated fruit at room temperature, and no decay was observed in cold storage. Overall, preharvest K + B spray and degreening, alone or in combination, are potent tools for achieving marketable peel color of 'Tango' fruit from HLB-affected trees. • K + B treatment increased fruit size in HLB-affected 'Tango' trees. • K + B treatment initiated uniform peel color formation in HLB-affected 'Tango' fruit. • Fruit from K + B treatment followed by degreening attained marketable peel color. • Degreening achieved acceptable peel color in fruit grown in CUPS and open field. • Fertilization strategies and degreening may need to be changed in the HLB-era. [ABSTRACT FROM AUTHOR]

Huanglongbing {HLB [Candidatus Liberibacter asiaticus (C Las)]} has been one of the biggest challenges in citrus (Citrus sp.) production in Florida and wherever it is present. HLB-affected trees show significant shoot and root dieback, fruit drop, and reduction in yield. Currently, there is no cure for HLB, and there is no commercial HLB-resistant germplasm. Nonetheless, intensive nutrient management has been promising for citrus growers. The nutrient requirement of HLB-affected trees seems to be greater than that of healthy citrus trees. By understanding the nutrient uptake potential of rootstocks, fertilizer programs can be customized accordingly to enhance the performance of a rootstock in existing groves. Moreover, a reduction in the application of nutrients is possible by planting rootstocks with a high nutrient absorption capacity. Use of rootstocks with good nutrient uptake efficiency can take some burden off the growers who are intensively managing HLB-affected citrus groves. Therefore, the objective of this study was to evaluate and understand the nutrient uptake potential of the citrus rootstocks. To achieve this objective, a 100% hydroponic greenhouse study was conducted with six rootstocks with a range of tolerance to HLB. Several physiological and molecular tools were applied to evaluate the rootstocks for their nutrient uptake potential. A+Volk × O-19 (HLB-tolerant) rootstock had greater nutrient uptake efficiency, whereas US-896 (HLB-susceptible) had lesser nutrient uptake efficiency. Swingle, one of the most popular pre-HLB rootstocks, had poor zinc uptake and the least expression of ZINC TRANSPORTER, suggesting that zinc applications should be emphasized in Swingle plantings. US-896 rootstock expressed the least level of nutrient transporter genes, such as IRON TRANSPORTER. UFR-4 (a good performer under HLB conditions) had a large root biomass, but the uptake efficiency for nutrients was poor, suggesting that the nutrient uptake potential is a complex process that is not solely dependent on root biomass. This study is unique because it is one of the first citrus studies to report nutrient uptake efficiency and the potential of rootstocks. The information presented can be used to improve performance or select better-performing rootstocks under HLB conditions. [ABSTRACT FROM AUTHOR]

Since the advent of Huanglongbing (HLB) disease, Florida's citrus production has dropped by 70%. HLB-affected trees decline rapidly under high pH growing conditions. Limited nutrient availability at high soil pH has been speculated to be the cause of such rapid decline in HLB-affected trees; however, such decline is not observed in healthy citrus trees. Therefore, the objective of this study was to understand the nutrient uptake, physiological, and molecular responses of healthy (HLY) and HLB-affected (HLB) sweet orange (Citrus sinensis (L.) Osbeck) trees when grown in media maintained at different pH levels 6.0, 7.0, or 8.0. Overall, the performance of HLY and HLB trees decreased with the increase in pH conditions. HLB trees showed a significant inverse correlation between growth parameters (leaf number, shoot growth, SPAD value, stem water potential) and growing media pH whereas no such correlation was observed for HLY trees. Interesting, superior performance of HLB trees at pH 6.0 coincided with higher accumulation of Fe, Mn, and N in whole plant body as compared to high pH treatments. In contrast, HLY tree performance was significantly better at pH 7.0 compared to other pH conditions. At pH 7.0, stress related genes were upregulated in HLB leaves as compared to HLY leaves, indicating the stress response in terms of leaf abscission, reduced growth, and natural aging process was exacerbated in HLB trees at higher pH treatments. Altogether, all the physiological and molecular observations supports an interaction between HLB trees and pH, where HLB trees at pH 6.0 perform better than at higher pH. [ABSTRACT FROM AUTHOR]

