First Report of Powdery Mildew Caused by Erysiphe alphitoides on Exochorda racemosa in China

Pearlbush (Exochorda racemosa, Rosaceae), named for its expanding flower buds that resemble pearls, is a popular ornamental plant in China. The flowers and tender leaves of pearlbush are also used as a high-quality foodstuff. In June 2017, approximately 70% of pearlbush plants in the Jigong Mountains (31°50ʹN, 114°05ʹE) in Xinyang, Henan Province, China, were affected by powdery mildew. Initially, the powdery mildew appeared as white, superficial patches on both sides of leaves. As the disease progressed, abundant masses of conidia covered the whole plant, resulting in reduced growth and extensive leaf and flower drop. Conidia were mostly produced singly, 21.50 to 32.50 µm (average 27.00 µm) × 11.00 to 18.00 µm (n = 100; average 14.50 µm). Conidiophores were unbranched, straight, cylindrical, 41.50 to 97.00 µm long (n = 100; average 69.75 µm). Lobed appressoria were formed on epiphytic mycelia. Foot cells were cylindrical or rarely curved, 10.00 to 41.00 × 1.50 to 7.50 μm (average 25.50 × 4.50 μm), followed by one to three shorter cells. Chasmothecia were not observed on the collected samples. These morphological characteristics were identified as being similar to those of Erysiphe alphitoides (Griffon & Maubl.) U. Braun & S. Takam (Takamatsu et al. 2007). To confirm the identification, the internal transcribed spacer (ITS) region of the isolate JGSBJM01 was amplified using the primers ITS1/ITS4. The ITS sequence deposited in GenBank (accession no. MF623857) showed 97 and 96% homology to the ITS sequences of two E. alphitoides isolates from Argentina and South Korea (AB292699 and KF734882). Additionally, the sequence of the 28S rDNA was amplified using primers NL1 and NLP2 (Takamatsu et al. 2007), which also shared 98% identity with three sequences of E. alphitoides isolates from Australia (AB237822, AB237825, and AB237826). Pathogenicity tests were performed by inoculating the infected plants to healthy leaves via leaf-to-leaf contact. Fifteen inoculated and noninoculated plants were kept in different growth chambers under the same growth conditions (22 ± 2°C, 50% relative humidity, and 16-h photoperiod). After 7 to 15 days, all the inoculated plants developed powdery mildew symptoms similar to those observed on the naturally infected plants, whereas no symptoms appeared on the controls. The fungus on the inoculated leaves was morphologically and molecularly identical to the samples collected in the mountain; thus, the disease cycle was replicated. Previous studies reported that E. alphitoides can infect a wide range of ornamental plants, including Euonymus japonica in China (Huang et al. 2017) and Korea (Lee and Nguyen 2017) and Sorbaria sorbifolia (Denton et al. 2013), Wisteria brachybotrys, and W. frutescens in the United Kingdom (Denton et al. 2016), which suggested that E. alphitoides is a worldwide threat to several ornamental plant species. According to the USDA Fungal Databases, E. alphitoides has not previously been reported on E. racemosa in China (Farr and Rossman 2017). Voucher specimens are available at the Key Laboratory of Plant Genetics and Molecular Breeding, Zhoukou Normal University, Zhoukou, China, as JGSBJM01.