The paper examines the effect of the application of three different preparations on the presence of four economically significant diseases in the crops of pot marigold, valerian and potato. On pot marigold, the intensity was determined of pot marigold powdery mildew infection of the marigold leaf (Podosphaera xanthii (Castagne) U. Braun & Shishkoff), whereas on valerian, it was the intensity of valerian stem canker and black mold (Alternaria alternata (Fr. ex Fr.) Keissel), and in potatoes, the intensity of early blight (Alternaria solani (Ellis & Mart.)) and that of late blight (Phytophthora infestans (Mont.) de Bary). Of the preparations, the synthetic preparation "Sekvenca" ("Sequence") (a.m. difenoconazole, 250 g/l) was applied at a dos...