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United States Department of Agriculture

Agricultural Research Service

Title: Using Survival Analysis to Predict the Risk of Infection in a Ctv Epidemic

Authors
item Gottwald, Timothy
item Taylor, Earl

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: November 7, 2004
Publication Date: May 4, 2005
Citation: Gottwald, T.R. 2005. Survival analysis of the spatio-temporal spread of citrus tristeza virus. 9th Biennial Meeting of the Florida Phytopathological Society, 2-4 May, 2005, University of Florida/IFAS, Mid-Florida Research and Education Center, Apopka, FL.

Interpretive Summary: Citrus Tristeza Virus (CTV) remains one of the most severe pathogens of citrus worldwide, causing large production losses and has caused the death of over 50 million trees. Understanding the disease dynamics is key to developing control measures. One aspect that is poorly understood is the interaction of various strains of CTV under field conditions. An analytical method called survival analysis, originating from the medical and engineering disciplines, was used for the first time to investigate the interaction of two field strains of the CTV virus and how they interact. The method was modified to be able to examine the effect of diseased trees within a near vicinity on the probability of a tree remaining disease free. An interaction between two strains of the virus was demonstrated that was never before detected. This method may be useful in evaluating the effect of one strain to protect the tree from infection with another or to slow the infection within the population of trees within an orchard, and thereby help develop alternative control measures.

Technical Abstract: The spatial and temporal aspects of Citrus Tristeza Virus (CTV) during epidemics have been described previously for both the CTV/Toxoptera citricida and CTV/Aphis gossypii pathosystems. For CTV/T. citricida pathosystem, aggregation of infected trees occurs presumably due to movement of viruliferous aphids within local areas of influence. In addition, long distance spread of CTV has been documented for both pathosystems. One question that arises is, What threat does individual CTV-infected trees present to neighboring trees? Survival analysis was used to examine the probability of survival (remaining in a non-infected state) of CTV-free trees when located at various distances of proximity to CTV-infected trees through time. A risk index was calculated via a modified Cox proportional hazards model to estimate the probability of survival through time of CTV-free trees when located at various distances to trees that became CTV-infected in prior years. The risk of becoming infected was related to previous reported 'local areas of influence'. It was found that a substantial proportion of newly infected trees within a planting could be accounted for by trees determined to be infected 6 months previous within a 'local area of influence' of approximately 24m radius and that 'survival' decreased significantly through time.

Last Modified: 9/21/2014
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