• Huanglongbing (HLB)-susceptible citrus had a higher drop rate of mature fruit than those with HLB tolerance. • Genes encoding antioxidants were upregulated in HLB-susceptible compared to tolerant citrus. • Gene ontology terms for cell death and senescence are enriched in HLB-susceptible citrus. • In HLB-susceptible citrus, elevated oxidative stress potentially leads to increase fruit drop. • HLB-tolerant citrus likely has intrinsic traits to mitigate oxidative stress, possibly contributing to its disease tolerance and low fruit drop. As Huanglongbing (HLB) becomes epidemic throughout Florida, there is a sharp rise in mature fruit drop prior to harvest in affected citrus, leading to a substantial reduction in yield. To investigate how HLB increases mature fruit abscission, this research evaluated the drop rate and compared the global gene expression using RNA sequencing in HLB-susceptible 'Hamlin' sweet orange (Citrus sinensis) versus HLB-tolerant 'LB8-9' mandarin {'Clementine' mandarin (C. reticulata) and 'Minneola' tangelo [hybrid of 'Duncan' grapefruit (C. paradisi) and 'Dancy' tangerine (C. reticulata)]}. Consistent with their susceptibility to HLB documented in the literature, the drop rate of mature fruit was significantly higher in 'Hamlin' sweet orange than 'LB8-9' mandarin. Between the two citrus, there were 368 differentially expressed genes (DEGs); 310 and 58 were upregulated and downregulated, respectively, in the abscission zone (AZ) of 'Hamlin' sweet orange compared to 'LB8-9' mandarin fruit. Although at the time of collection, fruits of both citrus were still attached to the tree branch (fruit detachment force > 6 kgf), upregulation of the DEGs related to cell wall loosening in 'Hamlin' sweet orange indicates that the process of abscission was already initiated in the AZ of this citrus. The pattern of DEGs related to the metabolism and signal transduction of hormones, specifically ethylene, auxin, cytokinin, abscisic acid, and gibberellic acid, also suggests that HLB likely brings about changes in endogenous hormone balance that promote mature fruit abscission. Interestingly, DEGs encoding antioxidants were upregulated, suggesting a demand for detoxification mechanisms against oxidative stress in the AZ of 'Hamlin' sweet orange fruit. Additionally, gene ontology (GO) terms for cell death and senescence, two of consequences of oxidative stress, were highly enriched. Together, the results of transcriptome analysis presented herein provide evidence that for HLB-susceptible 'Hamlin' sweet orange, oxidative stress caused by the pathogen infection likely results in cell wall modification, leading to cell separation and eventually fruit abscission. The results further suggest that 'LB8-9' mandarin may have an advanced antioxidant system to mitigate the pathogen-induced oxidative stress, thereby contributing to its tolerance to HLB. [ABSTRACT FROM AUTHOR]

