First report of powdery mildew caused by Erysiphe salmonii on Fraxinus excelsior and F. ornus in Austria

Abstract

In September and October 2021, prominent powdery mildew disease symptoms were seen on leaves of Fraxinus excelsior (common ash) at two localities in Vienna, and of F. ornus (manna or flowering ash) at three localities in southern Carinthia, Austria. The symptoms differed from those previously observed (caused by Phyllactinia fraxini) by mycelia appearing also on the upper leaf surface and by smaller chasmothecia bearing appendices with spirally curved apices. The disease was observed on small trees growing at the margins of mixed deciduous forests and in hedges. Disease symptoms started as small spots of white surface mycelium, that radially enlarged to form effuse patches, soon becoming confluent and then covering the entire upper leaf surface (Figures 1, 2). On the lower leaf surface, surface mycelia were less conspicuous (Figure 3a). On young surface mycelium, ellipsoid conidia (29–34 × 14–17 μm) were produced singly on conidiophores (Figure 4). Brown to black chasmothecia, 90–120 μm in diameter were formed abundantly (Figure 3), which had 10–23, 100–150 μm long and 5–9 μm wide, stiff, hyaline appendages with a brownish base and spirally curved tips. The chasmothecia contained 5–6 broadly ellipsoid-obovoid, mostly (5-)6-8 spored asci, 52–72 × 35–60 μm in size, with hyaline ascospores measuring 19–24 × 10–13 μm (Figure 5). Based on these characters, the powdery mildew was identified as Erysiphe salmonii (Braun & Cook, 2012; Heluta et al., 2017; Yamaguchi et al., 2021). To confirm the morphological identification, the ITS-LSU of samples WU 44779, WU 44780 and WU 44783 was sequenced. DNA extraction from chasmothecia and amplification of the ITS-partial LSU rDNA gene were done following Voglmayr et al. (2020). The obtained sequences were identical and deposited in GenBank (Accession Nos. OK324154, OK324155 and OK383397), and voucher specimens were deposited in the fungarium of the University of Vienna (WU). An nBLAST analysis of the ITS sequence revealed 99.3-100% identity to sequences of E. salmonii, with 100% identity to the ex-epitype sequence LC577619 from Japan (Yamaguchi et al., 2021). Native to East Asia, E. salmonii was first recorded in Europe from Ukraine on Fraxinus excelsior and F. pennsylvanica in 2015 (Heluta et al., 2017). In autumn 2020, E. salmonii was detected on F. ornus in Switzerland (Beenken & Brodtbeck, 2020) and on F. excelsior in Romania (Chinan & Dascălu, 2021), indicating a rapid spread. Although both F. ornus and F. excelsior are native and common in the area of southern Carinthia, we have not yet observed E. salmonii on common ash there. However, the Viennese records confirm earlier observations (Heluta et al., 2017; Chinan & Dascălu, 2021) that F. excelsior can also be severely infected, posing a potential additional threat to common ash, which is already severely suffering from ash dieback disease. Therefore, this emerging pathogen should be critically monitored for occurrence and impact on common ash.