Plant Science

Huanglongbing (HLB) or citrus greening disease is the most devastating disease of citrus worldwide. This disease, caused by the bacterium ‘Candidatus Liberibacter asiaticus’ (CLas), leads to low fruit quality and unproductive trees. In 2008, HLB was found in a residential citrus tree in Savannah, Georgia, and, as a result, the state has been quarantined for this disease since 2009. Nonetheless, little is known about the distribution of CLas within Georgia, even though the commercial planting of citrus in Georgia has increased exponentially in recent years. In 2019, 94 samples from commercial and residential citrus trees within 11 counties in coastal and southern Georgia were collected and tested for the presence of CLas. Molecular testing results revealed the presence of CLas in three counties where HLB had not been previously reported and in 9% of samples overall. This is the first definitive report confirming HLB in southern Georgia counties besides those along the coast.

Summary Citrus huanglongbing (HLB) is the most devastating citrus disease worldwide. ‘ Candidatus Liberibacter asiaticus’ (Las) is the most prevalent HLB causal agent that is yet to be cultured. Here, we analysed the flagellar genes of Las and Rhizobiaceae and observed two characteristics unique to the flagellar proteins of Las: (i) a shorter primary structure of the rod capping protein FlgJ than other Rhizobiaceae bacteria and (ii) Las contains only one flagellin‐encoding gene flaA (CLIBASIA_02090), whereas other Rhizobiaceae species carry at least three flagellin‐encoding genes. Only flgJ Atu but not flgJ Las restored the swimming motility of Agrobacterium tumefaciens flgJ mutant. Pull‐down assays demonstrated that FlgJ Las interacts with FlgB but not with FliE. Ectopic expression of flaA Las in A. tumefaciens mutants restored the swimming motility of ∆ flaA mutant and ∆ flaAD mutant, but not that of the null mutant ∆ flaABCD . No flagellum was observed for Las in citrus and dodder. The expression of flagellar genes was higher in psyllids than in planta . In addition, western blotting using flagellin‐specific antibody indicates that Las expresses flagellin protein in psyllids, but not in planta . The flagellar features of Las in planta suggest that Las movement in the phloem is not mediated by flagella. We also characterized the movement of Las after psyllid transmission into young flush. Our data support a model that Las remains inside young flush after psyllid transmission and before the flush matures. The delayed movement of Las out of young flush after psyllid transmission provides opportunities for targeted treatment of young flush for HLB control.

AbstractIn July 2017, a survey was conducted in a fig collection plot at Locorotondo (south of Italy) to investigate the possible presence of phytoplasmas in plants showing yellowing, deformed leaves, short internodes, mottling and mosaic. Samples were collected from symptomatic plants and tested by nested PCR assays using universal and specific primers to amplify the 16S rDNA of these prokaryotes. PCR results detected the presence of phytoplasma sequences in twenty plant samples that resulted clustering two phylogenetically distinct phytoplasmas, i.e., “Candidatus Phytoplasma asteris” and “Candidatus Phytoplasma solani” affiliated to 16SrI and 16SrXII ribosomal groups, respectively. The presence of phytoplasmas belonging to both ribosomal groups was confirmed with group specific quantitative PCR and RFLP assays on 16S ribosomal amplicons. Results of this study indicate for the first time the occurrence of phytoplasmas in fig; however, more work should be carried out to verify their association with the symptoms observed on diseased fig plants.

‘Candidatus Liberibacter solanacearum’ (Lso), transmitted by the potato psyllid (Bactericera cockerelli), is the putative causal agent of potato zebra chip disease. The bacterial pathogen infects a wide range of solanaceous plants (both wild and cultivated species), among which are peppers, potatoes, and tomatoes. Currently there are two commonly detected, genetically distinct haplotypes of Lso (A and B) identified from potatoes in the United States. To determine whether there are interactions between Lso haplotypes and different solanaceous hosts, experiments were conducted in the greenhouse in which pepper, potato, and tomato plants were infested with psyllids carrying Lso A, B, or an A and B mix (AB) or with psyllids free of Lso. Host plants were grown in pots in cages on the greenhouse benches and infested with six psyllids per plant. In addition, eight pepper cultivars were similarly infested for deeper understanding of host–haplotype interactions. Approximately 7 weeks after infestation, adult psyllids in each cage were counted to determine the impact of Lso haplotype–host interactions on psyllid survival and plants were sampled and tested molecularly for Lso. Individual psyllids carrying haplotypes B or AB and those free of Lso copiously reproduced on all three hosts, and leaf tissue from each plant tested positive for the respective Lso except those infested with Lso-negative psyllids. However, psyllids carrying Lso A did not survive on peppers but survived and abundantly reproduced on potatoes and tomatoes. In addition, samples from peppers infested with psyllids carrying Lso A tested negative for Lso. However, peppers infested with individual psyllids carrying Lso AB tested positive for Lso A, indicating that the presence of B may be required for infection by Lso A and psyllid survival on peppers. The different pepper cultivars infested with psyllids carrying Lso A showed similar results to the haplotype–host interaction tests, suggesting that cultivar may not be a factor in Lso A–pepper host interactions. Results from these studies suggest that Lso A may affect host selection by psyllids either for nutrition or laying of eggs. Mechanisms involved in preventing psyllid reproduction on peppers, once identified, will have significant implications for potential psyllid management.