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.

Automated summary

Two different Phyllactinia species are associated with powdery mildew on ash trees in Eurasia, but they have different distributions in Europe and Southeast Asia. The Asian species P. fraxinicola is not adapted to most of the environmental conditions in Europe, suggesting it does not pose an invasive threat to the continent as a whole. Models of potential distribution show no overlap in Europe, with some susceptibility to invasion by P. fraxinicola in the northwest.