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Purpose of the CCPP
2) To maintain blocks of trees that serve as the primary source of disease free, true to type budwood of all important fruit and rootstock varieties for California's citrus industry and citrus researchers.
Variety Introduction
Procedure and Timeline for the Introduction of New Varieties (Adobe Acrobat File)
Disease Testing
Therapy Methods for Clean-up
Thermaltherapy Also called heat treatment is performed by grafting infected buds into citrange seedlings. The newly grafted infected bud is tightly and completely wrapped with budding tape. The citrange seedlings, each with an infected bud grafted on it, are placed into a hot greenhouse kept at 28-40 °C daytime and 25 °C nighttime for preconditioning to high temperatures for 30 days. Following preconditioning the seedlings are placed in a controlled temperature chamber set for 16 hour days at 40 °C and 8 hour nights set at 30 °C for a period of 3 months. Upon removal of the plants from the temperature chamber the buds are unwrapped. The rootstock seedling is lopped over and the top of the seedling is pushed into the potting soil such that the grafted bud will become the terminal bud. The plants are then placed in the greenhouse until sufficient budwood growth is produced from the grafted bud for further indexing. All imports received by CCPP originating outside the U.S. are routinely subjected to thermaltherapy as a precaution.
Final Disease Testing
1) To provide a safe mechanism for the introduction into California, of citrus varieties from any citrus-growing area of the world for purposes of research, variety improvement, or direct use by the commercial industry.
Citrus quarantine in California is a cooperative venture involving federal, state, and county departments of agriculture and the University of California. The federal government represented by the USDA Animal and Plant Health Inspection Service (APHIS) is concerned with citrus pest exclusion from foreign sources entering the U.S. There are also instances where federal quarantines exist between states that likewise govern the movement of citrus. The director of the University's CCPP has a USDA/APHIS issued permit to import citrus budwood from foreign countries. There are specific stipulations spelled out on this permit regarding handling and treatment of materials in quarantine that must be followed when citrus material is imported. These stipulations are also required by the State of California and are enforced by the California Department of Food and Agriculture (CDFA). The agricultural code of the State of California states that any citrus material entering California regardless of its point of origin, foreign or domestic, must enter through the CCPP.
Imported budwood of all new varieties arrives at the CCPP quarantine facility in Riverside. The arrival of a new budwood shipment sets in motion a detailed process for the production and distribution of this new variety. This process starts with a comprehensive testing program, to detect graft-transmissible diseases that may arrive in an imported budline. Graft transmissible diseases may be caused by viruses, viroids or other pathogens and are vegetatively propagated with an infected budline. Graft-transmissible diseases can seriously affect fruit production and quality as well as tree health and longevity. Additionally, diseases from infected field propagations may be spread to neighboring orchards of healthy trees by insects or farming equipment. Detection of these graft-transmissible diseases of citrus is based primarily upon biological indexing that is accomplished by grafting tissue of the imported budwood into specific citrus indicator seedlings. The indicator varieties have been selected for their sensitivity to disease and ability to express specific diagnostic disease symptoms.
Complementing this biological testing are laboratory techniques including: enzyme linked immuno-sorbant assay (ELISA) used for the detection of citrus tristeza virus and sequential polyacrylamide gel electrophoresis (sPAGE) and polymerase chain reaction (PCR) used for the detection of citrus viroids like exocortis and cachexia. Typically, when a new import is received CCPP quarantine facility, four propagations are made of that budline on rough lemon rootstocks to preserve the budline and to produce budwood for future index and/or therapy. These propagations are placed in a warm greenhouse to push growth. The remaining portion of the same imported budwood is used to graft inoculate indicator seedlings in a screening index called the pre-index. The pre-index will indicate if the imported budline is infected with tristeza, psorosis or citrus viroids. A very high percentage of new imports arrive infected with one or more of these bud-transmissible diseases.
If the pre-index shows that the newly introduced variety is infected, it must be subjected to therapy procedures that can eliminate the disease or diseases from the budline. The CCPP employs two methods of therapy, thermaltherapy and shoot-tip-micrografting.
Shoot-tip-micrografting This is the other form of disease clean-up therapy employed by CCPP. Some pathogens, particularly the citrus viroids (exocortis/cachexia), are difficult or impossible to eliminate by thermaltherapy, and are more readily eliminated by shoot-tip-micrografting (STG). STG is a procedure where several new growth tips slightly less than 1cm in length are taken from one of the original infected import propagations. Then with the aid of a microscope, a very small portion of the shoot tip, about 0.15.mm, is removed from the infected growth tip and grafted onto a rootstock seedling growing in a test tube. If the shoot tip that is removed is small enough the disease agent will not have had sufficient time to infect this rapidly growing tip so the disease is not present in the micro-grafted propagation. STG propagations are returned to glass tubes and placed under light in a culture chamber. When the scion of the micrografted propagation reaches about 2cm, it is re-grafted onto a clean rough lemon seedling and moved to the greenhouse.
Following any therapy procedures, all propagations produced during therapy must undergo testing to determine their disease status. If this test shows that disease is still present then the plant material must be subjected again to therapy. This cycle of therapy and testing continues until the tests are negative. When a propagation of a budline tests negative following therapy it then enters the Variety Introduction (VI) index. Budwood from a selected therapy propagation of a budline, which has subsequently tested clean is grafted onto a host range of some 60 indicator seedlings and propagations in the complete VI Index. During the VI Index the budline is also tested for the presence of stubborn disease and is again double checked by ELISA, sPAGE and PCR. If a budline is shown to be free of all known diseases in the VI Index it is then considered ready for release from quarantine.
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University of California, Riverside Citrus Clonal Protection Program About CCPP |
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The Foundation-Evaluation Block is a field planting of about 14.5 acres containing over 1,000 trees and about 200 different scion and rootstock varieties. The Foundation Block contains about 80 trees representing 50 of the most popular citrus varieties planted in the ground. The block also contains about 500 container-growing trees representing an additional 150 varieties. The entire Foundation Block is enclosed in a 50,000+ square foot screenhouse. New budlines are added to the Foundation Block each year following quarantine release. When trees are planted in the Foundation Block, indexing and evaluation does not stop. Each tree is examined several times each year by CCPP personnel and interested University and industry people for trueness-to-type, fruit quality, budsports and chimeras, spontaneous genetic disorders, and symptoms of disease. If all is well with the tree it will then be registered with CDFA as a source tree for budwood. Registration by CDFA requires that after the trees have fruited satisfactorily for several years they are then re-indexed for tristeza, viroids and psorosis. If these tests are all negative, the tree is then assigned a CDFA registration number that must accompany any budwood distributed from that foundation tree. In order to remain as a registered budwood sources these trees must be tested annually for tristeza, every three years for viroids and every six years for psorosis. Any tree showing abnormal growth characteristics or which test positive for disease are immediately removed from the block. Budwood from these registered foundation block trees is then released to California nurserymen and growers. Limited quantities of budwood are available and recipients normally use Foundation Block budwood to produce their own nursery or grower owned registered trees or nursery increase blocks that are also regulated by CDFA.