TEMPORAL AND SPATIAL-ASPECTS OF THE EPIDEMIOLOGY OF SORGHUM DOWNY MILDEW ON MAIZE

The development of systemic disease from primary inoculum sources of s orghum downy mildew was studied on field-grown maize in Thailand. Data were recorded five times, from the first appearance of disease until 5 weeks after plant emergence. The incidence of diseased plants decrea sed with increasing distance from the primary inoculum sources, and th e slope of the gradient flattened as the epidemic progressed, The stee pest gradient of disease incidence was observed downwind. The progress in time and spread in space of disease about primary foci is describe d by three non-linear models which fit the data equally well. However, the resulting gradients at wider distances are different. With two mo dels the gradients decrease asymptotically to zero with increasing dis tance, whilst the other model leads to negative values above a certain distance. The rates of isopath movement of all models decrease with t ime, but the effect of distance on the isopathic rate is different; th e rate can decrease, stay constant or increase with distance.