First Report of Powdery Mildew Caused by Golovinomyces sonchicola on Sonchus wightianus in China

Abstract

Sonchus wightianus DC. is a perennial plant of the Asteraceae family, which is native to Nepal (Veena et al. 2021). In China, S. wightianus is distributed throughout Hainan Province and is commonly used for edible and medicinal purposes. During the spring of 2021 and 2022, powdery mildew symptoms were observed on 70% of S. wightianus plants on the Hainan Medical University campus (19° 58′ 53″ N; 110° 19′ 47″ E) in Haikou, Hainan Province, China. Powdery mildew colonies covered the leaf surfaces and stems of affected plants, causing discoloration and defoliation. Mycelia were superficial and hyphal appressoria were nipple-shaped, single or in pairs. Conidiophores (n =30) were unbranched, 81 to 209 × 15 to 19 µm. Foot cells (n =30) were cylindrical, straight or curved at the base, 14 to 78 µm long, followed by one to three shorter cells, and formed conidia in chains. Conidia (n =100) were ellipsoid to doliiform-cylindrical, 27 to 39 ×12 to 19 m (length/width ratio = 1.5 to 2.6), lacked fibrosin bodies, and produced germ tubes at their termini. The Eudoidium-type of conidial germination with a short germ tube was observed. Based on these morphological characteristics, the pathogen resembled Golovinomyces sp. (Braun and Cook 2012). The teleomorph was not observed. A specimen was deposited in the Hainan Medical University Plant Pathology Herbarium as HMSW-21. To confirm the genus identification and ascertain a putative species, genomic DNA was extracted from mycelium, conidiophores, and conidia using a fungal DNA kit (Omega Bio-Tek, USA). The rDNA internal transcribed spacer (ITS) region was amplified with primers ITS1/ITS4 (White et al. 1990) and sequenced directly. The resulting 607 bp sequence was deposited in GenBank (accession no. ON365772). A BLASTn search in GenBank of this sequence showed ≥99% similarity with the ITS sequences of G. sonchicola isolates from Mexico (MW425872), Slovenia (MK318522), and the United Kingdom (KY660772). Additionally, the 28S rDNA region was amplified using the primer pairs NL1 and NL4 (O´Donnell 1993; accession no. ON365785). This region shared 99% similarity with published G. sonchicola isolates (OM265397 and AB077624) as well. To confirm pathogenicity, five healthy potted plants of S. wightianus were inoculated by gently pressing a powdery mildew-infected leaf onto 15 young leaves. Five non-inoculated plants served as controls. All plants were maintained in a greenhouse at 24 to 30°C, 70% relative humidity, with a 16-h photoperiod. After 7 days, inoculated leaves showed powdery mildew symptoms whereas no symptoms were observed on control plants. The fungal colonies observed on inoculated plants were morphologically identical to those found on the originally infected leaves collected from Hainan Province. Based on the morphological characteristics and molecular identification, the fungus was identified as G. sonchicola. In different countries and regions, G. sonchicola has been previously reported on some Sonchus species, including S. oleraceus from Mexico (Beltrán-Peña et al. 2021) and Slovenia (Jakše et al. 2019), and S. asper in the United Kingdom. To our knowledge, this is the first record of G. sonchicola infecting S. wightianus in China. We are concerned that the pathogen could become a threat to the widespread planting of S. wightianus in the future.