Submitted to: Conference of International Organization of Citrus Virologists
Publication Type: Proceedings
Publication Acceptance Date: 5/22/1997
Publication Date: N/A
Interpretive Summary: Citrus Tristeza Virus (CTV) is one of the most destructive diseases of citrus worldwide. Some U.S. strains are mild and have no effect on citrus while others are severe and can cause tree decline and/or death. Occasionally mild forms of CTV can be used to protect trees from more severe forms similar to immunizations to protect animals from disease. Until recently in the U.S., the main vector has been the melon aphid. The most efficient vector of CTV is the brown citrus aphid (BCA), which was recently introduced into Florida in November 1995. This study contains data from plots in Costa Rica where the BCA exists. Both mild and severe strains of CTV exist there and are thought to have originated from Florida. The intent was to study the interaction of these two types of CTV, and determine if naturally occurring mild and severe strains compete/inhibit one another. We found that mild and severe strains co-infect and co-migrate independently with no apparent competition or interference. Therefore, many of the mild CTV strains will not protect citrus from infection by severe CTV strains. The results might be a prelude to what citrus growers in Florida and other citrus producing states might expect now that BCA has been introduced. Studies need to be conducted to determine if CTV disease development is caused by severe strains, which is independent of CTV infection, is inhibited by mild strains.
Technical Abstract: Three plots were established in commercial citrus plantings in Guanacaste, Costa Rica in October of 1992, each consisting of 400 trees of Valencia orange on CTV-tolerant rootstocks . All trees in each plot were sampled and assayed twice per year. Toxoptera citricida was introduced in Costa Rica in 1989, and is now the predominant aphid species. All samples were assayed first by ELISA DAS-I with a monoclonal antibody mixture to detect all isolates. Positive sources were assayed further with the monoclonal antibody (MC13) which detects most decline or stem-pitting CTV isolates. In two plots, mild and MC13-positive isolates existed initially. In the third plot, mild isolates predominated and MC13-positive isolates appeared and increased over time. Competition between MC13-positive and MC13-negative isolates was examined by calculating the probability of infection of previously infected vs. uninfected trees. Rates of infection for the two isolates were roughly the same within each plot. The MC13-positive isolate infected trees previously infected with mild isolates and uninfected trees without preference. Therefore, if isolate competition was occurring it was quite weak. Naturally occurring mild and MC13-positive CTV isolates in these plots apparently co-migrated independently without detectable competition or interference.