Occurrence of Powdery Mildew Caused by Erysiphe corylacearum on Hazelnuts in Israel

Abstract

Hazelnut (Corylus avellana L) is an emerging crop in Israel, primarily cultivated as a host plant to establish truffle plantations through symbiosis with ectomycorrhizal fungi. A significant damage and yield reduction is caused by the prevalent occurrence of powdery mildew in hazelnut trees (Sezer et al., 2017). Until recently, Phyllactinia guttata was considered the primary pathogen in Western Asia, the Caucasus region, and Europe (Abasova et al. 2018; Arzanlou et al. 2018; Mezzalama et al. 2021). However, in the last years, a new destructive species Erysiphe corylacearum has been identified as the pathogen of powdery mildew on hazelnuts in these regions (Meparishvili et al. 2019; Mezzalama et al. 2021; Kalmár et al. 2022; Zajc et al. 2023). In May 2022, powdery mildew symptoms were observed on hazelnut plants in the Ein-Zivan truffle plantation, gardens of Merom-Golan, and the adjacent garden of the packing house Pri-Beresheet in the northern Golan region of Israel. Symptoms were observed on the abaxial and adaxial leaf surfaces, fruits, and husks. Disease incidence and severity ranged between 30-70% and 5-90%, respectively. Disease severity was significantly greater on the leaves of the offshoots compared to those on the tree canopy. Morphological characterization of leaf samples from ten different trees showed the following characteristics: hyphal appressoria were lobed, solitary, 1–4 μm in diameter; mycelium was amphigenous, hyaline, and septate; conidiophores vertically elevated from the mycelium 50– 80 μm long. Conidia (n= 30) on conidiophores were hyaline, ellipsoid to ovoid, 24 – 34 μm long and 15.5 – 23 μm wide. Chasmothecia in several maturation degrees appeared in October on both sides of the leaves. They were spherical (n= 30) 86 – 125 μm in diameter, with 7 - 14 aseptate straight appendages, 67 - 96 μm long, 4.9 - 7.1 μm wide, dichotomous branched at the end 42 - 56 μm wide. In each chasmothecia, there were 3-5 asci (n=30) with a width of 38 – 43 μm and a length of 48–64 μm of oval-ellipsoid shape. Asci contained 4-8 ascospores (n=30), 18 - 26 μm long and 11 - 15.5 μm wide. A pathogenicity test was conducted to fulfill Koсh’s postulates. Both detached leaves and plants of C. avellana were artificially infected by brushing conidia from infected leaves. Inoculated leaves in Petri dishes on 2% water agar (n= 5), and plants (n= 5) were incubated under 25°C and 12-h photoperiod/day. Untreated leaves and plants served as control. Typical symptoms appeared on the upper surface of the leaves within 7-10 days after inoculation. No symptoms were found on untreated control plants or detached leaves. The fungus isolated from the inoculated leaves was morphologically identical to the original isolates from natural diseased plants. DNA was extracted and the rDNA internal transcribed spacer region of five isolates originated from leaves of the tree canopy and offshoots was amplified using primers ITS1 and ITS4, and sequenced. BLAST analysis of 595 bp fragments (all identical and represented by isolate Cora1, GenBank Accession No. OR752437) showed 99% identity to ITS rDNA sequences of E. corylacearum from Georgia (MK157199) and 100% identity to isolates from Azerbaijan, Turkey and Italy (LC270863, KY082910 and MW045425, respectively) and only 83% similarity to P. guttata (accession number AB080558). To the best of my knowledge this is the first report on E. corylacearum causing powdery mildew in Israel. Future control measures to manage the disease on hazelnuts in truffle plantations in Israel should be considered.