The bacterial pathogen associated with citrus huanglongbing (HLB) resides in the phloem of affected trees. The widespread abundance of the vector in Florida, the Asian citrus psyllid ( Diaphorina ...citri ), and the location of the pathogen in the tree vascular tissue limits the efficacy of foliar-applied therapies. Trunk injection is a crop protection strategy that applies therapeutic compounds directly into the tree vascular system, enabling their systemic distribution within the tree. However, limited information is available on the most effective methodology for implementing trunk injection at the commercial scale and the extent of damage inflicted by the injection. In this study, 5-year-old HLB-affected ‘Midsweet’ sweet orange ( Citrus sinensis ) trees were injected with the insecticide imidacloprid, the antibacterial oxytetracycline, or water. Injections occurred in Jun and Oct 2020 using three trunk injection techniques. Trees were monitored for external wounding and internal damage associated with injection, as well as tree health, bacterial titers, and yield for two production seasons. Low-pressure injection caused the least damage; however, it was less effective at delivering the tested compounds than medium- or high-pressure injection. Despite causing the greatest extent of external and internal damage at the injection site, injection of oxytetracycline significantly improved tree health, reduced bacterial titers, and increased yield in the two seasons of this study. Imidacloprid injection caused less wound damage but did not result in any lasting benefits to the trees. These results suggest that trunk injection of oxytetracycline could be an effective strategy for managing HLB and that the damage inflicted by this crop protection strategy can be reduced by selecting a suitable injection technique.
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.
Modern citrus nursery production makes use of potted-tree propagation in greenhouses. Supplemental lighting is one method by which nursery tree growth and profitability may be significantly improved, ...but limited specific information is available. Five replicated experiments were conducted to determine the utility and effects of increasing daylength during the winter months by supplemental illumination from light-emitting diode (LED) or high-pressure sodium (HPS) lights in citrus nursery propagation. Studies used ‘Valencia’ sweet orange scion, the most common citrus cultivar grown in Florida, and the commercially important rootstocks sour orange, ‘Cleopatra’ mandarin, ‘US-812’, ‘US-897’, ‘US-942’, and ‘US-1516’. Comparisons used the three common types of citrus rootstock propagation: seed, stem cuttings, and micropropagation. Six responses were measured in the lighting experiments, including vegetative growth before budding, scion bud survival, and scion bud growth after budding. Supplemental HPS or LED light to extend daylength to 16 h in the citrus nursery during short-day winter months was observed to be effective in increasing unbudded rootstock liner growth and ‘Valencia’ scion growth on all rootstocks and propagation types. Generally, the positive effect on vegetative growth from an increased daylength was stronger with the HPS light than with LED light, while increasing daylength with LED light, but not HPS light, provided some increased bud growth initiation. Use of HPS or LED supplemental lighting to extend daylength offers significant growth advantage for the citrus nursery industry in winter.
This new 4-page article provides an overview of a group of crop production materials termed “plant biostimulants,” which are frequently promoted as environment-friendly alternatives to chemical-based ...products. It explains their regulatory status and presents an overview of the most popular materials (e.g., beneficial fungi, seaweeds, and silicon) and their effects on plants. Written by Ute Albrecht and published by the UF/IFAS Horticultural Sciences Department. http://edis.ifas.ufl.edu/hs1330
Huanglongbing (HLB) is a major disease of citrus associated with phloem-limited bacteria in the genus Candidatus Liberibacter that affects all known citrus species and relatives, with many commercial ...cultivars being greatly damaged. Testing cultivar tolerance to HLB in field conditions is difficult because of the erratic spread of the bacteria, scion and rootstock interactions, and influence of many biotic and abiotic factors on the tree response to the disease. This study aimed to validate the effect of C Las infection on different citrus species and hybrids thought to have different levels of tolerance to the disease using C Las graft inoculation under controlled greenhouse conditions. Young potted seedlings from 12 different citrus germplasm selections were graft-inoculated with C Las or mock-inoculated. Plants were monitored periodically during 18 months for canopy growth, HLB and nutritional leaf symptoms, and leaf C Las titers. The leaf nutrient content was measured at the end of the experiment. Roots were also assessed at 18 months after inoculation (mai) for C Las titers and biomass distribution. There were significant differences in most analyzed variables of healthy and infected plants. Some plants of all cultivars were successfully infected; however, overall, the C Las transmission rate was low and inconsistent. Ct values of roots were generally higher than those in leaves at 18 mai. HLB symptoms were not observed on seedlings until 1 year after inoculation; at 18 mai, infected trees of all cultivars were HLB symptomatic. Significant shoot and root biomass reductions (44%–75%) in infected ‘Cleopatra’, ‘Duncan’, ‘Olinda Valencia’, ‘Sunburst’, and ‘Valencia 1-14-19’, considered susceptible to HLB, were measured. These cultivars also showed more severe HLB symptoms than the presumed tolerant cultivars such as Microcitrus inodora , Rich 16-6 trifoliate orange, and US-897. This study provides new knowledge of the efficacy and value of greenhouse screening of citrus germplasm for response to HLB to support the development of new cultivars with improved HLB tolerance or resistance.
