The present invention relates to inhibition of GTP cyclohydrolase II as target for the identification of fungicides. Binding assays are claimed and coupled enzymatic assays are disclosed involving the conversion of formate (product of the reaction catalyzed by GTP-cyclohydrolases) by formate dehydrogenase and spectroscopically measuring the NADH formed. Furthermore in vivo assays with organisms containing a GTP-cyclohydrolase II are claimed as well as a DNA sequence for a fungal GTP-cyclohydrolase II. 3-Butyl-10-(3-chlorophenyl)- 10H-pyrimido[4,5-b]quinoline-2,4-dione is identified by the enzymatic method as a fungicide. Finally, methods of controlling fungi (e.g. on plants, in soil) by applying this agent are claimed.
