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Ute Albrecht and Kim D. Bowman

Huanglongbing (HLB) is a destructive disease of citrus in most citrus-producing countries worldwide. The disease, presumably caused by phloem-limited bacteria of the genus Candidatus Liberibacter, affects all known citrus species and citrus relatives with little known resistance. Typical disease symptoms are the production of abnormal-looking fruit and chlorosis or blotchy mottle of the leaves followed at advanced stages by tree decline and death. Trifoliate orange (P. trifoliata L. Raf.) and some of its hybrids reportedly lack distinct disease symptoms despite infection with the pathogen. US-897 is a hybrid of trifoliate orange and ‘Cleopatra’ mandarin (C. reticulata Blanco), the latter being highly susceptible to HLB. This study investigated whether field-grown, naturally infected trees and greenhouse-grown, graft-inoculated seedlings of this genotype display tolerance or resistance to HLB. It was shown that naturally infected US-897 trees exhibited no distinct disease symptoms commonly associated with HLB, except for the occurrence of few mottled leaves in a small percentage of trees. Analysis of fruit and seed from infected trees did not detect any growth reduction or otherwise negative impact on development. Graft-inoculated US-897 seedlings became polymerase chain reaction (PCR)-positive for the pathogen but exhibited a superior performance compared with ‘Cleopatra’ mandarin seedlings, which displayed severe disease symptoms soon after inoculation. Despite infection, most US-897 seedlings did not develop leaf symptoms typical for HLB. Foliar symptoms observed in a small number of plants at later stages of the disease were faint and difficult to discern. Contrary to ‘Cleopatra’ seedlings, growth in stem diameter was only moderately reduced or unaffected in infected US-897 seedlings. The superior performance of US-897 plants in greenhouse and field locations suggest tolerance of this genotype to Ca. L. asiaticus.

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Ute Albrecht and Kim D. Bowman

Huanglongbing (HLB) is a devastating disease of citrus and threatens the citrus industry worldwide. The suspected causal agent of the disease is a phloem-limited bacterium of the genus Candidatus Liberibacter transmitted through insect vector or grafting with diseased budwood. Currently, most seed source trees for citrus rootstock propagation are located outdoors and unprotected from disease transmission. In addition, fruit from HLB-affected scion varieties in Florida containing seeds enter the commercial trade and move into other citrus-growing areas. The objective of this study was to determine how Ca. L. asiaticus infection affects seed quality and seedling development and whether the disease appears in seedlings grown from infected fruit. Two experiments were conducted involving thousands of seedlings produced from seeds from infected rootstock seed source trees and ‘Valencia’ sweet orange trees, respectively. Infection of trees and fruit with Ca. L. asiaticus significantly reduced seed weight, seed germination, and seedling height. Seedlings did not develop symptoms typical of HLB throughout the experiment. Polymerase chain reaction (PCR) analysis initially identified two of 686 rootstock seedlings and three of 431 sweet orange seedlings positive for the pathogen when they were very young. Resampling and PCR analysis of these five seedlings at older ages consistently indicated they were negative for the pathogen and none of these plants ever developed symptoms of HLB. It is suggested that Ca. L. asiaticus may have been translocated into some part of the embryo during seed development but that it was not present in cells or tissue, which permitted replication or disease development as the seedling grew.

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Aditi Satpute, Bryce Meyering and Ute Albrecht

Fresh-cut sweet basil (Ocimum basilicum L.) is highly sensitive to low temperatures during postharvest storage. This study investigates whether preharvest foliar application of different concentrations of abscisic acid (ABA) can increase tolerance of the commercial basil varieties ‘Di Genova’ and ‘Nufar’ to chilling injury (CI) during postharvest storage at 3.5 °C and at 7 °C. Experiments were conducted under greenhouse and commercial open-field conditions in southwest Florida during the 2017/2018 growing season. Our results showed that greenhouse-grown plants were less affected by CI during 9 days of storage at 3.5 °C when treated with 1000 mg/L or 1500 mg/L ABA and at 7 °C storage compared with the water control, but effects varied by experiment. Preharvest applications of 1000 mg/L ABA were sufficient in reducing CI during cold storage at 3.5 °C in basil grown under open-field conditions; however, at 7 °C postharvest storage, chilling-induced damage did not differ between ABA and untreated plants. Electrolyte leakage analysis of leaves confirmed the beneficial effects of ABA on alleviating chilling-induced injury. Under greenhouse conditions, preharvest applications of 1000 mg/L ABA were more effective when plants were harvested at 1300 or 1530 hr than at 1100 hr. Our results suggest that 1000 mg/L foliar preharvest applications of ABA in combination with afternoon harvest are an effective strategy to alleviate CI damage during postharvest storage at temperatures less than 4 °C and to extend the shelf life of greenhouse or field-grown, fresh-cut basil.

