Publications

4470

Abstract The potato psyllid, Bactericera cockerelli (Šulc), is a major pest of potato (Solanum tuberosum L.; Solanales: Solanaceae) as a vector of ‘Candidatus Liberibacter solanacearum’, the pathogen that causes zebra chip. Management of zebra chip is challenging in part because the noncrop sources of Liberibacter-infected psyllids arriving in potato remain unknown. Adding to this challenge is the occurrence of distinct genetic haplotypes of both potato psyllid and Liberibacter that differ in host range. Longleaf groundcherry (Physalis longifolia Nutt.) has been substantially overlooked in prior research as a potential noncrop source of Liberibacter-infected B. cockerelli colonizing fields of potato. The objective of this study was to assess the suitability of P. longifolia to the three common haplotypes of B. cockerelli (central, western, and northwestern haplotypes), and to two haplotypes of ‘Ca. L. solanacearum’ (Liberibacter A and B haplotypes). Greenhouse bioassays indicated that B. cockerelli of all three haplotypes produced more offspring on P. longifolia than on potato and preferred P. longifolia over potato during settling and egg-laying activities. Greenhouse and field trials showed that P. longifolia was also highly susceptible to Liberibacter. Additionally, we discovered that infected rhizomes survived winter and produced infected plants in late spring that could then be available for psyllid colonization and pathogen acquisition. Results show that P. longifolia is susceptible to both B. cockerelli and ‘Ca. L. solanacearum’ and must be considered as a potentially important source of infective B. cockerelli colonizing potato fields in the western United States.

Citrus huanglongbing (HLB) is a devastating disease for the citrus industry. The previous studies demonstrated that oxytetracycline and penicillin are effective antibiotics against Candidatus Liberibacter asiaticus (CLas). However, since CLas is uncultured, the mechanisms of action of antibiotics against CLas are still unclear. It was recently reported that the endophytic microbial communities are associated with the progression of citrus HLB after oxytetracycline and penicillin treatment. Therefore, we hypothesize that penicillin has greater antibacterial activity against CLas than oxytetracycline, which may be associated with the alteration of the structure and function of endophytic microbial communities in HLB-affected citrus in response to these antibiotics. To test this hypothesis, the microbiome of HLB-affected citrus leaves treated with these two antibiotics was analyzed using a metagenomic method. Our results indicate that the microbial structure and function in HLB-affected citrus were altered by these two antibiotics. The relative abundance of beneficial bacterial species, including Streptomyces avermitilis and Bradyrhizobium, was higher in penicillin-treated plants compared to those treated with oxytetracycline, and the relative abundance of the bacterial species (such as Propionibacterium acnes and Synechocystis sp PCC 6803) associated with CLas survival was lower for penicillin-treated plants compared to oxytetracycline-treated plants. These results indicate that penicillin has greater antibacterial activity against CLas. Based on the metagenomic analysis, this study elucidated the mechanism for the observed increase in antibacterial activity of penicillin against CLas. The data presented here are not only invaluable for developing eco-friendly and effective biocontrol strategies to combat citrus HLB, but also provide a method for revealing mechanism of antimicrobial against uncultured bacteria in host.

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.

AbstractCandidatus Liberibacter solanacearum haplotype B (CLsoB) is an economically important pathogen of tomato (Solanum lycopersicum L.) crops in New Zealand and Central and North America. Currently, resistant cultivars of tomato are not available as a management tactic because breeding programmes lack sources of resistance. Therefore, the objective of this study was to identify sources of resistance in tomato to CLsoB. Forty‐six landraces of tomato were collected from several states in Mexico and were inoculated with CLsoB using 20 infested adults of Bactericera cockerelli per plant. Two assays were done over two years under greenhouse conditions. In the first trial, landraces FC22 and FC44 showed a significantly higher proportion of resistant plants, less symptom severity and longer incubation time compared with the other forty‐four landraces and two susceptible cultivars. In the second assay, resistance to CLsoB of the landraces FC22 and FC44 was confirmed because they had again significantly greater numbers of resistant plants, less symptom severity, relative lower CLsoB titers and longer incubation time relative to the other genotypes. All plants considered resistant from both assays had DNA of CLsoB. Results indicate that all resistant plants from these landraces are promising sources for the development of tomato cultivars with resistance to CLsoB.

AbstractThe fast development of molecular taxonomy is impacting our knowledge of the world parasite diversity at an unprecedented level. A number of operational taxonomic units have been uncovered and new species described. However, it is not always that new parasite species are being described in compliance with the International Code of Zoological Nomenclature. This is the case of “Candidatus Dirofilaria hongkongensis”, a nematode found in dogs, jackals and humans in Hong Kong and parts of India. This name has been proposed without a formal description and without the designation of a holotype, and therefore is an unavailable name. Finally, we argue that using the provisional status Candidatus in zoological nomenclature is inappropriate, considering this term is not considered in the International Code of Zoological Nomenclature.