In this doctoral thesis, coumarin 1,2,4-triazoles with potential multiple biological effects related to application in plant protection were synthesized. The synthesis was carried out as a one-pot reaction of coumarin hydrazides (1 – 3) and differently substituted isothiocyanates in a deep eutectic solvent, choline chloride : urea (molar ratio 1 : 2), by stirring with a magnetic stirrer at 80 °C. The result is three series of compounds: the first in which the 1,2,4-triazole ring is connected by a -CH2- linker at position 4 and the hydroxyl group is at position 7 of the coumarin (1a – k), and the second series of derivatives with a 1,2,4-triazole ring connected by an ether linker at position 7 of the coumarin and a methyl group at position 4 of the coumarin (2a – o). The third, additional series of derivatives, differs from the second by the absence of a methyl group at position 4 of coumarin (3a – h). The use of a deep eutectic solvent enabled to carry out the reaction in one step without the need to use catalysts and environmentally harmful organic solvents. The simplified isolation and purification of the products shortened the duration of the process compared to conventional and other published methods. The derivatives of coumarin 1,2,4-triazoles were obtained in yields of 25 to 91 %, and their structures were confirmed by mass spectrometry and 1H and 13C NMR spectroscopy. All prepared compounds were evaluated in silico for their similarity to pesticides and their toxicity to humans and the environment. Most of the compounds were found to have a favorable toxicological profile compared to commercial triazole pesticides. The antifungal activity was tested in vitro on four phytopathogenic fungi, and the antibacterial activity on two phytopathogenic and two soil-beneficial bacteria. Coumarin 1,2,4-triazoles successfully inhibited the growth of fungal mycelia, with the second series of derivatives showing pronounced antifungal activity on Sclerotinia sclerotiorum and Fusarium oxysporum. The synthesized compounds did not show antibacterial activity, neither on the pathogens nor on the soil-beneficial bacteria. Based on the results of antifungal activity, a quantitative structure-activity relationship (QSAR) analysis of the compounds was performed. The model for the effect on S. sclerotiorum was developed using three descriptors (nR=Ct, RDF095m and HATS1e) and can explain 79 % of the inhibitory effect of coumarin 1,2,4-triazoles. The model for the effect on F. oxysporum was developed using four descriptors (R4u+, nAROR, RDF080e and Mor11u) and can explain 77 % of the inhibitory effect of the compounds. The reliability of the developed models was confirmed by internal and external validation methods. The potential mechanism of antifungal activity was investigated by means of molecular docking of the compounds to the enzymes important for mycelial growth (sterol 14α-demethylase, chitinases A and B, N-myristoyltransferase) and enzymes necessary for host cell wall destruction (proteinase K, endoglucanase I and endopolygalacturonase I and II). The compounds fit well into the active sites of demethylase, chitinases A and B, proteinase K, and endopolygalacturonase I and II. Despite good fitting energies, the compounds failed to achieve interactions in the active sites of N-myristoyltransferase and endoglucanase. From the obtained results, it can be concluded that the derivatives of the second series of coumarin triazoles are potential inhibitors of demethylase, chitinases or proteinase K, while the derivatives of the first series of compounds are inhibitors of endopolygalacturonases. Compound 2j stands out for its antifungal activity and its estimated low toxicity, which emphasized it as a good candidate for the development of a new active ingredient for plant protection products.