An ecologically, economically and generally useful genus of forest trees, which for generations has been spreading in the lowland areas of the Sava-Drava confluence and enriching the Posavina floodplain-swamp forests with biodiversity as the leading species of these stands, ash and its species common ash (Fraxinus excelsior L.) and narrow leaf ash (Fraxinus angustifolia Vahl) are rapidly dying out. Events from the past have shaped the current state of the lowland flood forests of central Posavina. Climatic changes, combined with anthropogenic influence, have left a long-lasting change in the entire ash habitat, altering the hydrological conditions in the wider catchment area, but also in the rivers themselves. In addition to abiotic factors, unknown biotic factors prevail in the process of the death of ash forest stands. In this paper, the causative agents of field ash diseases are described, in which, among numerous fungi and fungi- like organisms, the most pronounced symptoms have recently been the influence of the pathogenic species Hymenoscyphus fraxineus. The processes that lead to this phenomenon on the leaves and bark and later within the physiology of the trees are also possible effects of the disturbance of several habitat factors in ash forests, but we consider this fungus to be the main biotic factor that represents the greatest immediate danger to these forest stands