ASHS 2025 Conference

Citrus production is challenged by multiple biotic and abiotic stresses, with Huanglongbing (HLB) disease being the most damaging to global citrus production. Long-term field evaluations have revealed that certain citrus relatives have varying HLB tolerance. This study investigates the genetic relationships among citrus genotypes with differing HLB tolerance levels to inform future breeding strategies for citrus improvement. The genome sequencing data from 39 citrus species and hybrids were analyzed: 23 tolerant, 8 moderately tolerant, and 8 susceptible to HLB. Variant calling identified 445,222 high-quality SNPs, with chromosome 5 showing the highest contribution of unique variants. Polymorphic information content (PIC) ranged from 0.27 to 0.38, with the highest SNP frequency observed at a PIC value of 0.27 (21,500 loci). The mean heterozygosity across genotypes was 0.136, with Citrus aurantium showing the highest and Citrus polyandra the lowest heterozygosity. Phylogenetic analysis grouped the genotypes into six distinct clusters. Admixture analysis (K=5) was performed to explore ancestral genetic components, while PCA accounted for 62.76% of total genetic variation. These findings provide critical insights for developing HLB-tolerant citrus hybrids and highlight the underlying genetic diversity present in citrus germplasm.

The US third largest citrus industry located in Lower Rio Grande Valley (LRGV), South Texas is renowned for producing premium quality fresh fruit. However, the fresh-fruit industry dynamics have changed significantly as devastating Huanglongbing (HLB) disease become endemic in LRGV citrus groves. The primary reasons that affected the Texas citrus industry are unhealthy crop load, and unnatural and excessive fruit drops in HLB affected trees. Therefore, this study investigates HLB effects on citrus fruit development in the region's subtropical semi-arid climate, focusing on the period from bloom to harvest. Research was conducted on mature 'Rio Red' grapefruit and 'Marrs' sweet oranges grafted onto Sour orange rootstock, all exposed to HLB for over a decade. In both citruses multilocation trial and replicated number of trees were pre-screened for blotchy mottled symptoms on foliage and presence of presumptive causal bacterium of HLB, Candidatus Liberibacter asiaticus (CLas) using qPCR assay. The CLas ve trees were further evaluated for sectorial presence of HLB symptoms in 10 branches and divided into two sub-categories of HLB affected symptomatic (Symp) and apparently healthy, asymptomatic (Asymp) branches. In year-2024, monthly fruitlet retention analysis from April to July reveled significantly higher fruitlets drop in ‘Symp’ branches compared to ‘Asymp’ ones across both citrus varieties. In late-May, the ‘Symp’ grapefruit branches showed 28.5-35.3% drop, while sweet orange experienced 23.3-36.3% losses. While ‘Asymp’ branches had significantly lesser fruit drop ranged in 23.1-27.5% in grapefruit and 22.7-29.4% in sweet orange. Although late-May fruit drop is a natural phenomenon in citrus trees, the excessive fruit drop under HLB disease pressure highlights the negative effect of HLB in fruit retention and possibly in fruit size expansion. Ongoing research continues to identify vulnerable stages of fruit growth and expansion under HLB stress, aiming to develop targeted treatments for maintaining high-quality fruit production under HLB prevalence in LRGV citrus groves with region’s unique environmental and edaphic conditions.

Brassinosteroids (Brs), a class of plant steroidal hormones, protect various crop species, including maize, wheat, strawberries, and grapes, from biotic and abiotic stress. However, their impact on citrus remains unexplored. The Florida citrus industry faces a significant threat from Huanglongbing (HLB), a devastating disease that deteriorates tree health and affects overall yield. Given the efficacy of Brs in enhancing stress tolerance in other crops, their potential role in citrus resilience needs an investigation. To address this knowledge gap, we examined the plant's performance after foliar application of Brs at different intervals. Three-year-old Valencia sweet oranges grafted onto UF-R5 rootstock were grown in pots and treated with Brs at three intervals: every two weeks, once a month, and every other month. Key parameters, including shoot and root biomass, canopy volume, scion and rootstock diameters, specific leaf area, total phenolic content, and flavonoid content were assessed. After one year, there was a significant increase in flavonoid content in Brs-treated plants, regardless of application frequency. There was a positive trend on the response to the treatment of the other parameters studied, althoughnot statistically significant by the first year of treatment. This finding suggests that Brs may quickly enhance secondary metabolite production in citrus, but their impact on overall growth may require longer time of exposure to the treatment. Further investigation will elucidate this.

