Biological control is a very important factor as sustainable alternative or complement to conventional pesticides for fungal plant disease management. This study was carried out on cotton plants to investigate the roles of cyanobacteria in controlling plant fungal diseases and pesticides biodegradation. Therefore, twelve cyanobacterial  strains were isolated from soil samples that polluted with pesticides in Kafr El- Sheikh, Governorate and identified as fourgenera (i.e. Anabaena, Nostoc, Oscillatoria and Chroococcus). The in vitro study showed antagonistic activities of these cyanobacterial  strains against Fusarium oxysporum, Rhizoctoniasolani and Sclerotium rolfsii. The superior cyanobacterial strains, which antagonised the phytopathogenic fungi were Nostoc muscorum, Nostoc paludosum, Nostoc entophytum,  Nostoc verrucosum, Anabaena oryzae,  Anabaena variabilisandOscillatoria brevis.  These cyanobacterial strains showed different levels of efficiency in increasing the surviving seedlings dry weight and plant height in greenhouse tests, depending on the fungus under consideration; however, the cyanobacterial strains significantly increased these parameters. Under field conditions, also the cyanobacterial strains were effective in increasing the surviving seedlings and yield in 2021 and 2022 seasons. In addition, the results of bioagents materials showed that all cyanobacterial strains produced ammonia and catalase enzyme.  Whereas,  only Anabaena oryzae was found to be positive for  hydrogen cyanide and chitinase enzyme. Finally, the strong action of cyanobacteria is very important in development with fungicides against fungal pathogens.