A Geostatistical Analysis of the Spatio-Temporal Development of Downy Mildew Epidemics in Cabbage

Spatial and temporal data on disease incidence in an experimental field of red cabbage in which a downy mildew epidemic developed from a point source was analyzed by a geostatistical model. A two-step prediction procedure was defined to formulate a linear predictor for space and time variables based on a spatial semivariogram that changes in time. The model accommodated any pattern of spatial variables, including patterns of healthy and diseased plants. The spatial transposition of the epidemic focus was determined with high precision. Time of initiation of the epidemic was determined with lower precision due to extrapolation in time. The results were compared with data from three other experimental plots: one red cabbage plot from the previous year and two from the same year. For epidemics in white cabbage plots, a single-point source of the epidemic could not be identified. In fact, several small foci were present in the white cabbage plots. Thus, the procedures presented in this paper could distinguish between epidemics originating from a point source and those having more than one source. Optimal sampling plans based on a predetermined precision are proposed.