Modeling and Quantitative Analysis of Biological Control Mechanisms

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Abstract
Cellulose and chitin added to soil induced changes in inoculum potential-disease interactions in Rhizoctonia [R. solani] preemergence damping-off of radish [Raphanus sativus cv. Early Scarlet Globe]. Cellulose added to soil resulted in significant reduction in slope values of the inoculum density-disease incidence (ID-DI) curve when the logarithm of infections was plotted against inoculum density expressed as logarithm of propagules per gram (the log-log transformation). Slope values of the ID-DI curve were near 1.0 for experiments in nonamended soil which is the value predicted if a rhizosphere effect exists for the host-pathogen relationship. Values were not significantly different from 0.67 for experiments in soil to which cellulose had been added; this value is characteristic for a rhizoplane relationship between host and pathogen; i.e., propagules being able to infect only at the surface of the infection court. Thus, a mathematical model involving competition in biological control can be developed to describe the recduction of the rhizosphere to a rhizoplane for damping-off of radish when cellulose is added to soil. In contrast, slope values of ID-DI curves between nonamended and chitin-amended soil did not differ significantly, but the position of the curve was shifted to the right of that for the nonamended soil. This suggests that control is due to the presence of inhibitory compounds following additions of chitin since proportions of propagules participating in infection did not vary with changes in inoculum density. Neither cellulose nor chitin added to soil reduced the population of R. solani 9-14 days after application when compared with nontreated controls. Precise differences in efficiency among control measures may be obtained by determining how inoculum potential is influenced by treatments in ID-DI curves.