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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 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’.

A large (47.75 ± 3.56 µm in diameter) Thiovulum bacterial strain forming white veils is described from a marine mangrove ecosystem. High sulfide concentrations (up to 8 mM of H2S) were measured on sunken organic matter (wood/bone debris) under laboratory conditions. This sulfur-oxidizing bacterium colonized the organic matter, forming a white veil. According to conventional scanning electron microscope (SEM) observations, bacterial cells are ovoid and slightly motile by numerous small flagella present on the cell surface. Large intracytoplasmic internal sulfur granules were observed, suggesting a sulfidic-based metabolism. Observations were confirmed by elemental sulfur distribution detected by energy-dispersive X-ray spectroscopy (EDXS) analysis using an environmental scanning electron microscope (ESEM) on non-dehydrated samples. Phylogenetic analysis of the partial sequence of 16S rDNA obtained from purified fractions of this Epsilonproteobacteraeota strain indicates that this bacterium belongs to the Thiovulaceae cluster and could be one of the largest Thiovulum ever described. We propose to name this species Candidatus Thiovulum sp. strain imperiosus.

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.

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 .

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.

In Brazil, citrus huanglongbing (HLB) is associated with ‘Candidatus Liberibacter americanus’ (CLam) and ‘Ca. Liberibacter asiaticus’ (CLas). However, there are few studies about HLB epidemiology when both Liberibacter spp. and its insect vector, the Asian citrus psyllid (ACP, Diaphorina citri), are present. The objective of this work was to compare the transmission of HLB by ACP when both CLam and CLas are present as primary inoculum. Two experiments were performed under screenhouse conditions from April 2008 to January 2012 (experiment 1) and from February 2011 to December 2015 (experiment 2). The experiments were carried out with sweet orange plants infected with CLam or CLas as inoculum source surrounded by sweet orange healthy plants. One hundred Liberibacter-free adult psyllids were monthly confined to the source of inoculum plants for 7 days with subsequent free movement inside the screenhouse. Fortnightly, nymphs and adults of psyllids were monitored. Psyllid and leaf samples were collected periodically for Liberibacter detection by PCR or quantitative PCR. CLas was detected more frequently than CLam in both psyllid and leaf samples. No mixed infections were detected in the psyllids. A clear prevalence of CLas over CLam was observed in both experiments. The final HLB incidences were 16.7 and 14.5% of Liberibacter-positive test plants, and CLas was detected in 92.3 and 93.1% of these infected plants. Mixed infection was observed only in 3.8% of infected test plants in experiment 1. These results endorse the shift in the prevalence of CLam to CLas observed in citrus orchards of São Paulo, Brazil.