Submitted to: International Organization of Citrus Virologists Proceedings
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/11/2001
Publication Date: 2/2/2002
Citation: Gottwald, T., Garnsey, S. M., Riley, T. D., 2002. Latency of Systemic Infection in Young Field-grown Sweet Orange Trees Following Graft-inoculation with Citrus Tristeza Virus. Proceedings of the 15th Conference of the International Organization of Citrus Virologists Interpretive Summary: Citrus Tristeza Virus (CTV) is a serious disease of citrus that is not easily diagnosed visually. With over 1.5 million acres of citrus in the United States and several million worldwide, finding trees infected with CTV that can be eradicated prior to disease spread is a daunting problem. This experiment helps to describe the latency period, or the period between infection of CTV and the time that the disease either expresses symptoms or when the virus can be detected in young trees. By first inoculating citrus trees and them pruning below the inoculation, we were able to establish that pruning three weeks after inoculation stopped the infection but pruning 8 weeks after did not, indicating that the virus goes systemic in the tree between 3 and 8 weeks after infection. Also the virus was not detected until after the trees had over-wintered indicating that an over-wintering period is required before systemic infections become easily detected.
Technical Abstract: Experiments were conducted to determine the time required for Citrus tristeza virus (CTV) to begin migration from the site of inoculation, and the subsequent incubation period required for systemic infection to occur. Young CTV-free sweet orange trees propagated on Citrus macrophylla rootstocks were grown in the open in an area where commercial plantings are rare. They were bud-inoculated with CTV during the fall 1997 and spring 1998. Four branches were labeled on each tree; one received inoculum buds and three were uninoculated. Following inoculation, trees were pruned 15 cm below bud insertion at three or eight weeks or left unpruned. Control trees grafted with healthy buds were included to monitor potential natural CTV infections. Progress of infection was monitored by ELISA and tissue immunoblots. Inoculated branches were sampled at the time of pruning, and bark patches were taken from unpruned branches. Leaf petioles were collected periodically from uninoculated branches. All trees in the plot were sampled periodically following flushing to monitor the occurrence of systemic infections. When trees were inoculated in the fall of 1997 and left unpruned, 4 of 5 trees tested positive the following spring and all tested positive by the fall of 1999. Trees pruned three weeks post-inoculation remained CTV-free during the course of the test, while 4 of 5 trees pruned eight weeks post-inoculation were CTV-positive by the fall of 1998 and all were positive by the fall of 1999. Trees inoculated in spring 1998 showed systemic infection during the rest of that growing season, but all of 5 unpruned trees, 3 of 5 pruned three weeks post-inoculation, and 3 of 5 pruned eight weeks post-inoculation tested positive in spring 1999. The results indicate that initial movement is more rapid following spring inoculation and that an overwintering period is required before systemic infections become easily detected. Considerable variation in incubation period was also noted among groups of trees inoculated at the same time; this can affect analyses of rates of spread and the timing of detection assays employed to control the spread of CTV.