Diplodia fraxini: The Main Pathogen Involved in the Ash Dieback of Fraxinus angustifolia in Croatia

<i>Fraxinus angustifolia</i>, the main ash species in Croatia in terms of economic and ecological importance, is affected by a severe dieback initially attributed to the fungal pathogen <i>Hymenoscyphus fraxineus</i>. Recently, another pathogen, <i>Diplodia fraxini</i>, has been shown to play a key role in ash dieback in several European countries. Therefore, because the dieback symptoms of ash trees observed in Croatia are typical of <i>Botryosphaeriaceae</i> attacks, the aim of this study was to define the etiology of <i>F. angustifolia</i> dieback. To this end, symptomatic shoots and branches and cross-sections of the main stem were sampled from 20 symptomatic trees at eight locations and analyzed for the presence of <i>D. fraxini</i> and other possible fungal pathogens. <i>Diplodia fraxini</i> was the fungus most frequently associated with branch cankers and dieback; it was isolated from 17 trees in all sites monitored, and its pathogenicity towards <i>F. angustifolia</i> was confirmed. The fungus was also associated with wood necrosis at the base of the trunk in two trees. Other fungi, namely <i>H. fraxineus</i>, <i>Diaporthe eres</i>, <i>Diplodia seriata</i>, <i>Botryosphaeria dothidea</i>, <i>Armillaria gallica</i>, and <i>Lentinus tigrinus</i>, were isolated sporadically.