This article presents fungicidal properties of 9 synthesized nicotinamide-bromides with different alkyl side chain lengths toward Fusarium graminearum , Sclerotinia sclerotiorum , and Botrytis cinerea which were examined. The fungicidal properties were determined by the measurement of the radial growth of fungi, followed by the calculation of the antifungal index. The obtained results were correlated with the descriptors from DFT calculations to determine structural features that affect the fungicidal properties of nicotinamides. Based on the experimental and theoretical results, it was confirmed that F. graminearum is most resistant to the change of lipophilicity of compounds, while S. sclerotiorum is most sensitive. For all investigated compounds, the growth rate decreased with the increase of carbon atoms in the side chain until tetradecylnicotinamidium bromide, C14NicBr, while the further prolongation of the alkyl side chain increased the growth rate of fungus. This behavior was explained by the distinguished hydrophobic and hydrophilic surfaces in C14NicBr due to interactions between keto oxygen and bromide anion absent in the case of nicotinamides with a longer chain.