First Report of Powdery Mildew Caused by Podosphaera xanthii on Cosmos bipinnata in Northeastern China.

Abstract

Cosmos bipinnata (Cav.) is a common garden flowering plant with high ornamental value in China, and which is also widely distributed as a landscaping plant in northeastern China. During the summer of 2020, powdery mildew on C. bipinnata was observed in the garden of Best West Fortune Hotel, Harbin, Heilongjiang Province, China. More than 70% of plants in the garden were affected but at low severity, with only a few older leaves showing yellowing and wilting. Powdery mildew colonies began as small white spots on the upper surface of both young and mature leaves, and then spread to entire leaves. Conidia collected from infected plant tissues were ellipsoid-ovoid to barrel-shaped, with distinct fibrosin bodies visible in their cytoplasm, and measured 20 to 28 × 14 to 19 μm with a length/width ratio of 1.5 to 2.0 (n = 30). Conidiophores were unbranched, straight, 92 to 230 × 9 to 15 μm in size, and produced four to six immature conidia in chains (n = 30). Foot cells of conidiophores were cylindrical and 40 to 70 μm long, with light constriction at the basal septum, and followed by one to three short cells. Fungal hyphae were septate, branched, flexuous to straight, and up to 5 μm wide with indistinct to slightly nipple-shaped appressoria. Chasmothecia were absent in the collected infected samples. The morphological characteristics were consistent with Podosphaera xanthii reported by Braun and Cook (2012). For molecular identification, total gDNA was extracted from fungal colonies on infected leaves of three collections separately. For each DNA sample, the part of LSU and ITS regions were amplified using primers LSU1/LSU2 and ITS1/ITS4 (Scholin et al. 1994; White et al. 1990), respectively. BLASTn analysis of the 617bp (OP218411) and 563bp (MW166865) amplicons revealed 99.67% to 100% sequence identity with respective rDNA sequences of P. xanthii isolates present in GenBank (LSU: KY860729 and MK439610; ITS: MT242593 and MK439611). The phylogenetic trees were constructed using the neighbor-joining method in MEGA 5.0. Based on the ITS rDNA phylogenetic tree, the sequences retrieved from the specimens clustered within a strongly supported clade with P. xanthii. On the basis of morphological characteristics and molecular analysis, the isolate was identified as P. xanthii. Koch's postulates were carried out to prove the pathogenicity of the isolate. A pathogenicity test was performed by dusting conidia from an infected leaf onto young leaves of five healthy C. bipinnata plants in the greenhouse with five non-inoculated plants as a control. Powdery mildew symptoms were observed on inoculated leaves after 7 days of inoculation at 20/25°C (night/day) and 75% relative humidity, whereas control plants remained asymptomatic. The fungus on inoculated leaves was morphologically identified as that observed on the original diseased leaves. P. xanthii has been reported on several hosts in China, such as Zinnia elegans, Verbena bonariensis and Melothria indica (Fan et al. 2022; Hong et al. 2021; Zhong et al. 2022). To our knowledge, this is the first report of P. xanthii infecting C. bipinnata in northeastern China, and the information presented in this note will assist the horticultural industry on developing management strategies for this disease in China.