A growth assay was established for the barley pathogen Rhynchosporium commune with EGFP-tagged fungal mutants. This assay was used to study the effect of four antibiotics (hygromycin B, nourseothricin, kanamycin, phleomycin) and a herbicide (phosphinothricin) on fungal development. Fitting the growth curves to the modified Gompertz model allowed calculating growth parameters, such as lag periods of fungal colony formation and mycelial growth rates as well as EC(50) values. Except kanamycin all compounds were efficient inhibitors so that the corresponding resistance-conferring genes can be used as markers for selection of fungal transformation-based mutants. In addition the assay was used to quantify the inhibitory activity of a barley secondary metabolite, the indole alkaloid gramine.