An empirical method to estimate the effect of soil on the rate for transmission of damping-off disease

Abstract

The ability of some soils to suppress soil-borne diseases has been long recognised, but the underlying epidemiological mechanisms by which this occurs are largely unknown. Using damping-off disease caused by Rhizoctonia solani, spreading through replicated populations of radish (Raphanus sativus) seedlings growing in soil or sand, we introduce and test a method to show how the suppressive effects of soil affect the rates of primary and secondary infection, which control amplification and spread of disease. The method involves spatial mapping of disease over time combined with an epidemiological analysis to distinguish primary from secondary infection in a dynamically changing population of susceptible hosts available for infection. Analysis of the secondary transmission rates revealed three main trends: the transmission rate was lower for soil compared with sand; the transmission rate varied systematically with time, first increasing and then decreasing; and the transmission rate varied amongst replicate epidemics. The consequences of these findings for damping-off epidemics and the potential of this type of analysis to contribute to an epidemiological understanding of the effect of soil on the suppression of epidemics are discussed. © New Phytologist (2004).