Ash Trees (Fraxinus spp.) in Urban Greenery as Possible Invasion Gates of Non-Native Phyllactinia Species

Two Phyllactinia species have been associated with powdery mildew on leaves of ash trees (Fraxinus) in Eurasia, Phyllactinia fraxinicola U. Braun & H.D. Shin from Southeast Asia and Phyllactinia fraxini (DC.) Fuss from Europe. Non-native ash trees are planted in urban greeneries in both Europe and Southeast Asia, but so far, the two Phyllactinia species have not been reported from the same area. Our molecular analysis of European material consisting of 55 Phyllactinia specimens from 15 countries confirmed the absence of P. fraxinicola in Europe. In Europe, we confirmed P. fraxini on all three European native ash species and on the introduced Asian ash species, Fraxinus. chinensis ssp. Rhynchophylla (Hance) A.E. Murray and Fraxinus mandshurica Rupr, planted in arboreta. Among the 11 collections examined from Southeast Asia, 3 were identified as P. fraxini and 8 as P. fraxinicola. The environmental niches of the two Phyllactinia species do not show significant overlap in the multidimensional space defined by bioclimatic variables. This suggests that the Asian species P. fraxinicola is not adapted to conditions prevailing in most of Europe and does not represent an invasive threat across the continent. Models of the potential distribution of Phyllactinia species do not overlap in Europe, but there are some areas to the northwest that could be susceptible to invasion by P. fraxinicola.