Abstract: Black spot caused by Diplocarpon rosae Wolf is one of the most common and dangerous disease of roses grown in the field, but recently also under covering. During the vegetation season conidia are the major source of the pathogen spread, therefore the studies aimed at finding the most efficient method to reduce spores germination were undertaken. Experiments were carried out to determine the effect of vegetable oils: corn, olive, rapeseed (canola), sunflower, soybean, and grape; vegetable oils recommended as adjuvants: Dedal 90 EC (90% vegetable oil) and Olejan 80 EC (85% rapeseed oil); and mineral oils: Atpolan 80 EC (76% mineral oil SN), Ikar 95 EC (95% SAE mineral oil), Olemix 84 EC (84% DSA mineral oil), Promanal 60 EC (60% mineral oil), and Sunspray 850 EC (85% mineral oil) used at a concentration of 1% for a one-off spray treatment of rose bushes to control the germination of D. rosae spores. Observations of the number of germinating spores were conducted 1, 7 and 14 days after the treatment. In addition, 24 hours after spraying the bushes, an assessment of the effect of the tested oils on the spores of the fungus was carried out under a scanning electron microscope. The effectiveness of the tested oils in comparison with the control depended on the oil being tested, the time of observation and the experiment. After 1 day, the effectiveness of the tested oils ranged from 6.2% (olive oil) to 90.9% (Olemix 84 EC); after 7 days, from 4.2% (Promanal 60 EC) to 99% (Olemix 84 EC and Sunspray 850 EC); and after 14 days, from 0% (Atpolan 80 EC and Olemix 84 EC) to 94.1% (Ikar 95 EC).

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