Spatio-temporal analysis of an HLB epidemic in Florida and implications for spread.

Authors: Gottwald, Timothy; IREY, M.S. - Us Sugar Corporation; GAST, T. - Us Sugar Corporation; PARNELL, S. - Rothamsted Research; Taylor, Earl; Hilf, Mark

Submitted to: International Organization of Citrus Virologists Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 12/17/2008
Publication Date: 5/1/2010
Citation: Gottwald, T.R., Irey, M., Gast, T., Parnell, S., Taylor, E.L., Hilf, M.E. 2010. Spatio-temporal analysis of an HLB epidemic in Florida and implications for spread.. International Organization of Citrus Virologists Proceedings.

Interpretive Summary: Citrus huanglongbing (HLB) is a serious bacterial disease of citrus that first discovered in the Western Hemisphere in 2004. Since then it has been found in Florida, Brazil, throughout the Caribbean, and a few places in Central America including Mexico. In Florida as elsewhere in the world, this disease is not under adequate control. Therefore it is a great threat to the citrus industries of the United States. This article presents a number of analyses fostering a greater understanding of the epidemiology of HLB, that is it analyses that characterize how the disease increases and spreads using data from Florida, the only state with a current HLB epidemic. Key points of this study are comparisons of rates of increase and spread in Florida compared to other places around the world. This paper characterizes disease increase and spread among immediately adjacent trees as well as locally within individual plantings, and regionally across broad areas. It also discusses the implications of the latency of the disease, that is, the time during which the disease is infecting the tree and can be transmitted from a tree but is not yet showing symptoms. This is a very difficult stage of the disease to detect them greatly complicates control. This information serves as a knowledge base for all researchers, commercial industry, and regulatory agencies to better understand the implications of this disease and its epidemiology.

Technical Abstract: Data for Huanglongbing (HLB) epidemics was collected during 5 assessment dates over a 2-year period from 11, 4-ha commercial citrus blocks in Florida. Data were analyzed for regional spatial characteristics via Ripley’s K analyses. Data were fitted to the logistic and Gompertz temporal models, the latter proving to be the superior to represent the increase of HLB through time. Data were also examined using the spatio-temporal stochastic model for disease spread which was fitted using (MCMC) stochastic integration methods. The Ripley’s K analyses demonstrated a continuous relationship among HLB-diseased individuals over a broad range of spatial distances up to 3.5 km, demonstrating short range to regional components for HLB spread by psyllid vectors with a most common distance of 1.58 km which may indicate an average psyllid dispersal distance from a regional point of view. The results of the spatio-temporal analysis were viewed graphically in a two-dimensional parameter space representing a series of 'posterior density' contours of parameter densities. The spatio-temporal model suggested that HLB spread through a combination of random ‘background’ transmission and a ‘local’ transmission that operated over short distances. However, background versus local transmission often did not always occur simultaneously but often alternated. If we consider that the background transmission or primary infection is the result of inoculum sources outside the plots, then this is similar to the spatio-temporal stochastic model results for Citrus tristeza virus (CTV) and other pathosystems with vectors predominately of the migratory type. That is, infective psyllids periodically emigrated from outside the plots and infected test trees causing background or primary infection and then infective psyllids from within the plots caused local or secondary spread.