SPATIOTEMPORAL ANALYSIS AND ISOPATH DYNAMICS OF CITRUS SCAB IN NURSERY PLOTS

The Gompertz model was selected over three other models to describe temporal progress of disease incidence of citrus scab, caused by Elsinoe fawcettii, in four nurseries and eight disease control test plots of sour orange in Florida. Aggregation of disease was indicated by spatial auto-correlation analysis for nursery plots for all but one assessment date in one nursery. In three of the four nurseries, isopath boundaries moved predominately northward and away from a central focus of disease, presumably in response to splash dispersal of inoculum resulting from both rain showers and sprinkler irrigation. For the four nurseries and the eight disease control test plots, the spatial rate of spread (v) generally was greater early in the epidemics and when measured at distances further from the focus of infection. Rates of scab increase were reduced significantly in disease control plots by applications of captafol or copper at 30-day intervals compared to a water spray control. For all 12 plots, three-dimensional response surfaces of isopath-bounded areas (A(i)) versus isopath level (I-alpha) and time (t) in days were fit to complex models via nonlinear regression. The best spatio-temporal model (log-normal additive with synergy and intercept) accounted for >95% of the variation in the data for all 12 data sets. Comparison among observed and predicted spatiotemporal data sets demonstrated that captafol significantly affected the spatio-temporal dynamics of citrus scab epidemics by reducing both inoculum production and providing protection to susceptible new leaves, whereas other control strategies did not.