Essential nutrients for citrus ['Bingo' (Citrus reticulata, Blanco)] production are important for different functions, including photosynthesis, resistance to disease, and productivity. During the past 15 to 20 years, citrus production in Florida has significantly declined as a result of the devastating citrus greening disease also called huanglongbing (HLB). Therefore, a greenhouse study was conducted for 2 years, starting in 2018, at the University of Florida/Institute of Food and Agricultural Sciences Citrus Research and Education Center in Florida to evaluate the effect of varying rates of iron on the growth and development of 2-year-old HLB-affected 'Bingo' (Citrus reticulata, Blanco) trees on Kuharske citrange rootstock. Four treatments were used in a randomized complete block (HLB status) design with seven single tree replicates for each treatment. The treatments applied were 0.0 (control), 5.6 (standard fertilization, lx), 11.2 (2x), and 22.4 (4x) kgha21 iron on HLB-affected and healthy (non-HLB) citrus trees. Data including trunk diameter, tree height, and leaf samples were collected, processed, and analyzed at 3-month intervals for 2 years. At the end of the second year, trees were destructively sampled and processed as above-ground and below-ground biomass. Tree heights were different among iron rates of HLB-affected trees (P < 0.001); however, they were similar for non-HLB trees for both years. Higher average trunk diameters (P < 0.001) were observed for HLB-affected trees that received the 2x rate compared with the 1x rate and the control. In 2019, non-HLB trees showed 13% to 40% higher iron concentrations in leaves than HLB-affected trees. However, leaf iron concentrations were comparable for HLB-affected and non-HLB trees in 2020. Above-ground biomass for HLB-affected trees had between 33% and 44% more biomass (P < 0.01) than below-ground biomass for the corresponding iron fertilization. Iron accumulation correlated positively with all studied nutrients in the above-ground parts for both HLB-affected and non-HLB trees. A 95% confidence interval at which total biomass was nearly maximum corresponded to an iron rate of 9.6 to 11.8 kgha21, which was close to the 2x rate. Therefore, soil iron application using the aforementioned rates may be appropriate for better growth and development of young HLB-affected trees. [ABSTRACT FROM AUTHOR]

Improving nutrient uptake and tree health play an important role in managing Huanglongbing (HLB)-affected citrus trees in Florida. A greenhouse experiment was conducted to evaluate the effect of increasing rates of manganese (Mn) on growth and development of sweet orange [Citrus sinensis (L.) Osbeck] trees at the University of Florida's Institute of Food and Agricultural Sciences (UF/IFAS) Citrus Research and Education Center in Lake Alfred, FL. Half the trees were graft-inoculated with the HLB pathogen and the remainder were used as the HLB-free (non HLB) control trees. Four rates of Mn (0.0 kg·ha-1 Mn (Control), 5.6 kg·ha-1 Mn (1x--standard rate), 11.2 kg·ha-1 Mn (2x--standard rate), and 22.4 kg·ha-1 Mn (4x--standard rate) were split applied quarterly to both sets of the trees in a completely randomized design. There were seven single tree replicates for each treatment. Response variables measured were trunk diameter, tree height, leaf Mn concentration, plus above- and belowground biomass. The accumulated Mn in leaf tissues significantly increased trunk diameter but did not affect tree height for both HLB-affected and non-HLB trees, the 2x rate had the maximum value for trunk diameter relative to the 4x rate. This study established a positive correlation between soil available Mn with Fe and Cu, but negative correlation with B and Zn. A strong correlation of -0.76, -0.69, and 0.65 was observed between soil Mn and B, Zn, and Cu, respectively, as compared with 0.49 with Mn and Fe. Among HLB-affected trees, the 2x rate gave the most belowground dry matter, which was 3% greater than the control and 5% greater than 4x. Aboveground dry matter had at least 30% more biomass than belowground matter among all treatments within HLB-affected trees. For small and medium roots, Mn accumulation increased with Mn application until 2x rate and decreased thereafter for HLB-affected trees. The results from our study showed an Mn rate of 8.9-11.5 kg·ha-1 Mn, as the optimum Mn level for young 'Valencia' HLB-affected trees in Florida. [ABSTRACT FROM AUTHOR]

Fruit production of sweet orange (Citrus sinensis) in Florida has been declining with the presence of Huanglongbing [HLB; Candidatus Liberibacter asiaticus (CLas)] disease. Through disruption of the balance of endogenous hormone levels, the disease has negative impacts on fruit development, mature fruit retention, and overall tree health. Thus, the goal of this research was to determine whether plant growth regulator gibberellic acid (GA3) can be used to improve the production issues caused by HLB. For 'Valencia' sweet orange, although foliar applied GA3 from September to January (33 mg·L-1 for five applications) resulted in 50% decrease in bloom the following spring (results presented in Tang et al., 2021), this treatment did not cause reduction in yield of current and subsequent crops. Moreover, a 30% average increase in yield in GA3-treated trees was observed over a period of 4 years. The size of mature fruit was also increased (by 4% to 5%) with reduced fruit drop rate near harvest in GA3-treated trees compared with nontreated control trees. Furthermore, the canopy density, an indicator of HLB severity, was maintained in trees applied with GA3 (from 90.8% light interception to 90.4%). In contrast, there was a substantial decrease in canopy density for control trees (from 91.6% to 84.0%). Gene expression analysis of abscission zone and leaves indicated that GA3-treated trees had enhanced oxidative stress mitigation mechanism and plant defense response. Given that there is no cure for HLB, these results presented a possible remedy of using GA3 in sustaining tree health for field-grown sweet orange affected by HLB. [ABSTRACT FROM AUTHOR]

