1a. Objectives (from AD-416):
1. Evaluate citrus scion cultivars and advanced selections with potential for commercial use, identifying genotypes that will survive to bearing age and produce high quality fruit for the fresh and juice markets. 2. Determine horticultural and physiological factors affecting citrus trees propensity to succumb to HLB (huanglongbing).
1b. Approach (from AD-416):
Sustainability of the U.S. citrus industry, especially in Florida, is currently threatened by Huanglongbing (HLB), a disease associated with the bacterium Candidatus Liberibacter asiaticus (CLas) and its vector, the Asiatic citrus psyllid. HLB debilitates trees, reduces yield, ruins fruit quality, and eventually kills trees. Young trees are particularly susceptible to HLB, making the establishment of new orchards challenging. The foundation of sustainable citrus production in the face of HLB will be cultivars, both scion and rootstock, that are able to survive to bearing age and that produce high quality fruit for the fresh and juice markets. Identification of citrus types resistant or tolerant to HLB and acceptable to the market will require research at various stages of tree development; however, young tree survival, by default, is the first step in the process. If trees can survive to bearing age, the next step in the evaluation of promising genotypes is horticultural performance including susceptibility to other diseases, cropping ability, and fruit quality. Evaluation of fruit quality for genotypes which survive and produce a crop is essential to determine if new hybrids or transgenic plants meet established standards. Finally, when promising selections have been identified, replicated field trials will be the conducted to determine worthiness for eventual release. Protection and preservation of the invaluable advanced selections and breeding lines which exist only in the U.S. Horticultural Research Laboratory citrus breeding program are essential to this program; novel methods for pathogen elimination, scion evaluation, and germplasm conservation will be tested. Enhanced production efficiency, health, and value of the U.S. citrus crop will be outcomes of this research.
3. Progress Report:
Six hundred trees composed of ‘Valencia’ scion on US-812 rootstock were propagated, established in the greenhouse and are nearly ready for use in drought stress experiments; this relates to Objective 1: Determine the critical factors that limit standard and novel citrus cultivars’ ability to thrive and become productive in Florida, where Huanglongbing (HLB) and its vector are endemic, Subobjective 1a. Determine the effects of Huanglongbing on response to abiotic and biotic stresses. Two groups of trees consisting of 12 standard scion selections grafted onto US812 rootstock were prepared for use in greenhouse studies to determine susceptibility to Huanglongbing disease; this relates to Subobjective 1b. Determine the Huanglongbing susceptibility of various rootstock/scion combinations in greenhouse trials. Selections of elite germplasm identified by plant breeders were grafted onto rootstocks and maintained in the greenhouse. Data were collected for tree survival and tested for CLas and Citrus Tristeza Virus (CTV) periodically following grafting. Propagations that test negative for Candidatus Liberibacter asiaticus continue to be maintained in the greenhouse. Objective 2: Protect and/or rescue valuable and unique ARS citrus germplasm from infection by HLB through appropriate methods, including, micrografting, cyrotherapy, thermotherapy and antibiotics as necessary. Field trials, established with commercial growers in previous years to evaluate cultivar performance in areas impacted huanglongbing disease, have continued. Data was collected on tree survival, growth, and fruit quality characteristics. Additional field trials were established with commercial growers in orchards with diverse management practices to determine if it is possible to bring young trees, threatened by huanglongbing disease, into production. Arrangements were made with commercial nurserymen to prepare grafted trees for trials to be established in the upcoming fiscal year. This work relates to Objective 3: Conduct field trials to evaluate promising scion selections for tree health, productivity and fruit quality. Sensory and chemical analysis of juice prepared from Poncirus trifoliata and P. trifoliata hybrids was conducted. This work relates to Subobjective 3b: Determine fruit quality attributes of advanced selections of Poncirus trifoliata hybrids that show tolerance to Huanglongbing.
1. Rootstock impacts fruit quality parameters in grapefruit. A two year study to determine the effects of rootstock on grapefruit was completed by ARS scientists in Ft. Pierce, Florida. The devastating impact of Huanglongbing on citrus fruit production has spurred increased interest in the impact of rootstocks on the tree performance in huanglongbing affected trees. As part of a multiyear rootstock trial, data were collected to determine how rootstock affects grapefruit maturity and quality. Results indicate significant affects of rootstock on both time of maturity and standard fruit quality parameters (fruit size, peel thickness, fruit shape, total sugar and acid contents). The results of this study are of benefit to citrus producers who must make decisions regarding rootstock selection when establishing new orchards.
2. Expression of Huanglongbing symptoms is a function of Candidatus Liberibacter asiaticus titer. Based on analysis of data for 20,000 Huanglongbing diagnostic samples, provided by commercial collaborator, it was determined by ARS scientists at Fort Pierce, Florida that citrus leaves showing huanglongbing symptoms have a bacterial titer of approximately 10,000,000 to 100,000,000 cells per gram fresh weight. In addition the results show that Liberibacter infected but asymptomatic leaves have bacterial titers in the range of less than 10 to nearly 1,000,000 cells per gram fresh weight. These results have significant implications regarding kinetics of Liberibacter growth in citrus, how symptoms relate to titer, and the impacts of sample size on limits of pathogen detection.
3. Transmission of Candidatus Liberibacter asiaticus. Transmission of Candidatus Liberibacter asiaticus by infected psyllids in greenhouse plants is an effective and efficient means for determining the propensity of citrus cultivars to become infected with the pathogen. Most work to date by ARS scientists at Fort Pierce, Florida has utilized graft inoculation to infect experimental trees with Candidatus Liberibacter asiaticus; however, graft inoculation is not representative of how trees become infected in the field. Based on a series of greenhouse experiments, it was determined that placing citrus plants in a greenhouse containing free flying Asian citrus psyllids, the insect vector of Candidatus Liberibacter asiaticus, using infected plants as a source of inoculum, is an efficient method of inoculation and not prone to the artifacts associated with graft inoculation. In addition, it was determined that there were significant differences among citrus scions in the incidence of Candidatus Liberibacter asiaticus infection and the eventual titer of the pathogen. These results are important for designing experiments to test for susceptibility to huanglongbing.
Stover, E., Stange, R.R., McCollum, T.G., Jaynes, J., Irey, M., Mirkov, E. 2013. Screening antimicrobial peptides in-vitro for use in developing transgenic citrus resistant to huanglongbing and citrus canker. Journal of the American Society for Horticultural Science. 138:142-148.