Citrus australasica, commonly referred to as finger lime, is an Australian citrus species distinguished by its elongated, cylindrical fruits and numerous small, spherical juice vesicles that resemble caviar. It is highly valued in the culinary industry for its unique appearance, diverse color variations, and distinctive flavor. Finger lime is tolerant to Huanglongbing (HLB), a devastating citrus disease, making it a promising source of resistance genes for developing HLB-tolerant rootstocks and scions. In this study, we conducted a comparative analysis of four of the best University of Florida-developed white pulp finger lime selections - KB1, KB29, KB37, and KB48. These 7-year-old accessions, growing at the University of Florida’s Tropical Research and Education Center campus, have excellent fruit size, robust tree health, and remain HLB negative under endemic HLB conditions. The fruit quality assessment (fruit weight, edible rate, fruit length, diameter, and pearl diameter) revealed that the KB37 selection was significantly superior, followed by the other three finger lime selections, with the lowest values observed in the control, DPI-50-36. Finger lime juice analysis indicated that TSS (total soluble solids) and acid content were almost similar across the four fruit lines, and this was significantly lower than the DPI-50-36 control. The phenolic activity was not statistically different between the selections, while the highest flavonoid activity was observed in KB29, followed by KB1 and KB37. The volatile profiles obtained from the four-finger lime selections were also evaluated. Overall, morphological and physicochemical parameters identified KB37 as the best-performing selection amongst the 4 evaluated. This analysis underscores the nutritional profile of these novel white-pulp finger limes, positioning them as a strong candidate for commercial cultivation for the specialty citrus industry.

Huanglongbing (HLB), or citrus greening disease, caused by the phloem-limited bacterium Candidatus Liberibacter asiaticus (CaLas), poses a significant threat to citrus production worldwide. Identifying and developing HLB-tolerant cultivars has become necessary to mitigate its devastating impact. In Lake County, Florida, several Hamlin-type citrus trees, that’s are generally susceptible to HLB exhibited significant tolerance to the disease. The CaLas bacterial titer for most of the surviving trees was comparable to that of the susceptible control trees, and the qPCR Ct values ranged from 28 to 37. The visually tolerant citrus trees had increased chlorophyll and carotenoid content compared to the susceptible control trees. Juice quality assessments indicated a brix-to-acid ratio of 15-25, with other juice parameters meeting current U.S. Grade A standards. Interestingly, despite being labeled as Hamlin cultivars, several of these trees displayed variations in seed count, some aligning more with Parson Brown cultivars. To investigate their genetic origin, whole-genome DNA sequencing was performed on these trees, as well as on several Hamlin and Parson Brown clones. Genomic analysis revealed that the escape trees were genetically diverse; most were either likely Parson Brown types or Hamlin clones. Among them, two promising lines, R20-T30 (Parson Brown-type) and R21-T36 (Hamlin-type) were identified as superior and included in the 2024 Citrus Research and Field Trials (CRAFT) program. Both cultivars have demonstrated excellent yield and juice quality. Hamlin types, such as Hamlin 1-4-1 and Hamlin R21-T36, and Parson Brown types, such as Parson Brown F-56-2 and Parson Brown R20-T30, also displayed differences at the genomic level. Whole genome sequencing of the root DNA from these escape trees identified nucellar or zygotic Swingle as the predominant rootstock among escape trees with others similar to Carrizo and Poncirus trifoliata rootstocks. These findings underscore the role of genetic analysis in identifying HLB-tolerant cultivars, establishing a foundation for targeted breeding strategies aimed at improving citrus resilience to HLB.

Brassinosteroids (Brs) are a natural group of hormones known for their significant impact on responses to both biotic and abiotic stresses across various plant species. Brs can also influence plant growth and fruit maturation. With the recent availability of commercial formulations, this plant growth regulator is becoming an increasingly popular management tool in agriculture. Our team has been working on understanding BRs mode of action in citrus, because information is still scarce. In Florida, citrus industry is now in decline due mainly to the citrus greening disease (HLB), which has become endemic in the state, with no potential cure in the horizon. The disease reduces internal fruit quality and compromises viability of young, newly planted citrus trees. Our work has unveiled that Homobrassinolide (HBr) may increase internal maturation in sweet orange under citrus greening endemic conditions. We have determined the optimal application rate and the most effective timing for Hamlin and Valencia cultivars. We have shown that the beneficial effects of HBr are more pronounced in the healthier trees. This has led us to adapt the use of HBr in combination with oxytetracycline (OTC) injections, as growers in Florida are now injecting OTC in mature trees to reduce HLB. Our results show that HBr, when used on OTC-injected trees, improve internal fruit quality significantly. In addition, we have found that treating young, newly planted trees with HBr induces a strong immune response by activating the salicylic acid pathway, resulting in a delay in HLB infection. We have also shown that other Br analog, Epibrassinolide (EBL) improve the cold tolerance of citrus trees when young, treated plants were exposed to -4 ℃ for 24 hours. In addition, we have also observed EBL are helpful in improving the postharvest fruit quality. This is of special interest in the northern part of the state, where fresh market citrus industry is expanding, as region often experiences frequent chilling conditions during winter months. All in all, we are showing that Brs may be a ready-to-use tool in Florida citrus industry.