Open access

Bo Meyering, Adam Hoeffner and Ute Albrecht

Cilantro (Coriandrum sativum L.), also called coriander, is an herbaceous, annual plant that is cultivated worldwide for its leaves and seeds. Cilantro has a strong propensity to bolt quickly in hot weather and under long-day (LD) conditions, which affects the flavor and renders the crop unmarketable. High incidence of preharvest bolting in open-field production can cause significant economic loss. The phytohormone gibberellic acid (GA) regulates stem elongation and floral initiation in many LD rosette plants. In pilot experiments, we found that GA induced bolting in greenhouse-grown cilantro and that plant growth regulators (PGRs) with anti-GA activity can delay this process. We then explored the effects of different GA inhibitors on reducing the incidence of bolting in cilantro grown in a commercial open-field environment. Four field trials were conducted on a commercial farm near Clewiston in Florida between Fall 2016 and Spring 2018. Different growth regulators were applied at different times, ranging from 5 to 8 weeks after seeding (WAS), and plants were harvested 2 to 3 weeks thereafter. Applications of GA inhibitors significantly reduced the incidence of bolting in three of the four trials, but the extent depended on the type of inhibitor used. The results from one trial were inconclusive due to changes in weather that prevented bolting in the entire field. Overall, plots treated with prohexadione calcium and paclobutrazol were most effective and reduced bolting by up to 78%. Applying the PGRs at 5 and 6 WAS was more effective than at 7 or 8 WAS.

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Ute Albrecht, David G. Hall and Kim D. Bowman

Candidatus Liberibacter asiaticus (Las) is a phloem-limited bacterium associated with huanglongbing (HLB), one of the most destructive diseases of citrus in Florida and other citrus-producing countries. Natural transmission of Las occurs by the psyllid vector Diaphorina citri, but transmission can also occur through grafting with diseased budwood. As a result of the difficulty of maintaining Las in culture, screening of citrus germplasm for HLB resistance often relies on graft inoculation as the mode of pathogen transmission. This study evaluates transmission efficiencies and HLB progression in graft-inoculated and psyllid-inoculated citrus under greenhouse and natural conditions in the field. Frequencies of transmission in graft-inoculated greenhouse-grown plants varied between experiments and were as high as 90% in susceptible sweet orange plants 6 to 12 months after inoculation. Transmission frequency in a tolerant Citrus × Poncirus genotype (US-802) was 31% to 75%. In contrast, transmission of Las after controlled psyllid inoculation did not exceed 38% in any of four experiments in this study. Whereas the time from inoculation to detection of Las by polymerase chain reaction (PCR) was faster in psyllid-inoculated US-802 plants compared with graft-inoculated US-802 plants, it was similar in graft- and psyllid-inoculated sweet orange plants. HLB symptom expression was indistinguishable in graft- and psyllid-inoculated plants but was not always associated with the number of bacteria in affected leaves. The highest number of Las genomes per gram leaf tissue measured in sweet orange plants was one to four × 107 in graft-inoculated plants and one to two × 107 in psyllid-inoculated plants. Highest numbers measured in tolerant US-802 plants were one to three × 106 and two to six × 106, respectively. Compared with artificial inoculation in a greenhouse setting, natural inoculation of field-grown sweet orange trees occurred at a much slower pace, requiring more than 1 year for infection incidence to reach 50% and a minimum of 3 years to reach 100%.

