First Report of Powdery Mildew Caused by Podosphaera xanthii on Ixeris denticulata in China

Ixeris denticulata (Houtt.) Stebb is an annual herbaceous plant in the family of Asteraceae, which is native to Europe or central Asia. This plant is widely distributed in China and is commonly used for edible and medicinal purposes. In February 2019, typical symptoms of powdery mildew were observed on 70% of I. denticulata plants on the campus of Hainan University (20° 3' 25″ N; 110° 19' 4″ E) in Haikou, 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 nipple-shaped, and solitary. Conidiophores were straight, cylindrical, 109 to 259 × 9 to 16 µm (n = 50), and produced 3 to 5 immature conidia in chains with a crenate outline. Foot cells were cylindrical, straight or sometimes constricted at the basal septum, 30 to 62 µm long (n = 100). Conidia were ellipsoid-ovoid to doliiform, 23 to 33 × 15 to 23 µm (n = 100) with a length/width ratio of 1.1 to 1.9, with well developed fibrosin bodies, and produced germ tubes from the lateral position. Based on these morphological characteristics, this pathogen was provisionally identified as Podosphaera xanthii (Braun and Cook 2012). The teleomorph was not detected. A specimen was deposited in the Hainan University Plant Pathology Herbarium as HNID-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, USA). 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 577-bp sequence was deposited in GenBank (Accession No. MT739423). The GenBank BLAST analysis of the ITS sequence showed 100% similarity with P. xanthii on Bidens sp. from Thailand (LC270780), as well as with P. xanthii from Eclipta prostrata (MT260063) and Cyanthillium cinereum (MN203658) from China. Additionally, the 28S rDNA region was amplified using the primer pairs NL1 and NL4 (O'Donnell 1993, Accession No. MT739424). The amplicon was sequenced in both directions and shared 100% similarity with P. xanthii (MK357436, LC371333 and MH137264). To fulfill Koch's postulates, five healthy potted plants of I. denticulata 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