Modelling and Estimation of the Relative Potential for Infection of Winter Wheat by Inoculum of Gaeumannomyces graminis Derived from Propagules and Infected Roots

A model was developed to estimate the mean number of infections of seminal roots of wheat exposed to two sources of inoculum of the take‐all fungus Gaeumannomyces graminis var. tritici , in an experimental system. The sources comprise discrete propagules of initial, soil inoculum and infected roots of volunteer plants that had been infected by the initial inoculum, prior to the growth of crop plants. The model was based on the probability of escape from infection by individual roots ofthe crop plants. Parameter estimation was by maximum likelihood. A model was first fitted to data for infection of roots from the soil inoculum. This yielded estimates for the efficiency of soil inoculum to cause infection in the absence of volunteer plants. The parameter for efficiency of infection by soil inoculum was resolved into components for inoculum density, survival of inoculum and the probability of success of individual propagules. The model was extended to include simultaneous exposure of crop roots to soil inoculum and to root inoculum on the volunteer plants. The presence of volunteer seedlings prior to sowing of crop plants resulted in an increase in the effectiveness of inoculum to cause disease. Sowing date and soil condition, as affected by addition of sand, were shown to have significant effects on the efficiency of both sources of inoculum.