First report of Erysiphe salmonii on Fraxinus ornus and F. excelsior in Slovenia

In October 2022, powdery mildew symptoms were observed on young Fraxinus ornus and F. excelsior trees in multiple locations in central and western Slovenia. The affected F. ornus trees were growing in a mixed deciduous forest (western Slovenia) and in a mixed forest with an equal share of birch, sessile oak, spruce, sweet chestnut and sycamore (central Slovenia). The affected F. excelsior trees were growing in a mixed forest dominated by Fagus sylvatica. The chasmothecia were brown to black in colour (Figure 1), formed on the upper and lower leaf surfaces, measured 76–130 μm (average 96 μm), and had 15–22 appendages, 113–147 μm long, 6–8 μm wide (up to 10 μm towards the tip), hyaline and brownish at the base, with spirally curved tips. The chasmothecia contained 2–5 ovoid asci, measuring 45–62 × 27–44 μm with hyaline ascospores measuring 17–26 × 11–14 μm (Figure 1). Based on these morphological characteristics, the fungus was identified as Erysiphe salmonii (Braun & Cook, 2012). Conidiophores and conidia were not observed. Morphological identification of the powdery mildew from specimens collected was confirmed by sequencing the ITS rDNA region, and the obtained sequences (GenBank Accession Nos. OP962435 and OP962436) showed 100% identity with the reference ITS-rDNA sequences (e.g., MW265935 from Switzerland, OK383397 from Austria, and MW633028 from Romania). Voucher specimens were deposited in the Mycotheca and Herbarium of the Slovenian Forestry Institute (Accession Nos. LJF 8105, LJF 8107, LJF 8108). This is the first report of Erysiphe salmonii on F. excelsior and F. ornus in Slovenia. The disease is believed to originate from East Asia (Braun & Cook 2012) and has previously been observed in Ukraine (Heluta et al., 2017), Switzerland (Beenken & Brodtbeck, 2020), Austria (Voglmayr et al., 2021) and Romania (Chinan & Dascălu, 2022), suggesting that E. salmonii is spread throughout Europe. Further work is needed to determine the impact of E. salmonii on other Fraxinus species, including F. angustifolia. The work was funded by the Slovenian Research Agency (Programme P4-0107) and Ministry of Agriculture, Forestry and Food (Public Forestry Service).