First Report of Powdery Mildew Caused by <i>Erysiphe corylacearum</i> on Hazelnut ( <i>Corylus avellana</i> ) in Serbia

In Bački Jarak, Serbia, during a survey of a hazelnut orchard (0.5 ha) in May 2023, symptoms of powdery mildew were observed on 90% of trees. Unlike the common Phyllactinia guttata, white mycelia appeared mainly on leaf adaxial surfaces and husks. All trees of the varieties Tonda Gentile Romana, Hall's Giant, and Istrian Long exhibited signs corresponding to the newly reported agent, Erysiphe corylacearum, recently detected in the neighboring countries of Romania, Hungary, and Bulgaria (Rosati et al. 2021; Kalmár et al. 2022; Boneva et al. 2023). In late September, two morphotypes of chasmothecia were observed simultaneously on the same leaves: smaller, globose chasmothecia with branched appendages on 70% of trees, and larger chasmothecia of P. guttata bearing hyaline appendages with a bulbous base on 90% of trees. Morphological characterization was carried out on the samples belonging to the morphotype not consistent with P. guttata, revealing hyaline, branched, septate mycelium. Hyaline, ellipsoid conidia 22 to 38 µm long and 15 to 21 µm wide (n = 30) formed solitarily on conidiophores. Chasmothecia were spherical, 68 to 85 µm in diameter (n = 30), with 8 to 12 appendages. Each appendage had four to five times dichotomous branched apices with curved tips. Chasmothecia contained 3 to 5 ovoid asci (43 to 59 × 33 to 62 µm; n = 30), and each ascus contained 8 hyaline and ellipsoid ascospores (15 to 21 × 9 to 15 µm; n = 30). Molecular analysis was conducted on a single representative sample derived from chasmothecia morphologically consistent with E. corylacearum collected from the adaxial leaf surface. DNA extraction was performed using the NucleoSpin Plant II kit (Macherey-Nagel, Germany) following the manufacturer’s instructions specific for fungal tissues. PCR amplification targeted the rDNA internal transcribed spacer (ITS) region. The first reaction of semi-nested PCR was performed using the PMITS1/PMITS2 primer pair, followed by the PMITS1/ITS4 primer pair in the second PCR reaction (Csaba et al., 2022). Gel electrophoresis revealed a single amplicon of approximately 700 base pairs. The amplicon was sequenced (Macrogen Europe, The Netherlands) and the obtained sequence was uploaded to GenBank (accession no. PX249794). Sequence analysis using BLAST revealed 100% identity with E. corylacearum isolates from Switzerland (Beenken et al., 2020; MN822722), Romania (Rosati et al., 2021; MW423075), Turkey (Sezer et al., 2017; KY082910), and Italy (Mezzalama et al., 2021; MW045425). Pathogenicity test was carried out on one-year-old healthy plants of C. avellana cv. Tonda Gentile Romana in a controlled environment at 23 - 24 ±2 °C, 80% humidity, and a 12/12 h photoperiod. Plants were artificially inoculated by gently rubbing conidia from naturally infected leaves. White patches appeared on both sides of the leaves 9 to 10 days post inoculation, whereas symptoms were not observed in an untreated control (treated with distilled water). Reisolated samples were morphologically and molecularly identical to field samples, with ITS region analysis confirming E. corylacearum as the causal agent. In the South Bačka region of Serbia first symptoms of E. corylacearum appear in late April to early May, one to two months earlier than P. guttata, during the fruit development stage. It can affect 90% of the adaxial surface of the leaves, causing significant yield loss in this stage. To our knowledge, this is the first report of E. corylacearum on hazelnut in Serbia.