Location: Citrus and Other Subtropical Products Research
Project Number: 6034-21000-017-000-D
Project Type: In-House Appropriated
Start Date: Mar 1, 2019
End Date: Mar 17, 2020
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.
1a. Determine the effects of HLB on response to abiotic and biotic stresses.
1b. Determine the HLB susceptibility of various rootstock/scion combinations in
green house trials.
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.
2a: Rescue high-value, novel germplasm that is threatened by HLB as well as other
maladies.
2b: Develop improved methods for elimination of CLas from infected citrus.
3. Conduct field trials to evaluate promising scion selections for tree health,
productivity and fruit quality.
3a: Determine HLB tolerance for various combinations of rootstocks and scions.
3b: Determine fruit quality attributes of advanced selections of Poncirus trifoliata hybrids that show tolerance to HLB.
Approach:
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 especially 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, rootstock and the appropriate combinations thereof, that are able to survive to bearing age and produce high quality fruit for fresh and juice markets. Therefore, determination of susceptibility of various rootstock/scion combinations will be done in greenhouse and later evaluated in field trials trials. Complicating the HLB impact are the effects of other stress which may increase the impact of HLB. Thus, abiotic (drought) and biotic (nematode, fungal, and insect) stresses will be compared for HLB and healthy grafted trees. Citrus breeding is a long-term endeavor that requires growing and evaluation of trees in the field before promising selections can be identified. Historically, first stage testing of new hybrids has been conducted using own-rooted trees that are planted into the field, grown to maturity and evaluated for horticultural characteristics. We have been conducting projects for the last several years to rescue and protect these elite lines, and the process is ongoing. Selections are grafted onto seedling rootstocks and maintained in the greenhouse. Trees that survive are tested for CLas and Citrus Tristeza virus (CTV), trees identified as negative are maintained in the greenhouse for subsequent testing. Trees that continue to test negative for CLas after one year are submitted to the FDACS DPI Budwood program for entry into the parent tree program. Another approach is to investigate the ability of various treatments for the elimination of CLas from citrus budwood. Most research on elimination of CLas from citrus has focused on either shoot-tip grafting or whole plant thermotherapy. Such approaches are laborious and require lengthy periods for evaluation. We will collect buds from known HLB-positive branches to be subjected to heat, cold, antibiotics and/or other treatments that may come available.
