Citrus are susceptible to a number of serious graft transmissible diseases, which can be symptomless in some rootstock combinations but debilitating when infected buds are budded onto commercially used rootstocks. Infection of nursery trees with a virus/viroid such as exocortis can cause yield reductions of up to 50% and more over time, necessitating removal of the infected trees and replanting. In many cases a small number of infected trees in an orchard will result in widespread infection as the disease is spread through normal pruning and cultural operations.
Auscitrus budwood source trees are subjected to regular testing to ensure freedom from exocortis, psorosis and other graft transmissible pathogens. Grapefruit varieties are immunised with a mild protective strain of tristeza to protect against stem pitting. Strict hygiene procedures are adhered to in tree management, cutting of budwood and extraction of seed.