Abstract
The number of Septoria lycopersici pycnospores necessary for inducing tomato infection was reported. In addition to this, the influence of inoculum concentration and site of tomato infection as well as the influence of air humidity and temperature on diseasee expression have been researched. Studies upon the character of S. lycopersici penetration and host-plant parasite interaction were made with the susceptible Bounty tomato under greenhouse conditions. The results of observations concerning the appearance and development of septoria leaf spot in the fields of central Poland were also described.