Huanglongbing (HLB), an important citrus disease, causes many physiological and anatomical changes such as phloem dysfunction, imbalance in carbohydrate partitioning, decrease in leaf chlorophyll, ...and nutritional imbalances in the affected trees, ultimately resulting in tree decline. In Florida, HLB is associated with phloem-limited bacteria Candidatus Liberibacter asiaticus ( C Las), and it is vectored by the Asian citrus psyllid ( Diaphorina citri ). No cure for HLB has been found, and most of the HLB management efforts have been focused on vector control or exclusion, improved nutrient management, and the use of HLB-tolerant rootstocks. Individual protective covers (IPCs) are a type of psyllid exclusion tool that is increasingly used by growers for HLB management of newly planted citrus trees. However, no studies have evaluated their influence on citrus tree physiology. This study investigated the effect of IPCs and different rates of insecticides on C Las infection and different physiological attributes, including soluble (glucose, fructose, and sucrose) and nonsoluble (starch) carbohydrates, leaf chlorophyll, and leaf macronutrients and micronutrients over 2.5 years of field growth. The treatments (tree cover and insecticides rate) were applied in newly planted ‘Valencia’ sweet orange ( Citrus sinensis ) trees grafted on ‘Cleopatra’ ( C. reticulata ) rootstock. The IPCs prevented C Las transmission and accumulation of foliar starch, sucrose, and glucose commonly associated with HLB. IPC-covered trees had more leaf chlorophyll-a and chlorophyll-b than noncovered trees and more leaf nitrogen (N) and zinc (Zn). Our findings suggest that IPCs effectively prevent C Las infection and maintain the physiological health of young citrus trees under heavy HLB pressure. Therefore, IPCs are recommended as an important component of integrated pest management for this devastating disease.
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.
The citrus industry in Florida faces a destructive endemic disease, known as huanglongbing (HLB), associated with Candidatus Liberibacter asiaticus ( C Las), a phloem-limited bacterium, and ...transmitted by the Asian citrus psyllid (ACP). Rootstocks are regarded as critical to keep citrus production commercially viable and help trees cope with the disease. Although most scions are susceptible, some rootstocks are HLB-tolerant and may influence ACP infestation and C Las colonization and therefore the grafted tree tolerance. This study aimed to elucidate the relative influence of rootstock and scion on insect vector infestation and C Las colonization under natural HLB-endemic conditions. Seven commercial rootstock cultivars with different genetic backgrounds were grafted with ‘Valencia’ sweet orange ( Citrus sinensis ) or were self-grafted (non-‘Valencia’) and planted in an open field where ACP and C Las were abundant. ACP infestation was determined weekly during periods of leaf flushing, and leaves and roots were analyzed every 3 months to determine C Las titers. Trees with ‘Valencia’ scion were more attractive to the psyllids than non-‘Valencia’ scions. This was also associated with a higher number of bacteria and a larger abundance of foliar HLB symptoms. The influence of the rootstock on the psyllid attraction of grafted ‘Valencia’ scion was less evident, and leaf C Las titers were similar regardless of the rootstock. Among the non-‘Valencia’ scions, Carrizo had the lowest and US-942 the highest leaf C Las titers. Root C Las titers also varied among cultivars, and standard sour orange roots harbored more bacteria than some trifoliate orange hybrid rootstocks such as US-942. In some trees, C Las was detected first in the roots 4 months after planting, but root C Las titers remained low throughout the study. In contrast, leaf C Las titers increased over time and were considerably higher than root titers from 7 months until the end of the study, 15 months after planting. Overall, the results of this study demonstrate a greater relative influence of the scion than the rootstock on ACP infestation and C Las colonization during the early stages of infection. This suggests that other cultivar-specific traits, such as the ability to tolerate other stresses and to absorb water and nutrients more efficiently, along with influences on the scion phenology, may play a larger role in the rootstock influence on the grafted tree tolerance during the later stages of HLB progression.