Open access

Ute Albrecht, Shahrzad Bodaghi, Bo Meyering and Kim D. Bowman

The rootstock plays a large role in modern citrus production because of its influence on tolerance to adverse abiotic and biotic soil-borne stresses, and on the general horticultural characteristics of the grafted scion. In recent years, rootstock has received increased attention as a management strategy to alleviate the devastating effects of the bacterial disease huanglongbing (HLB), also known as “citrus greening.” In commercial citrus nursery production, rootstocks are typically propagated by seed. Because of the increased demand for HLB-tolerant rootstocks, seed supply is often inadequate for the most popular cultivars. Cuttings and tissue culture (TC) propagation are alternative methods to supply adequate quantities of genetically identical rootstocks to be used as liners for grafting. However, there are concerns among nursery owners and citrus growers regarding the possible inferiority of rootstocks that are not propagated by seed. This study investigates the influence of rootstock propagation method on traits of sweet orange trees grafted on four commercially important rootstock cultivars during the nursery stage and during the first year of growth in a commercial citrus orchard. Several of the measured traits during the nursery stage, including rootstock sprouting, grafted tree growth, and root mass distribution were significantly influenced by the rootstock propagation method, but traits were also influenced by the rootstock cultivar. Our results also suggest that for tissue culture-propagated plants, differences in the starting material and the culturing method can affect the grafted tree behavior. Except for canopy spread and scion to rootstock trunk diameter ratio, tree growth during the orchard stage was determined by the combination of propagation method and rootstock, rather than by propagation method alone.

Open access

Rayane Barcelos Bisi, Ute Albrecht and Kim D. Bowman

Six new hybrid rootstocks, ‘US-1279’, ‘US-1281’, ‘US-1282’, ‘US-1283’, ‘US-1284’, and ‘US-1516’, were released from the U.S. Department of Agriculture (USDA) citrus breeding program to provide improved tree tolerance to huanglongbing (HLB), the most destructive disease facing the citrus industry in the United States and many other parts of the world. Five of these new rootstocks were released based on field performance in trials with the rootstocks propagated by stem cuttings, rather than the traditional propagation using nucellar seedlings. In this study, we evaluated the fruit, seed, and seedling characteristics of these new rootstocks, along with four other USDA rootstocks of commercial importance. The study included a determination of the percentage of true-to-type and off-type seedlings by both plant morphology and simple sequence repeat (SSR) markers. All 10 rootstocks produced an acceptable number of seeds and good seedling emergence from those seeds. The rootstocks ‘Swingle’, ‘US-802’, ‘US-812’, ‘US-1283’, ‘US-1284’, and ‘US-1516’ had a high percentage of true-to-type seedlings and correspondingly good potential to be propagated by seeds. However, no true-to-type plants were observed among seedlings from the rootstocks ‘US-1279’, ‘US-1281’, and ‘US-1282’, indicating that economical seed propagation will be impossible for these cultivars. The 10 SSR marker sets used in this study were observed to easily differentiate the 10 rootstocks studied, and readily distinguished true-to-type and off-type seedlings among progeny from all 10 rootstock clones. This study presents information of significant value for commercial nurseries involved in propagation of citrus rootstocks, and those involved in citrus rootstock breeding and development around the world. We propose the use of these 10 SSR marker sets as readily applicable for accurate identification of most citrus rootstock cultivars and their true-to-type seedlings.

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Ute Albrecht, Mireia Bordas, Beth Lamb, Bo Meyering and Kim D. Bowman

There are generally inadequate supplies of seed for the newest rootstocks to satisfy the growing demand for the propagation material to be used in commercial citrus nurseries. Consequently, rootstock propagation, which is traditionally by seed, now often makes use of alternative methods such as cuttings and tissue culture (TC). Propagation through cuttings and TC will generate a root system that is largely composed of adventitious or lateral roots, compared with seed propagation, which will generally promote the formation of a well-defined taproot. In this study, we compared root architecture and growth of seven different rootstock plants, generated from seed, stem cuttings, or TC, during the early weeks of growth in the greenhouse. Based on total dry biomass, root mass fraction of plants generated from cuttings ranged from 11% to 16%, and from 16% to 29% and 21% to 30% for micropropagated plants and seedlings, respectively. Plants propagated through cuttings had the most primary roots (7–10), followed by tissue culture–propagated plants which developed an average of 2–6 primary roots. As expected, plants grown from seeds mostly developed a single and well-defined taproot during the first weeks. The total number of first order lateral roots was highest in the plants propagated as cuttings (108–185) compared with 53–103 and 43–78 for tissue culture–propagated plants and seedlings, respectively. Similarly, specific root length (SRL) was highest in plants derived from cuttings (21–43 m·g−1) and lowest in plants grown from seed (7–20 m·g−1). It is suggested that the larger number and length of roots on rootstock plants propagated through vegetative methods may be better suited for resource acquisition as compared with seed propagated plants.