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 dose of 0.5 l/ha, the registered biostimulant "Zlatno inje" ("Golden frost") (manure-based fertiliser) was applied at a dose of 3.0 l/ha, microbiological preparation (Bacillus amyloliquefaciens ssp. TMF3) at a dose of 3.0 l/ha. The fourth variant was the control variant, i.e. the variant without treatment. Field experiments were performed during 2019 at two locations in the wider area of the City of Pančevo. The experiments were performed by a randomised block system with three replications. The size of the main plot was 9.0 m2 (4.5 x 2.0 m). The following was used as plant material: the pot marigold variety of "Domaći oranž" ("Domestic orange"), the valerian variety of "Vojvođanski" ("Vojvodinian") and the potato variety of "Desire". The first assessment of disease intensity was performed one month following the treatment with the selected preparations, and the second one was performed two weeks after the first assessment. In the pot marigold crops, the smallest infection percentage had the plants treated with the type Bacillus amyloliquefaciens TMF3, in valerian, the fewest plants infected with the fungus A. alternata were recorded on small plots on which the synthetic preparation "Sekvenca" was applied, which was also the case with early blight and late blight. The weakest efficiency in the control of pot marigold powdery mildew infection and valerian stem canker and black mold was recorded with the use of the biostimulant "Zlatno inje", which showed the highest efficiency in the case of late blight. The highest yield of pot marigold seeds (632.7 kg/ha) had been treated with the type Bacillus amyloliquefaciens TMF3, with valerian, the highest seeds yield was recorded following the application of the preparation "Sekvenca" (186.1 kg/ha), while in potatoes, the highest amount of tubers was recorded with the preparation "Zlatno inje" (30.83 t/ha). However, if the agroecological and the agrotechnical aspects of production are taken into account, by the use of the type Bacillus amyloliquefaciens TMF3, a satisfactory basis was achieved for various kinds of further research of this and similar preparations in order to improve the existing production of medicinal and other types of plants.