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

Abstract

Data for Huanglongbing (HLB) epidemics were collected during five assessment dates over a 2-yr 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 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.