Under Florida conditions, sweet orange (Citrus sinensis) affected by Huanglongbing {HLB [Candidatus Liberibacter asiaticus (CLas)]} frequently exhibits irregular flowering patterns, including off-season flowering and prolonged bloom period. Such patterns can increase the opportunity for temporal and spatial proliferation of pathogens that infect flower petals, including the fungal causal agent for postbloom fruit drop (PFD) Colletotrichum acutatum J.H. Simmonds. For the development of strategies to manipulate flowering, the effects of floral inhibitor gibberellic acid (GA3) sprayed monthly at full- and half-strength rates (49 and 25 g·ha21, or 33 and 17 mg·L21, respectively) with different regimens, starting from September and ending in November, December, or January, on the pattern of spring bloom were evaluated in field-grown HLB-affected 'Valencia' sweet orange at two locations in subsequent February through April for two separate years in this study. To further examine whether GA3 effects on flowering patterns vary in different cultivars, early-maturing 'Navel' sweet orange trees receiving no GA3 or full-strength GA3 monthly in September through January were included. Overall, for 'Valencia' sweet orange, monthly applications of GA3 at 49 g·ha21 from September to December not only minimized the incidence of scattered emergence of flower buds and open flowers before the major bloom but also shortened the duration of flowering, compared with the untreated control trees. In addition, exogenous GA3 led to decreased leaf flowering locus t (FT) expression starting in December, as well as reduced expression of its downstream flower genes in buds during later months. When applied monthly from September through January at 49 g·ha21, similar influences of exogenous GA3 on repressing flower bud formation and compressing bloom period were observed in 'Navel' sweet orange. These results suggest that by effectively manipulating flowering in HLBaffected sweet orange trees under the Florida climate conditions, exogenous GA3 may be used to reduce early sporadic flowering and thereby shorten the window of C. acutatum infection that causes loss in fruit production. [ABSTRACT FROM AUTHOR]

Finger lime (Citrus australasica F. Muell.), a citrus species native to Australia, is a distinct finger-shaped fruit with unique caviar-like pulp and skin color variations. In addition to visual and sensory characteristics, finger lime has recently gained attention from the American citrus industry due to its tolerance to a devastating citrus disease, Huanglongbing (HLB). Here, we characterized the phytochemical profiles (including acid and sugar) of four Florida-grown finger lime selections: two with red pulp, one with pale pink pulp, and one with white pulp. Two-three-fold higher levels of phenolic compounds and antioxidant capacity were observed in the peel compared to the pulp in the four selections. Selections with red pulp had more antioxidant capacity and higher phenolic compound content in both types of tissues than in the selections with white pulp. Citric acid was found abundant in all four selections making the selections a major source of organic acid. The overall results suggested that finger limes are rich in health-benefiting and flavor contributing compounds; more specifically, Florida-grown red selections of finger lime are richest in phytochemicals, vitamin c, citric acid, and sugars among evaluated genotypes, therefore, making them strong alternative for production in Florida and consumption. • Finger lime are a rich source of total phenolics, vitamin C, acids and antioxidants. • Peel have higher level of phytochemicals and antioxidants compared to pulp. • Red pulp selection have higher phytochemicals and antioxidant capacity. • Florida-grown finger lime have higher content of phytochemicals. • Finger lime is a good alternative for production in USA as well as for consumers. [ABSTRACT FROM AUTHOR]