Key message
In a grafted citrus tree, the leaf and root metabolic composition and the response to the phloem-limited pathogenic bacterium
Candidatus Liberibacter asiaticus
are influenced by the ...rootstock.
Eleven different citrus rootstock varieties were investigated for their influence on the grafted tree response to
Candidatus Liberibacter asiaticus
(CLas), the pathogen associated with Huanglongbing (HLB). Rootstocks included mandarin and sweet orange cultivars, sour orange, and hybrids of citrus and trifoliate orange. Leaves and roots of healthy and CLas-infected greenhouse-grown plants were collected 6 months after graft inoculation and metabolite profiles were assessed by untargeted gas chromatography–time-of-flight-mass spectrometry (GC–TOF-MS). The genetic origin of the rootstock determined their metabolic profiles in the healthy state and influenced the root metabolic response to CLas. In the leaves, metabolic differences were determined by the tree response to CLas and to a lesser extent by the rootstock cultivar on which the plants were grafted. Among the leaf metabolites that responded most consistently to infection were citrulline, ornithine, and quinic acid, which were upregulated in infected plants. The reverse was found for cerotinic acid, cyanoalanine, hexadecane, and raffinose. In both leaves and roots, several metabolites associated with plant stress and disease tolerance such as benzoic acid and proline responded differently in plants on the mandarin × trifoliate hybrid rootstocks compared with the other rootstocks. This corresponds with the different HLB field tolerance of grafted trees on different rootstocks, which is generally higher in combination with some mandarin × trifoliate hybrids. Our results demonstrate the influence of rootstock on the grafted tree response to CLas and underscore the importance of rootstocks for disease management in an HLB-endemic environment.
Huanglongbing (HLB), which is associated with the phloem-limited bacteria Candidatus Liberibacter asiaticus ( C Las), is a devastating disease that affects citrus trees worldwide. Because of the ...pervasiveness of the bacteria and psyllid vector, the disease is considered endemic in Florida. Although the effects of C Las on tree growth and physiology have been investigated for decades, most studies compared infected and noninfected trees under greenhouse conditions. This study used newly planted field-grown ‘Valencia’ sweet orange ( Citrus sinensis ) trees on two different rootstocks to monitor the distribution and accumulation of C Las in aboveground and belowground tissues following natural psyllid colonization and assess tree physiological responses and biomass reductions under HLB-endemic conditions. Trees were transplanted into the field with individual protective covers (IPCs), which are used to exclude psyllids and prevent infection. Openings were cut in the IPCs of half of the trees; to promote infection, these IPCs were temporarily removed during the main vegetative flushing period when psyllid populations were high. All trees that were exposed to psyllids became infected and displayed the symptoms typically associated with HLB. Throughout the study, higher levels of C Las were detected in the leaves compared with those in the fibrous roots. Trees that were not exposed to psyllids remained noninfected and healthy. After 18 months, a subset of trees was excavated to assess biomass differences between infected and noninfected trees. Infected trees had root system reductions of 37% and shoot system reductions of 20%, thereby significantly reducing the belowground-to-aboveground biomass ratio. Fibrous root loss was 49% and more severe than the loss of the rest of the root tissue. This study is the first to demonstrate the full extent of damage caused by C Las infection under natural HLB-endemic conditions. The results confirm previous observations that suggested fibrous root loss as one of the major consequences of infection and colonization with C Las. They also reinforce the benefits of using IPCs to prevent infection of young citrus trees during the first years of growth in the field.
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