|Riley, K. M.|
Submitted to: International Organization of Citrus Virologists Proceedings
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/11/2001
Publication Date: 2/1/2002
Citation: Gottwald, T., Polek, M., Riley, K. M., 2002. History, Present Incidence and Spatial Distribution of Citrus Tristeza Virus in the California Central Valley. Proceedings of the 15th Conference of the International Organization of Citrus Virologists, 83-94. Interpretive Summary: Citrus Tristeza Virus (CTV) is a serious disease of citrus that is not easily diagnosed visually. 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. The Central California Tristeza Eradication Agency (CCTEA) uses a sampling/survey method developed by the first author for this purpose. This manuscript describes the use of this method from a practical standpoint and the operational plan used by the CCTEA to attempt to achieve eradication in California. It also outlines the historical perspective of CTV in California, what lead to the introduction of CTV into California's Central Valley, and the evolution of eradication through time.
Technical Abstract: Citrus tristeza virus (CTV) has been a primary concern in the San Joaquin Valley of California since its detection in 1956. Virus eradication was deemed necessary and was undertaken by the California Department of Food and Agriculture. Since 1963 the eradication program has been managed by the Central California Tristeza Eradication Agency (CCTEA) and funded by citrus growers. The replacement of biological indexing on lime by ELISA in 1990 revolutionized detection, and allowed the first systematic survey of all commercial acreage to be conducted from 1990 to 1995. It estimated a disease incidence of 0.367%. An international workshop held in 1995 recommended a three-year Operational Plan to remove over 50% of the CTV-infected trees annually to keep ahead of disease increase. In 1998 the aggressive tree removal plan was deemed a success and was replaced by a Maintenance Plan that is still in effect today. All commercial acreage is surveyed every four years using the Hierarchical Subsampling (HS) Method, which more accurately estimates low levels of disease incidence than previous methods used. Groves testing positive are prioritized from highest to lowest infection levels, and surveyed in preparation for tree removal as resources allow. Currently, the estimated incidence in areas practicing eradication is 0.120%. The HS method is based on the spatial distribution of CTV-positive trees and simulation modeling and was developed by the statistical examination of plot maps of 36 orchards at various times. The spatial patterns of the CTV-infected orchards were examined by Beta-binomial and spatial autocorrelation analyses. Since the brown citrus aphid (Toxoptera citricida) does not exist in California at this time, the spatial patterns of CTV spread are believed to be predominantly associated with the melon aphid (Aphis gossypii) or the use of infected propagation material. Further, these analyses indicated that spatial patterns associated with the Central California CTV/A. gossypii pathosystem are difficult to distinguish from random patterns, both on an individual tree basis and on larger spatial scales of groups of trees over distance. This is in contrast to situations where the CTV/Toxoptera citricida pathosystem has been similarly examined.