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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.

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.

The Asian citrus psyllid (Diaphorina citri) is a pest of citrus and the primary insect vector of the bacterial pathogen, ‘Candidatus Liberibacter asiaticus’ (CLas), which is associated with citrus greening disease. The citrus relative Murraya paniculata (orange jasmine) is a host plant of D. citri but is more resistant to CLas compared with all tested Citrus genotypes. The effect of host switching of D. citri between Citrus medica (citron) and M. paniculata plants on the acquisition and transmission of CLas was investigated. The psyllid CLas titer and the proportion of CLas-infected psyllids decreased in the generations after transfer from CLas-infected citron to healthy M. paniculata plants. Furthermore, after several generations of feeding on M. paniculata, pathogen acquisition (20 to 40% reduction) and transmission rates (15 to 20% reduction) in psyllids transferred to CLas-infected citron were reduced compared with psyllids continually maintained on infected citron. Top-down (difference gel electrophoresis) and bottom-up (shotgun MS/MS) proteomics methods were used to identify changes in D. citri protein expression resulting from host plant switching between Citrus macrophylla and M. paniculata. Changes in expression of insect metabolism, immunity, and cytoskeleton proteins were associated with host plant switching. Both transient and sustained feeding on M. paniculata induced distinct patterns of protein expression in D. citri compared with psyllids reared on C. macrophylla. The results point to complex interactions that affect vector competence and may lead to strategies to control the spread of citrus greening disease.

‘Candidatus Liberibacter asiaticus’ (Las) is the prominent species of Liberibacter associated with huanglongbing, a devastating disease of citrus worldwide. In this study, we report the identification of an ∼8.3-kb DNA region of the Las genome containing eight putative open reading frames flanked by two inverted repeats, which was not present in the Las str. psy62 genome. Comparisons with other genome sequences established this region as a unique genetic element associated with genome plasticity/instability. Primers specific for both the presence (Las wild type) and absence (Las mutant) of this region were designed to study the population dynamics and host adaptation of the two strains. Las populations with and/or without the wild-type strain were detected and differentiated in >2,300 samples that included psyllids, periwinkle, and several species of citrus. In psyllids, although a mixed population of the wild type and mutant was observed in most samples (88%), the wild-type Las was detected alone at a rate of 11%. In contrast, none of the infected citrus plants were positive for the wild type alone, which harbored either the mutant strain alone (8%) or a mixed population of the mutant and wild type (92%). Furthermore, the dynamics of these two major Las populations varied with different citrus hosts, whereas an in-depth study on grapefruit that did not rapidly succumb to disease revealed that the population of mutant alone increased with time, indicating that the absence of this genetic element is associated with the fitness of Las in planta under the selection pressure of its host.

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.