THEORETICAL APPROACH TO THE DYNAMICS OF THE INOCULUM DENSITY OF VERTICILLIUM-DAHLIAE IN THE SOIL - FIRST TEST OF A SIMPLE-MODEL

A mathematical equation was developed that describes the inoculum dens ities of microsclerotia of Verticillium dahliae in the soil over a lon g time span. The equation was based on measurable parameters and ecolo gically meaningful principles. In the model, the number of systemic in fections of plant roots during crop growth was related to soil inoculu m density. In turn, formation of microsclerotia in debris and reductio n of the amount of crop growth were related to the number of systemic infections. Finally, a gradual release and mortality of microsclerotia in the soil were included to calculate subsequent inoculum densities in the soil. Fitting the function to experimental data of potato cvs E lement, Ostara, Mirka and Astarte, flax, pea, barley, sugar beet, onio n and faba bean gave a very high correlation between observed and pred icted soil inoculum densities. The clear differences in inoculum produ ction among potato cultivars and other crops were expressed in quantit ative terms. The highest inoculum density after incorporation of the d ebris of a susceptible crop was estimated to occur at 2.3 thermal time units of 3600 degree days (base 0 degrees C). Ten per cent of the ini tial input of inoculum was still present after 4.5 thermal time units. The model was used to predict the dynamics of soil inoculum densities for V. dahliae under various cropping frequency schemes and performed satisfactorily.