Common Ash Stand Affected by Ash Dieback in the Wolica Nature Reserve in Poland

Abstract

The ash stand in Wolica reserve (Poland), affected with ash dieback, was studied. Isolations performed from collected ash tissues and rhizosphere soil samples revealed 28 isolates of Hymenoscyphus fraxineus and 27 isolates of Phytophthora spp., respectively. The vitality and defoliation of 198 and 176 trees were studied, respectively in 2012 and 2013. In 2013 only one tree was completely vital, while 83 trees were within the degeneration phase. These results suggested that trees being classified in this class are the most vital and the natural genetic resistance should be sought among this vitality class in the future. In total, 112 trees were classified to the stage 2 of vitality, for which large deformation of shoots is typical. Further, monitoring of defoliation in 2013 revealed that the largest number of trees in the stand (126) were moderately damaged trees (defoliation 26-60%), while 47 trees had over 60% of defoliation. The synthetic damage index was 1.58 in 2012 and 1.66 in 2013 indicating that advanced disease processes are occurring in this stand. In addition, sampling, isolation, morphological and molecular identifications of Phytophthora species were performed. After the isolation tests, P. megasperma, P. sp. hungarica, and P. plurivora were obtained. These results were confirmed after the PCR and ITS sequencing. This is the first report of P. sp. hungarica and P. megasperma in the stands of common ash in Poland. The natural genetic variation of the Fraxinus excelsior genome was studied to improve understanding of its role in the adaptation and tolerance processes facing ash dieback phenomenon. Six nuclear microsatellite markers and four chloroplast microsatellite markers have been used in order to assess the genetic diversity of Fraxinus excelsior stand in Poland, categorized into three different Roloff' classes of vitality 0+1, 2 and 3. We demonstrated lack of correlation between three different vitality classes of ash trees and their nuclear or chloroplast genetic differentiation. Nevertheless, the observed heterozygosity (H-O) value was significantly different between vitality classes 2 and 3 assessed with nuclear SSR markers (p = 0.000183 in HSD Tukey test, p lt 0.05). Also private (Ap) alleles distribution of chloroplast SSR markers significantly differ (p = 0.000 in HSD Tukey test, p lt 0.05) between the vitality classes 0+1 and 3 of ash trees. Those data suggest that DNA differentiation of F. excelsior at local spatial scale may be driven by gene based tolerance.