Based on the growth-differentiation balance theory (GDB) and the influence of tropospheric ozone (O 3 ) on plants, we hypothesized that pre-conditioning with elevated O 3 reduces adverse effects of the root rot pathogen Phytophthora citricola Sawada. To this end a 2-year phytotron study with juvenile European beech (Fagus sylvatica L.) and (Picea abies [L.] Karst.) grown in mixture was performed. The hypothesis was tested on phenological, leaf and root morphological as well as physiological aspects of plant performance. Contrasting with spruce, elevated O 3 limited leaf and root biomass development, photosynthetic performance and N uptake of beech. The growth limitation by O 3 conveyed increased resistance in beech against the pathogen. Conversely, spruce displayed enhanced susceptibility in the combined O 3 /P. citricola treatment. The hypothesis was supported in the case of beech rather than spruce. Nevertheless, conclusions support GDB regarding the trade-off between growth and stress defense, although compliance appears to be species-specific.