The present paper tries to demonstrate progress and gap of knowledge in plant pathology through the tomato - tomato powdery mildew host-pathogen interaction as a model. Tomato powdery mildew (Oidium lycopersici) has caused serious damages on glasshouse tomato (Lycopersicon esculentum) crops during the last approximately 15 years. Although the absence of tcleomorph stage did not allow exact taxonomic classification of the pathogen, comparative morphological studies using light and scanning electron microscopy revealed that Oidium lycopersici might be included to the Erysiphe sect. Erysiphe (close to Erysiphe aqui­ legiae var. ranunculi). Effective resistance sources to 0. lycopersici were found mainly in wi ld Lycopersicon hirsutum and L. penne/lii (confirmed by testing with four different 0. lycopersici isolates). Available information on the pathogenic variability of 0. lycopersici is given; host range experiments revealed considerable differences in abi lity of different 0. lycopersici isolates to infect cucumber and tobacco. postulating existence of different pathotypcs lformae specialist of pathogen. Similarly, some Lycopersicon spp. genotypes showed remarkable differential reactions with pathogen isolates, indicating existence of different pathogen races. Information regarding recently detected mechanisms and basis of resistance in Lycopersicon spp. are also men­ tioned. However, more research based on classical, biochemical and molecular approaches is also needed.