The object of the studies was to assess the effectiveness of the product Olejan 85 EC (85% rapeseed oil) against the disease casual agents of powdery mildew, rust, leaf spot and grey mould. Depending on a casual agent, Olejan 85 EC at the concentration of 1% was used for 2–6 spraying treatments at 7-day intervals. The results of performed experiments revealed that Olejan 85 EC used as a curative spraying treatment on rose and pansy plants significantly reduced the development of powdery mildew. After 4 treatments, it caused a 3.3-fold reduction to complete inhibition of Sphaerotheca pannosa var. rosae development on roses. In the case of pansy plants, it caused a complete inhibition of the development of the fungus S. humuli. The preparation Olejan 85 EC applied curatively against leaf spot, significantly inhibited the development of this disease on rose and pansy. After 6 spray treatments, the product caused at least a 1.9-fold reduction in the development of Diplocarpon rosae on rose. On pansy plants, after 4 applications, it caused a 3.1-to-4-fold reduction in the development of Colletotrichum spp. Olejan 85 EC applied as a curative spraying treatment on chrysanthemum and willow inhibited the development of rust. After 4 applications on chrysanthemum, the product caused at least a 2-fold reduction in the development of Puccinia horiana, with 58% of brown and malformed telia. In the protection of willow, after 2 applications, Olejan 85 EC caused at least a 2.3-fold reduction in the development of Melampsora epitea, and from 32 to 75% of the uredinia turned brown and malformed.