Matsiakh, I., Solheim, H, Hietala, A.M., Nagy, N.E. and Kramarets, V. 2017. Assessment of Seasonal Patterns in Tissue-specific Occurrence of Hymenoscyphus fraxineus in Stems of Fraxinus excelsior. Baltic Forestry 23(1): 20-24.

   Dieback of European ash (Fraxinus excelsior L.), a disease caused by the ascomycete Hymenoscyphus fraxineus (previ-ously referred to as H. pseudoalbidus or Chalara fraxinea), was first observed in Poland in the early 1990ies, and is currently present almost throughout the entire distribution area of European ash. The characteristic symptoms of the disease include dead shoots with necrotic lesions in the bark and discoloration of xylem and pith but the seasonal dynamics of pathogen spread in shoot tissues remain poorly understood. To investigate whether the internal spread of the fungus involves season-specific pat-terns, saplings with necrotic bark lesions in 1-2 -year-old stem regions were collected during 2014-2015 at time intervals in spring, summer, autumn and winter at several localities in western Ukraine and at two localities in south-eastern Norway. Tissue-specific presence of H. fraxineus was determined by a highly sensitive quantitative real-time PCR assay that is specific to DNA of H. fraxineus. The relatively high proportion of bark samples positive for H. fraxineus in the saplings collected during spring provides support to a model that H. fraxineus can be a primary causative agent of bark lesions and that other fungi may eventual-ly replace it in old infection areas.

Keywords: ash dieback, Hymenoscyphus fraxineus, bark, phloem, pith, qPCR (Real-Time PCR).