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by applied methodology, thus not supporting the hypothesis that fungus has spread from infected seeds to seedlings planted in the field from the local nursery. Still, these findings do not exclude the possibility that the fungus could be present and thus spread on the surface of samaras, since this aspect of transmission was not investigated. The fact that this pathogen has been confirmed in both symptomatic and visually healthy seeds from trees of various levels of susceptibility to the fungus in similar research conducted on Fraxinus excelsior (Cleary et al. 2013, Hayatgheibi 2013, Marčiulynienė et al. 2018) and not in the Fraxinus angustifolia seeds analysed in this research, could be due to high summer temperatures (July and August 2017 maximum > 35 °C) (DHMZ 2017b, a) characteristic for the narrow-leaved ash distribution area in the Republic of Croatia, which seems to be a limiting factor for the spread of pathogen (Hauptman et al. 2013, Grosdidier et al. 2018) or due to seed collection method, where only seeds from visually healthy narrow-leaved ash trees from registered seed stands and natural stands registered as seed sources are collected for further purpose of nursery seedling production. In addition, recent surveys conducted by Marčiulynienė et al. (2018) found no evidence of fungus being able to spread from infected seeds to grown plants, which still doesn’t exclude this possibility in the opinion of authors.
Analysed narrow-leaved ash seeds collected from visually healthy trees from registered seed sources and seed stand revealed relatively low level fungal presence in comparison to other similar studies, indicating good health status and usability for further nursery seedling production regarding this particular aspect. Identified fungal species haven’t caused visible symptoms on seeds after one to two months of storage, not excluding their possible negative effect on seeds after longer period of storage or storage in unfavourable conditions, since some of them are known as seed pathogens and some are reported as opportunistic parasites in necrotic tissues of Fraxinus spp. Presence of pathogenic fungus Hymenoscyphus fraxineus in seeds was not confirmed, so it can be concluded that potential dieback of seedlings caused by this pathogen in nurseries or in the field is a consequence of infections from affected narrow-leaved ash stands in the vicinity rather than spread of fungus from infected seeds.
This research was carried out with the financial support of the Croatian Science Foundation under the project „The role of biotic agents on vitality of narrow-leaved ash (Fraxinus angustifolia Vahl) in Croatian floodplain forests“ – FRAXINPRO (IP-11-2013). We thank Croatian Forests Ltd. for providing samples for this research and Mirjana Grahovac-Tremski for kindly providing all the necessary information regarding seed origin, collection and storage.
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