The incidence of fungal diseases affecting olive trees increased in Tunisia. Mycoflora isolated from diseased olive trees revealed their colonisation by several fungal species such as Fusarium oxysporum, F. solani, F. lateritium, Macrophomina phaseolina, Rhizoctonia solani, Athelia rolfsii, Neonectria radicicola, Verticillium dahliae, Armillaria mellea, Phytophthora megasperma, P. palmivora and Pythium sp. These fungi were also isolated from roots and collars of young olive cuttings in the nursery, where they caused severe losses. Most of the fungi were pathogenic and caused typical symptoms of dieback upon their inoculation on olive plants, which fullfill the Koch’s postulate. These pathogenic fungi showed the ability to infect, either separately or jointly the olive tree. Moreover, cropping of olive in high clay soil and irrigation with high saline water increase the fungal incidence and plays a major role in the disease outbreak.