INOCULUM DENSITY AND SPATIAL PATTERN OF RHIZOCTONIA-SOLANI IN-FIELD PLOTS OF SOLANUM-TUBEROSUM - EFFECTS OF CROPPING FREQUENCY

Bait plants, comprising micropropagated and commercial seed tubers, we re used to estimate the effects of rotation on the density and spatial pattern of inoculum of Rhizoctonia solani in large field plots of pot atoes. Soilborne inoculum of R. solani produced economically significa nt levels of stem canker and its incidence and severity varied with ro tation, with most disease in 2-year and less in 4- and 6-year rotation s. The rates of loss of inoculum during intercrop periods differed amo ngst rotations with a rapid fall to low levels occurring after 1 year in a 6-year rotation and after 2 years in a 4-year relation. Replenish ment of inoculum to soil was rapid following the growth of a susceptib le crop, with comparatively high levels of infection and disease, even in long rotations. Disease occurred in patches and the size of patche s and the density of R. solani within patches differed with cropping f requency. The degree of spatial autocorrelation also differed amongst rotations but there was no evidence for any significant differences in the rate of change of spatial autocorrelation during intercrop period s in the three rotations.