In phytopathological laboratory of Tobacco Institute-Prilep in vitro investigations were

made on the presence of toxic matters in inoculum of the pathogen Phytophthora parasitica var. nicotianae and their impact on tobacco plant, to determine whether these toxic metabolites cause necrotic reactions and other damages on the host-plant. Comparison was made between symptoms resulting from the effect of toxic metabolites and symptoms caused by the pathogen itself. Inoculum was prepared from suspension of fungus mycelium and filtrate as selective agent obtained from the fungus grown on liquid medium and after that filtrated through filter paper and bacteriological filter of 0.22 μ porosity. Leaves of Nicotiana nesophila species and leaves of two oriental and one Burley variety of Nicotiana tabacum were used in the investigation. Inoculation was made by two methods: adding a drop of the inoculum on leaf surface and submerging the base of the leaf in the inoculum. In leaves inoculated by both methods, differences in necrosis development on leaf surface could be noticed. The necrosis developed more rapidly in leaves inoculated with a suspension of pathogen’s mycelium, where percentage of damaged leaf tissue was also higher compared to the leaves inoculated with selective agent. There were no differences, however, between leaf symptoms caused by toxic metabolites and those caused by the pathogen itself. The aim of the investigation was to determine the infectivity of toxic compounds released by the pathogen and the effects of two methods of inoculation on the percentage of damaged leaf tissue.

Key words: tobacco, pathogen, toxins, inoculum, suspension