The general situation regarding ash dieback in Germany and investigations concerning the invasion and distribution strategies of Chalara fraxinea in woody tissue 1

Ash dieback is currently one of the most important tree diseases in Germany. A countrywide distribution is already assumed. Trees of all ages on various site types are affected in forests and the landscape, as well as in nurseries and urban plantings. Initially economic losses were most severe in Northern Germany, but damage has increased in all regions. In several parts of Germany, surveys have been started during the past few years. In North‐East Germany, where the disease was first observed, conditions on various sites were compared to identify the characteristics which influence the disease occurrence and severity. In addition, laboratory investigations were carried out with a large number of coeval ash saplings to study the infection rate, invasion and spread strategy of the causal agent inside plant tissues, as well as the role of soil‐borne Oomycetes as possible primary or accompanying organisms in the disease process. The experiments presented here show that the fungus is able to spread very effectively in wood as a parasite and does not originate from the root system. The results confirm the dominant role of Chalara fraxinea and rule out the importance of pathogenic Oomycetes .