1a. Objectives (from AD-416)
1) Strategically expand and improve collections of priority genetic resources of citrus and date palm and associated information. 2) Conserve and propagate citrus and date genetic resources efficiently and effectively, and distribute pathogen-tested samples and associated information worldwide. 3) Strategically characterize (“genotype”) and evaluate (“phenotype”) citrus and date palm genetic resources for priority genetic and horticultural traits. 4) Develop more rapid and accurate diagnostic methods for priority graft-transmissible pathogens of citrus to promote exchange of pathogen-tested stock and efficiently screen for host-plant resistance.
1b. Approach (from AD-416)
New accessions will be acquired through exchange with university breeders, foreign country national programs, botanical gardens, or by plant exploration. New accessions will be quarantined, indexed and therapied before being available for distribution, and they will be characterized using the ‘Descriptors for Citrus’ published by IPGRI. In addition to distribution of germplasm, information on the accessions is disseminated via the GRIN website/server and the Repository website. The ‘genotype’ and ‘phenotype’ of citrus and date palm genetic resources will be characterized for priority genetic and horticultural traits, such as level of antioxidants and the tolerance/resistance to selected diseases. Using SSR markers and by sequence analyses of selected regions of the chromosomal DNA, the genetic variability of core accessions of orange hybrids and major groups will be examined and used to determine ancestral origin. Laboratory-based diagnostic methods will be developed for citrus vein enation and citrus concave gum disease, presently detectable only by biological indexing on indicator plants. Diagnostic tests for huanglongbing disease will be utilized to screen subsets of citrus genetic resources so as to identify new sources of host-plant resistance/tolerance to this disease. Replaces 5310-21000-008-00D (3/08).
3. Progress Report
The primary mission of the Repository is to acquire, maintain, evaluate, and distribute germplasm of citrus, citrus relatives, date palm and closely related genera of palm, and to conduct research to enable the unit to better fulfill its mission. The protected collection, from which citrus germplasm distributions of budwood are made, now has over 450 accessions. The protected collection has been assayed for Citrus tristeza virus and Cytoplasmic leprosis virus this past year. The date palm collection has been increased with 11 new accessions acquired from Arizona, the first new accessions by side shoots made to the collection in many years. The Citrus Variety Collection, which is owned by University of California, Riverside, but utilized by the Repository for seed collection, observations, evaluations, DNA extracts, and pollen collection, now has over 1000 accessions. The Citrus Relatives Collection is now consolidated into a single block planting at Southwest Coast Field Research Station, Irvine, and is being planted also at the Coachella Valley Agricultural Research Station. A total of 915 distributions were made in CY2007. This accomplishment addresses National Program 301 component 1, plant and microbial genetic resource management, problem statement 1A, efficiently and effectively manage plant and microbial genetic resources.
1. Development of a real time PCR assay for detection of huanglongbing in psyllids. Huanglongbing (HLB), or citrus greening disease, is probably the most destructive disease of citrus and is spread in nature by psyllid vector, Diaphorina citri. Candidatus Liberibacter asiaticus and Ca. L. americanus have been found to be associated with the disease in Brazil in 2004 while in the USA (Florida in 2005 and Louisiana in 2008) only Ca. L. asiaticus has been reported. In cooperation with scientists from the Florida Division of Plant Industry and the FUNDECITRUS, Brazil, the use of real time PCR to detect HLB in psyllids has been applied to monitoring effectiveness of different management strategies by ARS scientists in the National Clonal Germplasm Repository for Citrus in Riverside, CA. Selected blocks of citrus trees from three orchards practicing varying levels of disease management practices were selected; weekly samples of psyllids were collected from these blocks over a period of one year and analyzed for the presence of both species of Liberibacters, and the trees were monitored for development of disease symptoms. The results show that the presence of Ca. Liberibacters can be identified in psyllids long before symptoms become visible in plants and show the usefulness of psyllid analysis in monitoring different management practices. This accomplishment addresses National Program 301 component 1, plant and microbial genetic resource management, problem statement 1A, efficiently and effectively manage plant and microbial genetic resources.
2. Development of antibodies specific for Citrus leaf blotch virus (CLBV). CLBV has been reported to be seed transmitted thus increasing the risk of importing this virus when citrus seed is imported. ARS scientists in the National Clonal Germplasm Repository for Citrus in Riverside, CA, in cooperation with the Department of Plant Pathology at the University of California, Riverside, have developed polyclonal antibodies which recognize CLBV. These appear to work well in DASI ELISA format assays to detect CLBV. The antibodies will enable serological detection of this seed borne virus which will allow rapid screening of trees used as seed source trees. This accomplishment addresses National Program 301 component 1, plant and microbial genetic resource management, problem statement 1A, efficiently and effectively manage plant and microbial genetic resources.
3. Citrus leprosis disease, caused by Cytoplasmic citrus leprosis virus (CCLiV) and vectored by Brevipalpus species mites, is an emerging disease in Central America. ARS scientists in the National Clonal Germplasm Repository for Citrus in Riverside, CA, in cooperation with University of Florida, have further characterized the genome of CCLiV including the subgenomic RNAs. A sensitive RT-PCR assay has been developed and polyclonal antibodies have been raised which are specific for CCLiV in DASI-ELISA format assays. The sensitive RT-PCR and serological assays will enable rapid screening of plants and mites for presence of CCLiV. This accomplishment addresses National Program 301 component 1, plant and microbial genetic resource management, problem statement 1A, efficiently and effectively manage plant and microbial genetic resources.
5. Significant Activities that Support Special Target Populations