The objective of undertaken studies was evaluation in vitro conditions efficiency of eight fungicides from different chemical groups and three preparations of natural origin i.e. Biosept Active, Beta-chikol and Bioczos towards isolate M 365 Boeremia strasseri, obtained from peppermint rhizomes showing symptoms of black rot. Test were performed by poisoning the culture media. The percentage of inhibition of the growth of four- and eight-day-old colonies on the medium with preparations in the control colonies was a measure of activity of the preparations. Moreover, the microscopic observations of morphological structures of fungus were conducted. Within chemical compounds trifloxystrobin, tiophanate methyl and mancozeb pointed to be the most effective. The effectiveness of Beta-chikol and Biosept Active in limiting B. strasseri colony growth was higher than the efficiency of Bioczos.