Mixed infections of plant viruses are common in nature as well as in agriculturally important plants. We were interested in relationship between Potato virus Y (PVY) which is wide-spread in Slovenia and Pepino mosaic virus (PepMV) which is spreading fast across Europe. These two viruses have partially overlapping host ranges and can theoretically appear in the same plant. Effects of mixed infections differ in different hosts, thus we used 3 test hosts: potato (Solanum tuberosum), tomato (Solanum lycopersicum) and tobacco species Nicotiana benthamiana. On different varieties of potato, we established 3 treatments (only PVY, only PepMV and mixed infection); on tomato and tobacco we esatblished 8 inoculation treatments, to investigate if different time course of infection has an effect on relationship bewtween viruses in mixed infection and on the developed disease symptoms. 4 weeks after mechanical inoculation we checked disease symptoms, which differed between inoculation treatments, and isolated total RNA from plant samples. We determined virus titer in samples using quantitative polymerase chain reaction, and normalised the amount of viruses in two different ways: using reference gene COX and the amount of total RNA measured with fluorometry. We confirmed that the normalization methods are comparable. None of the potato varieties was infected with PepMV but in tomato and tobacco we observed mixed infection in case of coinfection and superinfection. In mixed infected plants we observed the strongest disease symptoms. There were differences in virus accumulation between some of the different inoculation treatments.