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.