Steps in predicting the relationship of yield on fungicide dose

A set of hypothetical steps has been defined, which links fungicide dose to marketable yield, whereby (i) increasing dose decreases symptom area, acco rding to a dose-response curve, (ii) decreased symptom area increases crop green area index (GAI), (iii) increasing GAI increases fractional intercept ion of photosynthetically active radiation, (iv) increased fractional inter ception increases crop dry matter accumulation, and (v) yield increases, de pending on the partitioning of dry matter to the marketable fraction. One e quation represented all five steps. By integrating this equation for light interception during the yield forming period and differentiating with respe ct to the ratio of fungicide cost over yield value, an analytical solution was obtained for the economic optimum dose. Taking published ranges of para meter values for the Septoria tritici wheat pathosystem as an example, yiel d-response curves and optimum doses were biologically plausible when compar ed with data from four field experiments. The analytical and empirical resu lts imply that the dose required to optimize economic return will vary subs tantially between sites, seasons. and cultivars. Sensitivity analyses ident ified parameters describing specific facets of disease severity, fungicide efficacy, and assimilate partitioning as most influential in determining th e dose optimum.