Submitted to: Citrograph
Publication Type: Trade Journal
Publication Acceptance Date: 12/3/2013
Publication Date: 6/1/2014
Citation: Lee, R.F., Volk, G.M., Hartung, J.S. 2014. Developing cryotherapy to eliminate graf-transmissible pathogens in citrus. Citrograph. 5(3):50-57.
Technical Abstract: This article summarizes research being conducted as part of a project funded by the California Citrus Research Board to develop cryotherapy (freezing buds in liquid nitrogen, and then recovering them) as a viable method for elimination of graft transmissible pathogens from Citrus. There are currently two methods available for therapy of Citrus from graft transmissible pathogens: thermotherapy and shoot tip grafting (STG). Thermotherapy does not eliminate viroids, and is inefficient at eliminating huanglongbing (HLB) and Citrus tatterleaf virus (CTLV). Although STG has been shown to eliminate all graft transmissible pathogens, pathogens such as viroids and CTLV are difficult. However, the major drawback to STG is the requirement for a high level of expertise since the technique is mastered only after many months of practice and requires a steady hand. A new technology, cryotherapy, has been successfully implemented in potato, sweet potato, grapevine, raspberry, and Prunus to eliminate pathogens that have been challenging using traditional methods. Recently cryotherapy has been shown to be capable of eliminating HLB from infected Citrus. Buds up to 1 mm long (including three leaf primordia) are excised and used in cryotherapy; in contrast the STG method uses buds with one leaf primordium (about 0.1 mm long). Use of larger shoot tips in cryotherapy should increase the survival rate and make the procedure more reproducible. As in the case of thermotherapy and STG, this method (cryotherapy) does avoid juvenility. If cryotherapy is proven to be easier and more reliable than traditional pathogen elimination methods, application of cryotherapy in California would help ensure availability of pathogen free materials in the threat of HLB and other exotic diseases.