First Report of Powdery Mildew Caused by Erysiphe corylacearum on Hazelnuts in Georgia

Abstract

Hazelnuts (Corylus avellana) are widely grown in the Republic of Georgia and considered a major agricultural export product (Gurcan et al. 2010). Powdery mildew in hazelnut trees is a major disease that causes significant yield loss (Sezer et al. 2017). Until recently, Phyllactinia guttata was considered the main causal agent in the neighboring countries of Western Asia and the Caucasus (Abasova et al. 2018; Arzanlou et al. 2018). However, in the last 3 years a new destructive species, Erysiphe corylacearum, has been identified as a pathogen of powdery mildew on hazelnut in the neighboring countries of Iran, Azerbaijan, and Turkey (Abasova et al. 2018; Arzanlou et al. 2018; Sezer et al. 2017). During May and July 2018, powdery mildew symptoms were observed in hazelnut plantations of the cultivar Anakliuri in Adjara region (western Georgia). Symptoms were observed on the upper leaf surface and fruit clusters including husks. Disease incidence reached 100%, and severity ranged between 10 and 70%. On heavily affected leaves, necrotic lesions were observed 9 to 13 days after the first symptoms appeared, followed by leaf curling and defoliation. Morphological characterization of leaf and husk samples showed the following characters: hyphal appressoria (n = 30) were lobed, solitary, 1 to 4 μm in diameter; mycelium was amphigenous, hyaline, branched, septate 1.8 to 5.3 μm wide; conidiophores (n = 35) vertically elevated above the mycelium 53 to 82 μm long and 5 to 12 μm wide; produced solitary conidia (n = 50) on conidiophores, hyaline, ellipsoid to ovoid, 24 to 36 μm long, 14 to 24 μm wide. Chasmothecia appeared in early October. They were spherical, single or in groups, 73 to 104 μm in diameter, appendages six to 15 straight, 72 to 101 μm long, four to five times dichotomous branched, aseptate or one with a septum at the base. In each chasmothecium, there were three to five asci (n = 50) with a width of 38 to 53 μm and a length of 26 to 38 μm of oval-ellipsoid shape. Asci contained four to eight ascospores (n = 50), 18 to 25 μm long and 8 to 12 μm wide. Pathogenicity testing was conducted according to Koсh’s postulates. Two-year-old plants of C. avellana, cultivar Anakliuri, were artificially infected by dusting conidia from infected leaves (n = 25). Inoculated plants were incubated under 20 to 28°C and 70 to 80% humidity. Typical symptoms (fluffy white bloom) appeared on the upper surface of the leaves within 8 to 10 days after inoculation. No symptoms were found on control plants treated with sterile water. The fungus isolated from the inoculated leaves (n = 25) was morphologically identical to the original isolates from natural diseased plants (n = 10). DNA was extracted, and the rDNA internal transcribed spacer region of four isolates was amplified using primers ITS1 and ITS4 and sequenced. BLAST analysis of our 593-bp fragments (all identical and represented by GenBank accession no. MK157199) showed 99% identity to ITS rDNA sequences of E. corylacearum from Azerbaijan (LC270863) and Turkey (KY082910) and only 83% similarity to P. guttata (accession no. AB080558).