Role of Root and Stem Base Fungi inFraxinus angustifolia(Vahl) Dieback in Croatian Floodplain Forests

Large-scale ash (Fraxinusspp.) dieback caused by the fungusHymenoscyphus fraxineushas been a major concern throughout Europe for more than two decades. Most of the related research has been focused onFraxinus excelsiorL., and there is still little information on fungal involvement in the dieback ofFraxinus angustifoliaVahl, especially in roots and stem bases, which play an important role in decline progress and tree stability. The objectives of this study were to identify fungi present in visually healthy and symptomatic wood tissues in basal parts of narrow-leaved ash trees in different decline phases, in order to determine the possible role of these fungi and their importance in the dieback process. The stem bases and roots of 90 trees in three different health categories, determined based on crown defoliation, were sampled in natural stands affected by ash dieback. Isolated fungal cultures were identified based on the rDNA ITS (Internal transcribed spacer) region and their association with tree health status was analyzed. In total, 68 different fungal taxa were confirmed, includingHymenoscyphus fraxineusandArmillariaspp., which were mainly present in roots, although overall in lower frequencies than on common ash in other studies. Most frequently isolated fungal taxa, which encompassed 51% of all obtained isolates, wereTrichodermaspp.,Ilyonectria robusta,Fusarium solani,Cladosporium cladosporioides, andDiaporthe cotoneastri. Their associations with tree health categories and presence in both symptomatic and visually healthy wood indicate that they act as opportunistic pathogens and early colonizers of weakened ash tissues. Research also revealed that, although the extent of crown defoliation and presence of root and stem necroses were associated, basal symptoms occurred on a number of trees with healthy looking crowns, meaning they can develop independently.