Computer Modelling of Damping-Off Epidemics in Relation to Host Density and Number of Disease Foci

Abstract

In a previous experimental study of post-emergence damping-off epidemics in populations of cress seedlings, rate of advance ( A ) of spreading disease fronts and apparent infection rates ( r ) were measured separately in relation to differences in host density. In this study, a computer model was built to simulate those damping-off epidemics, with the aim of relating A and r. Over a range of host densities it was found that r could be predicted from the known quantitative relationship between A and host density, provided that the number of primary infection foci was also taken into account. Scrutiny of the earlier experimental results revealed that the number of primary infection foci separately influenced the sub- sequent infection rate. The importance of number of foci in this instance is attributed to the very steep inoculum gradient characteristic of this disease, which limits inoculum transmission to plants around the periphery of disease patches so that disease increase is controlled more by the sum of disease perimeters than by the sum of diseased infectious plants as in air-borne diseases. It is expected that other slow-spreading soil-borne diseases with steep inoculum gradients will follow this same pattern.