Solanum tuberosumaspartic proteases (StAPs) with antimicrobial activity are induced after abiotic and biotic stress. In this study the ability ofStAPs to produce a direct antimicrobial effect was investigated. Viability assays demonstrated thatStAPs are able to kill spores ofFusarium solaniandPhytophthora infestansin a dose-dependent manner. Localization experiments with FITC-labelledStAPs proved that the proteins interact directly with the surface of spores and hyphae ofF. solaniandP. infestans. Moreover, incubation of spores and hyphae withStAPs resulted in membrane permeabilization, as shown by the uptake of the fluorescent dye SYTOX Green. It is concluded that the antimicrobial effect ofStAPs againstF. solaniandP. infestansis caused by a direct interaction with the microbial surfaces followed by membrane permeabilization.