Occurrence of common mistletoe (Viscum album subsp. album) on alien tree species in the urban green areas of Wrocław (Poland)

Common mistletoe (Viscum album) is an evergreen hemiparasitic plant that parasitizes around 200 taxa of trees and shrubs in Poland, including both native and alien species. Most reports from Poland regarding alien host species of mistletoe consist of historical records, mainly from arboreta. The aim of this study was to assess the occurrence and degree of infestation by common mistletoe of alien tree species considered rare in the historic urban green areas of Wroc & lstrok;aw. The research encompassed all green areas where the occurrence of these rare species was recorded, noting the number of sites in which infested specimens were found. The study also included an analysis of the frequency of infestation for each tree species. The presence of common mistletoe was found on 19 out of 57 examined taxa. The infested species belonged to 8 genera and 7 families. The results indicate that Quercus palustris, Celtis occidentalis, Carya ovata, Juglans nigra, Aesculus glabra, and Acer tataricum were the species most frequently colonised by Viscum album under urban conditions in Wroc & lstrok;aw. In contrast, unlike on other taxa of the Tilia L. genus, no mistletoe was found on Tilia xeuchlora nor on species in the Fabaceae family, even though a member of that family - Robinia pseudoacacia - is a common host in Poland. This work can serve as valuable baseline data on the dynamics of mistletoe spread on selected alien tree species not only in Wroc & lstrok;aw and western Poland, but also in Central and Eastern Europe.