Grafting a scion onto a rootstock results in physical and physiological changes in plant growth and development, which can affect tree vigor, productivity, and tolerance to stress and disease. Huanglongbing (HLB) is one of the most destructive citrus diseases and has become endemic in Florida since its introduction in 2005. It is associated with the phloem-limited bacteria Candidatus Liberibacter asiaticus (CLas), which cause severe metabolic disruptions in affected plants. Although most scion cultivars are highly susceptible, some rootstock cultivars are tolerant and allow the grafted tree to cope better with the disease. The objectives of this study were to identify rootstock traits that can be used to assess cultivars under controlled greenhouse conditions in advance of longer-term field trials. We used 10 commercially important rootstocks with different genetic backgrounds and known field performance in graft combination with ‘Valencia’ sweet orange scion. Trees were graft-inoculated with CLas and compared against mock-inoculated trees. Tree health and CLas populations were assessed regularly, and root growth was monitored using a minirhizotron imaging system. Plants were excavated and destructively sampled 21 months after inoculation to assess biomass distributions and other CLas-induced effects. We found significant differences between healthy and infected trees for most variables measured, regardless of the rootstock. In contrast to leaf CLas titers, root titers were significantly influenced by the rootstock, and highest levels were measured for ‘Ridge’ sweet orange and sour orange. Root growth and root biomasses were reduced upon infection but differences among rootstocks did not always agree with reported field performances. Despite severe biomass reductions plants maintained their relative distribution of biomass among different components of the root system, and no dead roots were observed. Root respiration was reduced by CLas infection and was overall higher in tolerant cultivars suggesting its potential as a physiological marker. This study improves our knowledge about the strengths and weaknesses of assessing rootstock traits of grafted trees in a controlled greenhouse setting. Results from the study suggest that in addition to HLB tolerance, other rootstock traits will ultimately have major contributions to field survival and productivity of the grafted trees in an HLB endemic production environment.
The devastating citrus disease huanglongbing (HLB) associated with the phloem-limited bacteria Candidatus Liberibacter asiaticus (CLas) has caused a more than 70% reduction in citrus production since its discovery in Florida in 2005. Most citrus scion cultivars are sensitive to HLB, whereas some cultivars used as rootstocks are tolerant. Using such tolerant rootstocks can help trees to cope better with the disease’s impact. Evaluating rootstock effects on a grafted scion in the field takes many years, but shorter-term evaluation is imperative to aid in rootstock selection for an HLB-endemic production environment. In this study, we investigated grafted healthy and CLas-infected citrus trees under controlled greenhouse conditions. The objectives were to identify traits suitable for assessing grafted tree tolerance in advance of longer-term field studies and aiding in the selection of superior rootstock cultivars. We assessed 10 commercially important rootstocks grafted with ‘Valencia’ sweet orange scion and with known field performance. At 6, 9, 15, and 21 months after graft inoculation (mai), leaf CLas titers were determined and canopy health was evaluated. Plants were destructively sampled at 21 mai to assess plant biomasses and other physiological and horticultural variables. There was little influence of the rootstock cultivar on CLas titers. Surprisingly, few HLB foliar disease symptoms and no differences in soluble and nonsoluble carbohydrate concentrations were measured in infected compared with healthy plants, despite high CLas titers and significant reductions in plant biomasses. Most trees on rootstocks with trifoliate orange parentage were less damaged by HLB than other rootstocks, although results did not always agree with reported field performance. Among the different variables measured, leaf size appeared to be most predictive for grafted tree assessment of HLB sensitivity. The results of this study provide a better understanding of the strengths and weaknesses of assessing rootstock influence on grafted tree performance in a controlled greenhouse environment. Although such studies provide valuable information for cultivar tolerance to HLB, other rootstock traits will ultimately contribute to field survival and productivity in an HLB endemic production environment.
Health, growth, fruit production, and fruit quality of citrus crops are severely affected by tree infection with Candidatus Liberibacter asiaticus (CLas) and subsequent development of the disease huanglongbing (HLB). The use of HLB-tolerant rootstocks is one strategy that is used to ameliorate the effects of HLB in commercial production. Although there is a clear long-term rootstock effect to improve tree performance, such field trials take many years for establishment and evaluation, and this long time-period is very limiting to expeditious evaluation of new rootstocks. In this study, we have conducted a 50-week greenhouse experiment to evaluate rootstock influences on Valencia sweet orange tree response to CLas infection. The infection of trees with CLas reduced scion and rootstock growth, increased leaf yellowing, and reduced the number of leaves per tree and leaf area, regardless of rootstock. There were clear rootstock influences on some traits during the 50-week study. In general, infected trees on US-942 rootstock had lower CLas root titers, less reduction of the number of leaves, less reduction of leaf area, and less leaf yellowing, as compared with some of the other rootstocks. The 50-week greenhouse evaluation method provided results that corresponded well with results from long-term field testing, indicating this may be a useful tool to accelerate evaluation and selection of new rootstocks, as well as in testing other HLB management strategies.
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%.
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.