Abstract

Dieback of Fraxinus excelsior, Fraxinus angustifolia and other Fraxinus species is an emerging infectious disease caused by the ascomycete fungus Hymenoscyphus pseudoalbidus(anamorph Chalara fraxinea). Investigations in five forest nurseries in Austria from 2008 to 2011 showed that ash dieback is a common and important disease of nursery seedlings. H. pseudoalbidus was consistently isolated at high frequencies from symptomatic F. excelsior andF. angustifolia seedlings, confirming that this fungus is associated with ash dieback. Symptom observations on potted seedlings revealed progression of the disease outside the vegetation period and thus a long incubation period in the disease cycle of ash dieback. Apothecia of H. pseudoalbidus were occasionally observed on dead ligneous parts of common ash, including stems of nursery seedlings. This suggests that a low portion of diseased seedlings can initiate new infections, which may be important for moving the pathogen to new areas. Based on our observations and studies in forest nurseries and the present knowledge on the disease cycle of ash dieback, recommendations for disease management in tree nurseries and concerning artificial regeneration are presented.

Key words: Chalara fraxinea, Fraxinus excelsior, Fraxinus angustifolia, emerging forest disease, disease management.