First Report of Powdery Mildew Caused by Erysiphe alphitoides on Ipomoea obscura L. in China

Obscure morning glory (Ipomoea obscura L.) is a perennial vine with attractive flowers in the family of Convolvulaceae. In China, this plant is widely distributed throughout the tropical and subtropical regions and is commonly used for ornamental and medicinal purposes. In November 2018, typical symptoms of powdery mildew were observed on I. obscura plants in Hainan University campus (20° 3′ 25″ N; 110° 19′ 4″ E) in the city of Haikou in Hainan Province, China. White, superficial mycelia and conidia covered the leaf surfaces of affected plants, resulting in leaf curling, discoloration, and defoliation. Hyphal appressoria were lobed, solitary or in opposite pairs. Conidiophores were unbranched, 61 to 92 × 9 to 12 µm (average: 78.0 × 11.0 µm) (n = 50). Foot cells were cylindric, straight or rarely curved, 26 to 47 µm long (average: 33.7 µm), followed by one to three shorter cells. Conidia were formed singly and were obovoid-ellipsoid to doliiform, 31 to 52 × 15 to 22 µm (average: 40.3 × 18.3 µm) (n = 100) with a length/width ratio of 1.6 to 2.9 (average: 2.2), without fibrosin bodies, and produced germ tubes from the terminal position. Primary conidia had a rounded top with a subtruncate base. Secondary conidia were doliiform but did not have rounded ends. The outer walls of fresh conidia featured an angular/rectangular pattern. Based on these morphological characteristics, this pathogen was provisionally identified as Erysiphe alphitoides (Takamatsu et al. 2007). The teleomorph was not detected. A specimen was deposited in the Hainan University Plant Pathology Herbarium as HNIO-18. In order to confirm the identification, genomic DNA was extracted from mycelium and conidia collected from a single leaf using a Fungal DNA kit (Omega Bio-Tek, U.S.A.). The rDNA internal transcribed spacer (ITS) region and 28S rDNA were amplified with the primer pairs ITS1 and ITS4 (White et al. 1990) and sequenced directly. The resulting 658-bp sequence was deposited in GenBank (accession no. MN186769). The GenBank BLAST analysis of the ITS sequence showed 99% similarity with E. alphitoides on Quercus sp. from Argentina (AB292699 and AB292702), the United Kingdom (KJ845645 and KJ845646), and Australia (AB292704), as well as with E. alphitoides from Wisteria sinensis (EF183498) and Sorbaria sorbifolia (KC489094) from the United Kingdom. Additionally, the 28S rDNA region was amplified using the primer pairs NL1 and NL4 (O’Donnell 1993, accession no. MN186771). The amplicon was sequenced in both directions and shared 99% similarity with E. alphitoides (AB292699, AB292702, and AB292704). To fulfill Koch’s postulates, five healthy potted plants of I. obscura were inoculated by gently pressing a powdery mildew-infected leaf onto 15 young leaves. Five noninoculated plants served as controls. All plants were maintained in a greenhouse at 24 to 30°C, 70% relative humidity, and a 16-h photoperiod. After 7 days, inoculated leaves showed signs and symptoms of powdery mildew, whereas no signs or symptoms were observed on the control plants. The fungus observed on the inoculated plants was identical morphologically to that on the originally infected leaves. Denton et al. (2016) reported that E. alphitoides has a wide host range, which includes Quercus, Wisteria, and Sorbaria. Recently, Euonymus japonica (Lee and Nguyen 2017) and Exochorda racemosa (Zhang et al. 2018) have been determined as additional hosts for E. alphitoides. This study indicates that E. alphitoides has extended its host range to I. obscura in China. To our knowledge, this is the first record of E. alphitoides infecting I. obscura in China.