Publications

4499

‘Candidatus Liberibacter asiaticus’, the putative causal agent of citrus greening, is not available in pure culture yet. In addition to trees of citrus and citrus relatives, ‘Ca. L. asiaticus’ can grow in Madagascar periwinkle (Catharanthus roseus). Using gas chromatography-mass spectrometry, we compared the phloem sap composition in sweet orange ‘Valencia’ (Citrus sinensis) and periwinkle plants after the infection with ‘Ca. L. asiaticus’. Interestingly, in contrast to our previous studies of total leaf metabolites, we found that, compared with uninfected phloem sap, the organic acids implicated in the tricarboxylic acid cycle (TCA) cycle including citrate, isocitrate, succinate, fumarate, and malate were reduced significantly in the infected phloem saps of both species. As a result of the reduction of organic acids content, the pH of infected phloem saps was increased. We hypothesize that the bacterial growth induces the mitochondrial TCA cycle in parenchyma cells to produce more of these compounds to be used as a bacterial carbon source. Once these compounds reach a low level in the phloem sap, the bacterium may send a signal, yet to be identified, to initiate a feedback loop to further induce the TCA cycle. Phloem blockage might be another reason behind the reduced translocation of TCA cycle intermediates within the phloem. The net result, localized availability of organic acids, likely benefits bacterial growth and may explain the unequal distribution of ‘Ca. L. asiaticus’ within infected trees. These findings may help in designing media for the pure culturing of ‘Ca. L. asiaticus’.

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.

A decade ago, shoot proliferation symptoms (i.e., witches’ broom) in carrots were believed to be the cause of ‘Candidatus Phytoplasma’ and Spiroplasma infection, yet in recent years this association appeared to have weakened, and a closer association was found with the yet-unculturable, psyllid-transmitted Gram-negative bacterium ‘Candidatus Liberibacter solanacearum’. In Israel, carrots are grown throughout the year, yet shoot proliferation symptoms tend to appear only in mature plants and mostly in late spring to early summer. We hypothesized that factors such as plant age, temperature, and vector load, which vary during the year, have a critical effect on symptom development and examined these factors under controlled conditions. Here we show that young carrot seedlings are as prone as older plants to develop shoot proliferation symptoms after ‘Ca. L. solanacearum’ inoculation. Surprisingly, we found that the local ‘Ca. L. solanacearum’ haplotype was extremely sensitive to constant temperature of 30°C, which led to a significant reduction in bacterial growth and symptom development compared with 18°C, which was very conducive to symptom development. We have also found that inoculations with 10 or 20 psyllids per plant results in faster symptom development compared with inoculations with two psyllids per plant; however, the difference between vector loads in disease progress rate was not significant. These data provide important insights to the effects of plant age, growth temperature, and vector load on ‘Ca. L. solanacearum’ and its associated symptoms and further strengthen the notion that ‘Ca. L. solanacearum’ is the main responsible agent for carrot witches’ broom in Israel. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

A survey of weeds was undertaken in a palm nursery affected by lethal bronzing (LB) to identify a reservoir host of the causal phytoplasma. Three common species were identified; Urochloa maxima (Guineagrass), Sporobolus indicus (smut grass), and Cyperus esculentus (yellow nutsedge) and sampled over a period of 2 years. Each species was sampled 36 times and all three species were negative for the LB phytoplasma. However, three specimens of C. esculentus tested positive for the phytoplasma species ‘Candidatus Phytoplasma brasiliense’. These findings represent the first documented case of ‘Ca. P. brasiliense’ in North America, specifically in Florida, U.S.A., as well as a new host record for the phytoplasma and the first monocot host documented. Because of the impact this phytoplasma has on papaya and hibiscus in South America, it presents a unique threat to ornamental and agricultural sectors in south Florida. An area-wide survey for the phytoplasma and potential vectors is recommended.

It has been nearly 100 years since citrus growers in two distinct regions in the northern provinces of South Africa noticed unusual symptoms in their citrus trees, causing significant crop losses. They had no idea that these symptoms would later become part of an almost global pandemic of a disease called greening or huanglongbing (HLB). The rapid spread of the disease indicated that it might be caused by a transmissible pathogen, but it took >50 years to identify the causative agent as ‘Candidatus Liberibacter africanus’. Recently, the disease appeared in more African countries, spreading by both infected planting material and Trioza erytreae. To date, five ‘Ca. L. africanus’ subspecies have been identified in various rutaceous species, with ‘Ca. L. africanus subsp. clausenae’ the only subspecies for which a biovar was detected in citrus. Efforts to detect and differentiate HLB-causing Liberibacter species are ongoing, and recent developments are discussed here. This review focuses on aspects of the African form of HLB, including its specific bacterial species and subspecies, its main insect vector, its geographic distribution, and current management strategies.

‘Candidatus Liberibacter’ species are associated with severe, economically important diseases. Nearly all known species are putatively insect transmitted, specifically by psyllids. Detection of ‘Ca. Liberibacter’ in plants is complicated by their uneven distribution in host plants and largely fastidius nature. The death of black (Fraxinus nigra) and mancana (Fraxinus mandshurica) ash trees in Saskatchewan, Canada has been associated with infestation by the cottony ash psyllid (Psyllopsis discrepans). A combination of conventional PCR amplification and Sanger sequencing of the 16S recombinant DNA was used to detect and identify ‘Ca. Liberibacter’ in psyllids collected from ash trees in Saskatchewan. BLAST analysis of two 16S sequences that were 1,058 and 1,085 bp long (NTHA 5, GenBank accession number MK942379 and NTHA 6, GenBank accession number MK937570, respectively) revealed they were 99 to 100% similar to a ‘Ca. Liberibacter solanacearum’ sequence (GenBank accession number KX197200) isolated from the Nearctic psyllid (Bactericera maculipennis) of U.S. provenance. Sequencing the psyllid genes CO1 and Cyt-b confirmed that the psyllids from which the bacterial DNA was isolated were P. discrepans, based on comparisons with sequences in GenBank and BOLD and a reference sample from the United Kingdom. These results provide the first evidence that ‘Ca. Liberibacter solanacearum’ species are associated with psyllids collected from ash trees and specifically P. discrepans. The recent episodes of dieback of ash in Saskatchewan associated with psyllid feeding are consistent with disease symptoms caused by ‘Ca. Liberibacter’ pathogens, and this possibility warrants further study.