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Ten saplings of European ash ( Fraxinus excelsior L.) naturally infected by the invasive ash dieback pathogen Hymenoscyphus fraxineus were collected in Ukraine and Norway and examined for bark necrosis and extension of discoloration in sapwood and pith in a stem region. Tissue-specific colonization profiles were determined by spatial analyses of symptomatic and visually healthy stem tissues using a H. fraxineus-specific qPCR assay and light microscopy. Our data suggest that hyphal growth in the starch-rich perimedullary pith is of particular importance for both axial and radial spread of H. fraxineus, but that most of its biomass accumulates in sapwood parenchyma. The study confirms the results from earlier work and presents new information that refines the current stem invasion model. [ABSTRACT FROM AUTHOR]

Abstract: In order to compare Norway spruce (Picea abies) and silver fir (Abies alba) with respect to susceptibility to wound colonizing fungi, wood stains and rots, experimental cortical lesions were created in situ on mature trees. The study was performed with 40 Norway spruce and 39 silver fir. Two years after the infliction of the lesions the experiment was evaluated with respect to fungal colonization. The most common fungi isolated from xylem beneath the wound area of Norway spruce were Neonectria fuckeliana, Stereum sanguinolentum and Leptodontium beauverioides. Overall, fungal wound infection rate of Norway spruce was 4.6-fold higher compared to silver fir. Only 7% of the Norway spruce stems were sterile in opposite to 67% sterile silver fir trees. Wood decay fungi were isolated from 28% of Norway spruce trees compared to only 8% of silver fir. Accordingly, discolorations in the wood of Norway spruce were clearly larger in their extent. Silver fir mostly displayed only superficial discoloration. On the other hand, the impact on xylem of both tree species was only slightly influenced by the season of wounding (May versus October). Histological examination revealed that in Norway spruce fungal infections occurred through regular resin channels which appeared to be easily colonized by fungi facilitating long distance growth of fungal hyphae. In contrast, traumatic resin ducts formed by both tree species were not found to be infected by fungi. Very obviously, silver fir – being void of primary resin channels – was able to seal injuries by deposition of accessory substances close to the surface of the exposed xylem, thus inhibiting sapwood dehydration and impeding xylem colonization by fungi. For Norway spruce, computer tomography strongly indicated specific sapwood desiccation in the wound region, while in silver fir no such desiccation could be found. [Copyright &y& Elsevier]