First record of Sawadaea polyfida causing powdery mildew on Acer palmatum and A. japonicum in Switzerland and Europe

Abstract

The Japanese maples, Acer japonicum and A. palmatum, and their cultivars are popular ornamental trees in Europe. The powdery mildew, Sawadaea polyfida, occurs natively on these and some other Asian maple species in China, Japan, and Korea (Hirose et al., 2005; Braun & Cook, 2012; Wan et al., 2022), and was introduced into Australia on A. palmatum in 2018 (Kiss et al., 2020). The pathogen was previously unknown in Europe, but Sawadaea tulasnei has been reported on A. japonicum and A. palmatum (Braun & Cook, 2012; Ing, 1990; author's observation in Geneva in 2015). In October 2022, S. polyfida was identified for the first time on A. palmatum in a cemetery in Zurich. After this first finding, further randomly selected sites in Switzerland were monitored. In total, S. polyfida was detected on two A. japonicum and 38 A. palmatum trees. The records were registered at the Swissfungi online distribution atlas (https://swissfungi.wsl.ch/en/distribution-data/distribution-atlas.html.) The morphology of our samples (Figures. 1-3) fitted well with the description of Braun & Cook (2012): white mycelium on leaves, amphigenous, in patches or effuse. Conidiophores erect, 70–150 μm long, septate; macroconidia doliiform, 20–30×10–15 μm, with fibrosin bodies; microconidia broadly ellipsoid 7–13.5 × 5–9 μm, with fibrosin bodies. Chasmothecia dark brown, globose, 170–240 μm in diameter, with a white wreath of many densely standing appendages in the upper half; appendages up to 110 μm long, multiple branched, with uncinate tips; asci numerous 80–100 × 40–50 μm, ascospores (6–)8, up to 25 × 15 μm. The main difference compared to S. bicornis and S. tulasnei is the much higher number of appendages (Braun & Cook, 2012). Collections without or with poorly developed chasmothecia could therefore be misidentified. We confirmed the identification molecularly from a collection subset, including the record on A. palmatum from Geneva, which had been determined morphologically as S. tulasnei in 2015 (L. Beenken, unpublished). Powdery mildews on A. campestre, A. platanoides, and A. pseudoplatanus trees in close vicinity to infected Japanese maples were also sampled. Vouchers were deposited in the herbarium of ETH Zurich (ZT Myc 66412–ZT Myc 66430). DNA was extracted from infected leaves. PCR and sequencing of the ITS region were performed using the PMITS1/PMITS2 primer pair (Cunnington et al., 2003). Sequences were checked by Blast and deposited in GenBank (OQ309157–OQ309175). They were aligned to ITS sequences of S. bicornis, S. polyfida, and S. tulasnei from GenBank to perform a Maximum Likelihood analysis (Figure 4). The results show that all powdery mildews sampled from Japanese maples belong to S. polyfida. In contrast, S. polyfida could not be found on the three European maple species, these trees were infected with S. bicornis or S. tulasnei. The point of first introduction of S. polyfida into Europe is difficult to determine due to possible misidentifications in the past. Therefore, older powdery mildew findings on Japanese maples should be re-determined (e.g., Ing, 1990). The 2015 collection from the Botanical Garden of Geneva was such a misidentification as S. tulasnei, this has now turned out to be the oldest known record of S. polyfida for Switzerland and Europe. The species is unlikely to have occurred much earlier in Switzerland, as Japanese maples have not previously been reported as hosts of powdery mildew from Switzerland (Bolay, 2005; Swissfungi online distribution atlas), not even from the very well-studied Botanical Garden of Geneva (Bolay, 2013). Sawadaea polyfida is probably much more widespread in Europe and can be expected wherever Japanese maples are planted. We thank Prof. Dr. Ottmar Holdenrieder (Zurich), who made the first identification of S. polyfida in Zurich in October 2022, for reporting it to WSL. We are grateful to the technical staff of the WSL plant pathology laboratory for the molecular analyses.