Huanglongbing (HLB), a devastating bacterial disease presumed to be caused by Candidatus Liberibacter asiaticus, disrupts normal plant physiology by inducing hormonal imbalances that impair growth, fruit development, and defense responses. With no known citrus germplasm exhibiting resistance to HLB, effective management strategies are essential for sustaining citrus production. This study focuses on the combined application of methyl salicylate (MeSA), a systemic acquired resistance (SAR) inducer, and gibberellic acid (GA₃), a growth-promoting hormone, as a potential strategy to improve tree health and mitigating symptoms in HLB-affected citrus orchards. The study is being conducted at UF-CREC in Lake Alfred, Florida, on 15-year-old Valencia trees grafted onto Swingle rootstock. Four treatments are being evaluated: Control, GA₃, MeSA, and GA₃ MeSA. Trees were sprayed every 45 days from July to November 2024 with GA₃ and MeSA at 10 ounces per acre. The study assesses key production parameters, including yield, fruit drop, canopy density, and fruit quality. Preliminary results indicate that GA₃ application reduced preharvest fruit drop by 57% compared to the control. The leaf water content was significantly higher in Ga MeSa (58.42%) as compared to Control. No significant differences in canopy density were observed across treatments. Leaves were collected for molecular analysis on Days 7, 14, 30, 46, and 60 after the first treatment application. Differential gene expression analysis revealed significantly lower transcript levels of Rboh and LOX2 within 7 to 14 days post-treatment in GA MeSA, suggesting a potential reduction in oxidative stress. In contrast, CDR1 expression was significantly higher in MeSA treated leaves between days 30 and 45, likely indicating SAR activation. These physiological responses may contribute to improved tree performance under HLB pressure. The study will continue into the next year to refine application strategies and better understand the sustained impact of these treatments on tree health and productivity.

Pre-harvest fruit drop significantly challenges citrus orchard yield and economic viability, especially those affected by Huanglongbing (HLB). This issue is relevant in Florida’s citrus industry. This study investigated the efficacy of propionic acid he experiments applied three monthly doses of propionic acid in two consecutive years—in 2023 treated doses were 300 ppm, 1,100 ppm and 2,200 ppm— to trees, with an untreated group serving as the control. For this experiment, results indicated that the 1,100 ppm of propionic acid was most effective in controlling fruit drop, significantly reducing losses by 30% compared to the control. In 2024, the second experiment aimed to fine-tune doses. Trees were treated with three monthly doses of propionic acid utilizing 700 ppm, 1,100 ppm, and 1,400 ppm, with an untreated group serving as the control. Results showed that the 700 ppm dose was the most effective treatment in preventing fruit drop, retaining 15% more fruit than untreated trees. In both years, propionic acid increased average fruit weight. The 700 ppm dose improved yield by 36% compared to the control in 2024, while the 1,100 ppm dose led to a 17% increase in 2023 and 6% in 2024. Propionic acid did not alter fruit quality parameters, including °Brix and acidity. These findings suggest that propionic acid is a promising management tool for reducing pre-harvest fruit drop under HLB conditions.

Huanglongbing (HLB), also known as citrus greening, is a devastating disease of citrus and has severely impacted the citrus industry in Florida, causing substantial economic losses. With no known cure, management practices just aim to mitigate the harmful effects on the tree. Recent studies have shown that the 5 monthly foliar applications (September-January) of gibberellic acid (GA) can reduce preharvest fruit drop, improve fruit set, and enhance tree health. The potential impact of these findings is huge for commercial citrus production; however, growers are looking for alternatives to foliar application as foliar sprays are highly weather dependent and expensive, currently an air blast sprayer application can range anywhere from $30 to $50 per acre. Therefore, it becomes critical to find alternatives to foliar GA application. Supply of GA to the plants through irrigation could be a valid alterative instead of foliar application. This study explores the feasibility of GA application through irrigation as an alternative to foliar sprays. Specifically, we investigated GA uptake efficiency through citrus roots, its effects on overall plant growth, and transcriptomic changes in leaves and roots. Preliminary results indicate that both foliar (GF) and root applications (GR) induced tree responses, altering the hormone profiles in leaves and roots. Fifteen days post application, GR showed significantly higher concentration of GA₁ (950 ng/g fresh weight (FW)), GA₃ (450 ng/g FW), and GA8(112 ng/g FW) in the roots whereas these hormones were undetectable in the GF or untreated controlled. In the leaves, GA3 was significantly higher (210 ng/g FW) in GF but undetectable in untreated and GR. At 30 days post treatment, an enhanced leaf growth response was observed in GF trees with a mean new growth of 35 leaves as compared to 20.2 in GR and 17.8 in untreated control. Similarly, a 60-day hydroponic study demonstrated that root GA application significantly increased leaf production (p = 0.04). Bud growth was also significantly higher in root-treated plants compared to controls at both 10 days (p = 0.01) and 20 days (p = 0.01) post-application These preliminary findings suggest that root-applied GA does not mimic foliar application effects but offers some benefits for plant growth. An in-depth, long-term analysis is underway to validate the potential of GA delivery via irrigation.

