In this graduation thesis we are trying to determine the causes of sessile oak(Quercus petraea) dieback in the local unit Adlešiči and in this way contribute to better forest management. In the fall of 2006 and 2007 we visited the stand several times with special attention to presence of fungi that form their fruit bodies at that time of the year. In the spring 2007 we also performed sample gathering of potential disease agents on the declining trees in the time of their cut down. At the Slovenian Forestry Institute we microscoped samples of fungal fruitbodies that were gathered, took their pictures, and on the base of their characteristics and with the help of determination keys we found out their identity. Nine different fungal species were determined, which are known to accompany the process of oak decline. The current alarming state of the stand is also the consequence of the anthropogenic origin and other negative anthropogenic influences on the forest. As the primary pathogen on oak roots we determined the fungus Collybiafusipes for the first time in Slovenia. With unusually fast development on sessile oak and its large frequency we considered chestnut blight (Cryphonectira parasitica) to be an important factor of dieback. The other pathogens and saprophytic fungi were found on dead and dying thin branches on crown tops: Colpoma quercinum, Coryneum elevatum, Cryphonectria parasitica, Fusicoccum quercus, Hercospora taleola and Botryosphaeria melanops. Some of these fungi are beneficial in healthy stands, but are also frequent companions of decaying oaks and are causing injuries on branches in crowns of the weakened trees.