Brassinosteroids (BRs) are essential plant steroid hormones that regulate a wide range of developmental and physiological processes. In addition, these hormones play a significant role in helping plants cope with stress by modulating responses to various stressors, including heat, cold, drought, and heavy metals. Freezing temperatures during winter and spring pose a serious threat to citrus production, putting the sustainability of this newly emerging citrus industry in north Florida. Based on the protective effects of BRs, we hypothesized that exogenous application of BRs could enhance the basal tolerance of citrus plants exposed to freezing temperatures. To test this hypothesis, we evaluated the photosynthetic performance and cellular damage of cold sensitive grapefruit (cv. Ray Ruby) plants sprayed with BRs (6 ppm) under freezing stress. Plants were treated with BRs (0 and 6 ppm) at 12, 24, and 48 hours before freezing stress (hbfs). Plants maintained at 20°C and sprayed with BRs (0 and 6 ppm) served as controls. Following treatment, the plants were placed in a freezing chamber and subjected to a gradual temperature decrease until reaching -6°C, where they were maintained for 1.5 hours. Freezing stress induced greater cellular damage in exposed plants compared to those kept at 20°C. However, BRs-treated plants at 12, 24, and 48 hbfs showed approximately 30% less cellular damage compared to water-sprayed plants. Photosynthetic parameters were affected by freezing stress, leading to reductions in CO₂ assimilation (

The Georgia citrus industry is rapidly growing. The citrus farmgate value increased by ~70% from 2022 ($22.4M) to 2021 ($13.2M). More acres of citrus are put in every year and, as of 2024, there are nearly 5,000 acres of citrus across the southern half of the state. Of this acreage approximately 60% are satsumas. This has generated a bottleneck for packinghouses as a majority of the citrus grown in the state reach an acceptable maturity at the same time. Georgia citrus growers have expressed an interest in expanding the harvest window of their existing plantings so more fruit can reach the market. The plant growth regulator, gibberellic acid (GA), has historically been used to delay color break in Florida citrus. Unfortunately, the existing recommendations from Florida are not applicable to the citrus varieties grown in Georgia. The present works aims to develop GA recommendations for Georgia satsumas. GA was applied at a rate of 0, 8, 10, 15, 20 ppm (n = 4) prior to color break in August 2024. The 15 and 20 ppm concentrations saw significantly less red and yellow color development by mid-December (p

The citrus industry experiences considerable losses due to a physiological rind disorder that compromises mandarin fruit quality and marketability—particularly in cultivars like Satsuma Owari. This disorder, characterized by water-soaked areas that progress to brown lesions, is likely triggered by multiple factors, including environmental conditions such as high relative humidity and rainfall. It typically appears after fall rain events and is more prevalent in fruit from the outer canopy. Our study, conducted in a commercial orchard in California’s San Joaquin Valley, explored the role of environmental conditions, rind water status, and potential management strategies. We found that treatments applied at the color break stage, 2,4-dichlorophenoxyacetic acid (2,4-D, 16 mg/L), gibberellic acid (GA₃, 20 mg/L), and Vapor Gard® (0.5% v/v)—significantly reduced rind damage. However, GA₃ delayed external color development by approximately four weeks. Susceptibility to rind disorder varied by cultivar, with Kishu being the most susceptible, followed by Satsuma Owari and Gold Nugget, while Page, Tango, and W. Murcott exhibited greater tolerance. We also investigated biochemical, hormonal, and molecular changes associated with the disorder. Damaged tissues showed reduced cellulose concentrations but similar levels of pectin and hemicellulose compared to healthy tissue. Additionally, damaged rinds had higher hydrogen peroxide (H₂O₂) content and increased peroxidase (POD) and polyphenol oxidase (PPO) activities. Metabolomic and hormonal profiling identified 76 metabolites, with 43 differing significantly between healthy and damaged tissues, highlighting the involvement of jasmonic acid and related compounds in the disorder’s development. Transcriptomic analysis further revealed differential expression of genes involved in stress responses, cell wall degradation, and hormone signaling between healthy and affected tissues. These findings suggest that delaying rind senescence using targeted treatments may reduce the incidence of this disorder. This research provides new insights into the physiological, biochemical, and molecular basis of mandarin rind disorder and offers practical management strategies to mitigate fruit losses across different varieties.